wireless communications - World Institute Of Technology(WIT)

wireless communications - World Institute Of Technology(WIT)
LECTURE NOTES
WIRELESS
COMMUNICATION
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UNIT 1
INTRODUCTION TO WIRELESS COMMUNICATION SYSTEMS
Evolution of mobile radio communications
During The first wire line telephone system was introduced in the year 1877. Mobile
communication systems as early as 1934 were based on Amplitude Modulation (AM) schemes
and only certain public organizations maintained such systems. With the demand for newer and
better mobile radio communication systems during the World War II and the development of
Frequency Modulation (FM) technique by Edwin Armstrong, the mobile radio communication
systems began to witness many new changes. Mobile telephone was introduced in the year 1946.
However, during its initial three and a half decades it found very less market penetration owing to
high
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Figure: The worldwide mobile subscriber chart.
Costs and numerous technological drawbacks. But with the development of the cellular concept in
the 1960s at the Bell Laboratories, mobile communications began to be a promising field of
expanse which could serve wider populations. Initially, mobile communication was restricted to
certain official users and the cellular concept was never even dreamt of being made commercially
available. Moreover, even the growth in the cellular networks was very slow. However, with the
development of newer and better technologies starting from the 1970s and with the mobile users
now connected to the Public Switched Telephone Network (PSTN), there has been an
astronomical growth in the cellular radio and the personal communication systems. Advanced
Mobile Phone System (AMPS) was the first U.S. cellular telephone system and it was deployed in
1983. Wireless services have since then been experiencing a 50% per year growth rate. The
number of cellular telephone users grew from 25000 in 1984 to around 3 billion in the year 2007
and the demand rate is increasing day by day. A schematic of the subscribers is shown in
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Figure: Basic mobile communication structure.
Examples of wireless comm. Systems.
Wireless is a term used to describe telecommunications in which electromagnetic waves (rather
than some form of wire) carry the signal over part or all of the communication path. Some
monitoring devices, such as intrusion alarms, employ acoustic waves at frequencies above the
range of human hearing; these are also sometimes classified as wireless.
The wireless method of communication uses low-powered radio waves to transmit data
between devices. High powered transmission sources usually require government licenses to
broadcast on a specific wavelength. This platform has historically carried voice and has grown
into a large industry, carrying many thousands of broadcasts around the world. Radio waves are
now increasingly being used by unregulated computer users.
Humans communicate in order to share knowledge and experiences. Common forms of
human communication include sign language, speaking, writing, gestures, and broadcasting.
Communication can be interactive, transitive, intentional, or unintentional; it can also be verbal or
nonverbal. In addition, communication can be intrapersonal or interpersonal.
We owe much to the Romans that in the field of communication it did not end with the Latin
root communicate. They devised what might be described as the first real mail, or postal system,
in order to centralize control of the empire from Rome. This allowed Rome to gather knowledge
about events in its many widespread provinces.
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The first wireless transmitters went on the air in the early 20th century using radiotelegraphy
(Morse code). Later, as modulation made it possible to transmit voices and music via wireless, the
medium came to be called "radio." With the advent of television, fax, data communication, and
the effective use of a larger portion of the spectrum, the term "wireless" has been resurrected.
Common examples of wireless equipment in use today
(1) Cellular phones and pagers: provide connectivity for portable and mobile applications,
both personal and business
(2) Global Positioning System (GPS): allows drivers of cars and trucks, captains of boats and
ships, and pilots of aircraft to ascertain their location anywhere on earth
(3) Cordless computer peripherals: the cordless mouse is a common example; keyboards and
printers can also be linked to a computer via wireless
(4) Cordless telephone sets: these are limited-range devices, not to be confused with cell
phones
(5) Home-entertainment-system control boxes: the VCR control and the TV channel control
are the most common examples; some hi-fi sound systems and FM broadcast receivers
also use this technology
(6) Remote garage-door openers: one of the oldest wireless devices in common use by
consumers; usually operates at radio frequencies
(7) Two-way radios: this includes Amateur and Citizens Radio Service, as well as business,
marine, and military communications
(8) Baby monitors: these devices are simplified radio transmitter/receiver units with limited
range
(9) Satellite television: allows viewers in almost any location to select from hundreds of
channels
(10) Wireless LANs or local area networks: provide flexibility and reliability for
business computer users
Wireless technology is rapidly evolving, and is playing an increasing role in the lives of people
throughout the world. In addition, ever-larger numbers of people are relying on the technology
directly or indirectly. (It has been suggested that wireless is overused in some situations, creating
a social nuisance.) More specialized and exotic examples of wireless communications and control
include:
(11)
Global System for Mobile Communication (GSM): a digital mobile telephone
system used in Europe and other parts of the world; the de facto wireless telephone
standard in Europe
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(12)
General Packet Radio Service (GPRS): a packet-based wireless communication
service that provides continuous connection to the Internet for mobile phone and computer
users
(13) Enhanced Data GSM Environment (EDGE): a faster version of the Global System
for
Mobile (GSM) wireless service
(14) Universal Mobile Telecommunications System (UMTS): a broadband, packet-based
system offering a consistent set of services to mobile computer and phone users no matter
where they are located in the world
(15) Wireless Application Protocol (WAP): a set of communication protocols to standardize
the way that wireless devices, such as cellular telephones and radio transceivers, can be
used for Internet access
(16) i-Mode: the world's first "smart phone" for Web browsing, first introduced
in Japan; provides color and video over telephone sets
PAGING SYSTEMS
Paging Systems are wireless communication systems that are designed to send brief messages to a
subscriber. It's a one-way messaging system in which Base Station send messages to all
subscribers. The Paging System transmits the message also known as Page, along with Paging
System access number, throughout the service area using Base Station, which broadcast the page
on a radio link.
Types of Paging Systems
The Paging Systems can be of two types.
Manual Paging System: In a manual paging system, a message is sent to the paging operator
through telephone call by the caller. The message is then delivers to the pager through paging
network by the operator.
Automatic Paging System: In an automatic paging system, the incoming requests are
automatically processed by the paging terminal and then this information is delivers to the pager.
Automatic Paging Systems are mostly used.
Messages in Paging Systems
One of the following four types of information messages can be delivered in a Paging System.
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•
•
•
•
Alert Tone Message
Voice Message
Digital String Message
Text String Message
Alert Tone Message: In the alert tone message, a dedicated telephone number is assigned to the
receiver, which is also known as Tone Pager. The pager is triggered by dialing the number. To
generate tone-type messages, the advantage of tone paging is that it utilizes a small amount of
airtime.
Voice Message: In the voice message, a voice message can be transmitted in some tone paging
systems after the beep.
Digital String Message: In digital string message, the receiver is a Numeric Pager. The string can
be the telephone number of the caller or a coded message. This coded message is generated on
request of the caller by the paging center and is decoded by a codebook built into the pager. This
type of paging takes less amount of airtime.
Text String Message: In the text string message, the receiver is an Alphanumeric Pager, which
has large screen to display the text strings. This type of messaging is becoming more popular than
numeric messaging.
Cordless telephone systems:
All cordless phones operate in the frequency range of 43-50 MHz. Cordless phone systems are
used inside a building or in a particular coverage area of tee signal.
Base Unit of Cordless Phone
Base unit is always plugged into the telephone jack, mostly on the walls. It consists of the
following
components.
Phone line interface unit
radio unit
power unit
Phone Line Interface
All the telephone signals are being sent and received through the phone line by this interface.
These are the two major functions of the phone line interface. It sends the ringing signal to the
bell or to the radio components which shows there is an incoming call. It also sends the signals
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out from the handset to the person to whom the conversation is made. It happens like the voice
signals are converted into electrical signals which are the changes and these changes are finally
sent.
Radio Unit/Radio Component
This unit receives the signals which are electrical signals from the keypad buttons and also from
the phone line interface. Vital function of this unit is to convert the electrical signals into radio
waves suitable to be broadcasted through the antenna. Quartz crystals are used to send and receive
the signals, which function as quartz crystal oscillator
Power Unit
Low voltage required by the electrical parts is supplied by a direct current dc power cube
transformer, a type of power supply transformer. It also consists of rechargeable battery. Other
luxury parts are liquid crystal displays like organic liquid crystal display, dialing keypads and
memory like solid state memory.
Cordless Phone Handset
Since it is cordless, one can carry the handset everywhere inside the house or within the frequency
range of transmitter. Handset consists of
a speaker to convert the electrical signals into audio signals
a microphone to convert voice(audio) signals into electrical signals
keypad to dial
buzzer to give incoming call alerts
light emitting diodes as indicators
battery which is rechargeable
Comparison of various wireless systems
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Early radio systems transmitted analog signals. Today most radio systems transmit digital signals
composed of binary bits, where the bits are obtained directly from a data signal or by digitizing an
analog signal. A digital1radio can transmit a continuous bit stream or it can group the bits into
packets. The latter type of radio is called a packet radio and is characterized by bursty
transmissions: the radio is idle except when it transmits a packet. The first network based on
packet radio, ALOHANET, was developed at the University of Hawaii in 1971.
This network enabled computer sites at seven campuses spread out over four islands to
communicate with a central computer on Oahu via radio transmission. The network architecture
used a star topology with the central computer at its hub. Any two computers could establish a bidirectional communications link between them by going through the central hub. ALOHANET
incorporated the first set of protocols for channel access and routing in packet radio systems, and
many of the underlying principles in these protocols are still in use today. The U.S. military was
extremely interested in the combination of packet data and broadcast radio inherent to
ALOHANET. Throughout the 1970’s and early 1980’s the Defense Advanced Research Projects
Agency (DARPA) invested significant resources to develop networks using packet radios for
tactical communications in the battlefield. The nodes in these ad hoc wireless networks had the
ability to self-configure (or reconfigure) into a network without the aid of any established
infrastructure. DARPA’s investment in ad hoc networks peaked in the mid 1980’s, but the
resulting networks fell far short of expectations in terms of speed and performance.
These networks continue to be developed for military use. Packet radio networks also found
commercial application in supporting wide-area wireless data services. These services, first
introduced in the early 1990’s, enable wireless data access (including email, file transfer, and web
browsing) at fairly low speeds, on the order of 20 Kbps. A strong market for these wide-area
wireless data services never really materialized, due mainly to their low data rates, high cost, and
lack of “killer applications”. These services mostly disappeared in the 1990s, supplanted by the
wireless data capabilities of cellular telephones and wireless local area networks (LANs). The
introduction of wired Ethernet technology in the 1970’s steered many commercial companies
away from radio-based networking.
Ethernet’s 10 Mbps data rate far exceeded anything available using radio, and companies did not
mind running cables within and between their facilities to take advantage of these high rates. In
1985 the Federal Communications Commission (FCC) enabled the commercial development of
wireless LANs by authorizing the public use of the Industrial, Scientific, and Medical (ISM)
frequency bands for wireless LAN products. The ISM band was very attractive to wireless LAN
vendors since they did not need to obtain an FCC license to operate in this band. However, the
wireless LAN systems could not interfere with the primary ISM band users, which forced them to
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use a low power profile and an inefficient signaling scheme. Moreover, the interference from
primary users within this frequency band was quite high. As a result these initial wireless LANs
had very poor performance in terms of data rates and coverage. This poor performance, coupled
with concerns about security, lack of standardization, and high cost (the first wireless LAN access
points listed for $1,400 as compared to a few hundred dollars for a wired Ethernet card) resulted
in weak sales. Few of these systems were actually used for data networking: they were relegated
to low-tech applications like inventory control. The current generation of wireless LANs, based
on the family of IEEE 802.11 standards, have better performance, although the data rates are still
relatively low (maximum collective data rates of tens of Mbps) and the coverage area is still small
Wired Ethernets today offer data rates of 100 Mbps, and the performance gap between wired and
wireless LANs is likely to increase over time without additional spectrum allocation. Despite the
big data rate differences, wireless LANs is becoming the preferred Internet access method in
many homes, offices, and campus environments due to their convenience and freedom from
wires. However, most wireless LANs support applications such as email and web browsing that
are not bandwidth-intensive.
The challenge for future wireless LANs will be to support many users simultaneously with
bandwidth-intensive and delay-constrained applications such as video. Range extension is also a
critical goal for future wireless LAN systems. By far the most successful application of wireless
networking has been the cellular telephone system. The roots of this system began in 1915, when
wireless voice transmission between New York and San Francisco was first established. In 1946
public mobile telephone service was introduced in 25 cities across the United States. These initial
systems used a central transmitter to cover an entire metropolitan area. This inefficient use of the
radio spectrum coupled with the state of radio technology at that time severely limited the system
capacity: thirty years after the introduction of mobile telephone service the New York system
could only support 543 users. A solution to this capacity problem emerged during the 50’s and
60’s when researchers at AT&T Bell Laboratories developed the cellular concept.
Cellular systems exploit the fact that the power of a transmitted signal falls off with distance.
Thus, two users can operate on the same frequency at spatially-separate locations with minimal
interference between them. This allows very efficient use of cellular spectrum so that a large
number of users can be accommodated. The evolution of cellular systems from initial concept to
implementation was glacial.
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UNIT 2
MODERN WIRELESS COMMUNICATION SYSTEMS
2G: Second Generation Cellular Networks
Digital modulation formats were introduced in this generation with the main technology as
TDMA/FDD and CDMA/FDD. The 2G systems introduced three popular TDMA standards and
one popular CDMA standard in the market. These are as
1. TDMA/FDD Standards
(a) Global System for Mobile (GSM): The GSM standard, introduced by Group Special Mobile,
was aimed at designing a uniform pan-European mobile system. It was the first fully digital
system utilizing the 900 MHz frequency band. The initial GSM had 200 KHz radio channels, 8
full-rate or 16 half-rate TDMA channels per carrier, encryption of speech, low speed data services
and support for SMS for which it gained quick popularity.
(b) Interim Standard 136 (IS-136): It was popularly known as North American Digital Cellular
(NADC) system. In this system, there were 3 full-rate TDMA users over each 30 KHz channel.
The need of this system was mainly to increase the capacity over the earlier analog (AMPS)
system.
(c) Pacific Digital Cellular (PDC): This standard was developed as the counterpart of NADC in
Japan. The main advantage of this standard was its low transmission bit rate which led to its better
spectrum utilization.
2. CDMA/FDD Standard
Interim Standard 95 (IS-95): The IS-95 standard, also popularly known as CDMA One, uses 64
orthogonally coded users and codeword’s are transmitted simultaneously on each of 1.25 MHz
channels. Certain services that have been standardized as a part of IS-95 standard are: short
messaging service, slotted paging, over-the-air activation (meaning the mobile can be activated by
the service provider without any third party intervention), enhanced mobile station identities etc.
3. 2.5G Mobile Networks
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In an effort to retrofits the 2G standards for compatibility with increased throughput rates to
support modern Internet application, the new data centric standards were developed to be overlaid
on 2G standards and this is known as 2.5G standard.
Here, the main up gradation techniques are: supporting higher data rate transmission for web
browsing 2.5G networks also brought into the market some popular application, a few of which
are: Wireless Application Protocol (WAP), General Packet Radio Service (GPRS), High Speed
Circuit Switched Dada (HSCSD), Enhanced Data rates for GSM Evolution (EDGE) etc.
3G: Third Generation Networks
3G is the third generation of mobile phone standards and technology, superseding 2.5G. It is
based on the International Telecommunication Union (ITU) family of standards under the
International Mobile Telecommunications-2000 (IMT-2000). ITU launched IMT-2000 program,
which, together with the main industry and standardization bodies worldwide, targets to
implement a global frequency band that
Would support a single, ubiquitous wireless communication standard for all countries, to provide
the framework for the definition of the 3G mobile systems. Several radio access technologies
have been accepted by ITU as part of the IMT-2000 framework. 3G networks enable network
operators to offer users a wider range of more advanced services while achieving greater network
capacity through improved spectral efficiency. Services include wide-area wireless voice
telephony, video calls, and broadband
Wireless data, all in a mobile environment. Additional features also include HSPA data
transmission capabilities able to deliver speeds up to 14.4Mbit/s on the down link and 5.8Mbit/s
on the uplink.
3G networks are wide area cellular telephone networks which evolved to incorporate high-speed
internet access and video telephony. IMT-2000 defines a set of technical requirements for the
realization of such targets, which can be summarized as follows:
1. High data rates: 144 kbps in all environments and 2 Mbps in low-mobility and indoor
environments
Symmetrical and asymmetrical data transmission
2. circuit-switched and packet-switched-based services
3. Speech quality comparable to wire-line quality improved spectral efficiency
Several simultaneous services to end users for multimedia services
Seamless incorporation of second-generation cellular systems
Global roaming
Open architecture for the rapid introduction of new services and technology.
3G Standards and Access Technologies
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As mentioned before, there is several different radio access technologies defined within ITU,
based on either CDMA or TDMA technology. An organization called 3rd Generation Partnership
Project (3GPP) has continued that work by defining a mobile system that fulfills the IMT-2000
standard. This system is called Universal Mobile Telecommunications System (UMTS). After
trying to establish a single 3G
Standard, ITU finally approved a family of 3G standards, which are part of the 3G framework
known as IMT-2000:
W-CDMA
CDMA2000
TD-SCDMA
Europe, Japan, and Asia have agreed upon a 3G standard called the Universal Mobile
Telecommunications System (UMTS), which is WCDMA operating at 2.1 GHz. UMTS and
WCDMA are often used as synonyms. In the USA and other parts of America, WCDMA will
have to use another part of the radio spectrum.
3G W-CDMA (UMTS)
WCDMA is based on DS-CDMA (direct sequence code division multiple access) technology in
which user-information bits are spread over a wide bandwidth (much larger than the information
signal bandwidth) by multiplying the user data with the spreading code. The chip (symbol rate)
rate of the spreading sequence is 3.84Mcps, which, in the WCDMA system deployment is used
together with the 5-MHz carrier spacing. The processing gain term refers to the relationship
between the signal bandwidth and the information bandwidth. Thus, the name wideband is
derived to differentiate it from the 2GCDMA (IS-95), which has a chip rate of 1.2288 Mcps. In a
CDMA system, all users are active at the same time on the same frequency and are separated from
each other with the use of user specific spreading
Codes. The wide carrier bandwidth of WCDMA allows supporting high user-data rates and also
has certain performance benefits, such as increased multipath diversity. The actual carrier spacing
to be used by the operator may vary on a 200-kHz grid between approximately 4.4 and 5 MHz,
depending on spectrum arrangement and the interference situation.
In WCDMA each user is allocated frames of 10 ms duration, during which the user-data rate is
kept constant. However, the data rate among the users can change from frame to frame. This fast
radio capacity allocation (or the limits for variation in the uplink) is controlled and coordinated by
the radio resource management (RRM) functions in the network to achieve optimum throughput
for packet data services and to ensure sufficient quality of service (QoS) for circuit-switched
users. WCDMA
Supports two basic modes of operation: FDD and TDD. In the FDD mode, separate 5-MHz
carrier frequencies with duplex spacing are used for the uplink and downlink, respectively,
whereas in TDD only one 5-MHz carrier is time shared between the uplink and the downlink.
WCDMA uses coherent detection based on the pilot symbols and/or common pilot. WCDMA
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allows many performance- enhancement methods to be used, such as transmit diversity or
advanced CDMA receiver concepts. Table
Summaries the main WCDMA parameters. The support for handovers (HO) between GSM and
WCDMA is part of the first standard version. This means that all multi-mode WCDMA/GSM
terminals will support measurements from the one system while camped on the other one. This
allows networks using both WCDMA and GSM to balance the load between the networks and
base the HO on actual measurements from the terminals for different radio conditions in addition
to other criteria available
Multiple access method DS-CDMA
Duplexing method Frequency division duplex/time division duplex Base station synchronisation
Asynchronous operation Chip rate 3.84 Mcps Frame length 10 ms Service multiplexing Multiple
services with different quality of service requirements multiplexed on one connection Multi-rate
concept Variable spreading factor and multicode Detection Coherent using pilot symbols or
common Pilot Multi-user detection, smart antennas Supported by the standard, optional in the
Implementation The world's first commercial W-CDMA service, FoMA, was launched by NTT
DoCoMo in Japan in 2001. FoMA is the short name for Freedom of Mobile Multimedia Access,
is the brand name for the 3G services being offered by Japanese mobile phone operator NTT
DoCoMo. Elsewhere, W-CDMA deployments have been exclusively UMTS based. UMTS or WCDMA, assures backward compatibility with the second generation GSM, IS-136 and PDC
TDMA technologies, as well as all 2.5G TDMA technologies. The network structure and bit level
packaging of GSM data is retained by W-CDMA, with additional capacity and bandwidth
provided by a new CDMA air interface
Wireless Local Loop (WLL)
Microwave wireless links can be used to create a wireless local loop. The local loop can be
thought of as the "last mile" of the telecommunication network that resides between the central
office (CO) and the individual homes and business in close proximity to the CO. An advantage of
WLL technology is that once the wireless equipment is paid for, there are no additional costs for
transport between the CO and the customer premises equipment. Many new services have been
proposed and this includes the concept of Local Multipoint Distribution Service (LMDS), which
provides broadband telecommunication access in the local exchange
Wireless local area networks
A wireless local area network (WLAN) links two or more devices using some wireless
distribution method (typically spread-spectrum or OFDM radio), and usually providing a
connection through an access point to the wider internet. This gives users the mobility to move
around within a local coverage area and still be connected to the network. Most modern WLANs
are based on IEEE 802.11 standards, marketed under the Wi-Fi brand name.
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Architecture
Stations :All components that can connect into a wireless medium in a network are referred to as
stations. All stations are equipped with wireless network interface controllers (WNICs). Wireless
stations fall into one of two categories: access points, and clients. Access points (APs), normally
routers, are base stations for the wireless network. They transmit and receive radio frequencies for
wireless enabled devices to communicate with. Wireless clients can be mobile devices such as
laptops, personal digital assistants, IP phones and other smartphones, or fixed devices such as
desktops and workstations that are equipped with a wireless network interface.
Basic service set :The basic service set (BSS) is a set of all stations that can communicate with
each other. Every BSS has an identification (ID) called the BSSID, which is the MAC address of
the access point servicing the BSS. There are two types of BSS: Independent BSS (also referred
to as IBSS), and infrastructure BSS. An independent BSS (IBSS) is an ad-hoc network that
contains no access points, which means they can not connect to any other basic service set. An
infrastructure BSS can communicate with other stations not in the same BSS by communicating
through access points.
Extended service set :An extended service set (ESS) is a set of connected BSSs. Access points in
an ESS are connected by a distribution system. Each ESS has an ID called the SSID which is a
32-byte (maximum) character string.
Distribution system: A distribution system (DS) connects access points in an extended service
set. The concept of a DS can be used to increase network coverage through roaming between
cells. DS can be wired or wireless. Current wireless distribution systems are mostly based on
WDS or MESH protocols, though other systems are in use.
Types of wireless LANs
Peer-to-Peer or ad-hoc wireless LAN
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An ad-hoc
hoc network is a network where stations communicate only peer to peer (P2P). There is no
base and no one gives permission to talk. This is accomplished using the Independent Basic
Service Set (IBSS).
A peer-to-peer (P2P) network allows wireless de
devices
vices to directly communicate with each other.
Wireless devices within range of each other can discover and communicate directly without
involving central access points. This method is typically used by two computers so that they can
connect to each other to form a network. If a signal strength meter is used in this situation, it may
not read the strength accurately and can be misleading, because it registers the strength of the
strongest signal, which may be the closest computer.
Hidden node problem:: Devices A and C are both communicating with B, but are unaware of each
other IEEE 802.11 defines the physical layer (PHY) and MAC (Media Access Control) layers
based on CSMA/CA (Carrier Sense Multipl
Multiplee Access with Collision Avoidance). The 802.11
specification includes provisions designed to minimize collisions, because two mobile units may
both be in range of a common access point, but out of range of each other.
The 802.11 has two basic modes of operation:
operation: Ad hoc mode enables peer-to-peer
peer
transmission
between mobile units. Infrastructure mode in which mobile units communicate through an access
point that serves as a bridge to a wired network infrastructure is the more common wireless LAN
application the
he one being covered. Since wireless communication uses a more open medium for
communication in comparison to wired LANs, the 802.11 designers also included shared-key
shared
encryption mechanisms: Wired Equivalent Privacy (WEP), Wi-Fi
Fi Protected Access (WPA,
WPA2), to secure wireless computer networks.
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Bridge:
A bridge can be used to connect networks, typically of different types. A wireless Ethernet bridge
allows the connection of devices on a wired Ethernet network to a wireless network. The bridge
bri
acts as the connection point to the Wireless LAN.
Wireless distribution system
A Wireless Distribution System enables the wireless interconnection of access points in an IEEE
802.11 network. It allows a wireless network to be expanded
expanded using multiple access
ac
points without
the need for a wired backbone to link them, as is traditionally required. The notable advantage of
WDS over other solutions is that it preserves the MAC addresses of client packets across links
between access points. An access point can be either a main, relay or remote base station. A main
base station is typically connected to the wired Ethernet. A relay base station relays data between
remote base stations, wireless clients or other relay stations to either a main or another relay base
bas
station. A remote base station accepts connections from wireless clients and passes them to relay
or main stations. Connections between "clients" are made using MAC addresses rather than by
specifying IP assignments.
All base stations in a Wireless Distribution System must be configured to use the same radio
channel, and share WEP keys or WPA keys if they are used. They can be configured to different
service set identifiers. WDS also requires that every base station be configured to forward to
others in the system.
WDS may also be referred to as repeater mode because it appears to bridge and accept wireless
clients at the same time (unlike traditional bridging). It should be noted; however, that throughput
in this method is halved for all clients connected
con
wirelessly. When it is difficult to connect all of
the access points in a network by wires, it is also possible to put up access points as repeaters.
Roaming
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Roaming among Wireless Local Area Networks
There are two definitions for wireless LAN roaming:
•
Internal Roaming (1): The Mobile Station (MS) moves from one access point (AP) to
another AP within a home network because the signal strength is too weak. An
authentication server (RADIUS) presumes the re-authentication of MS via 802.1x (e.g.
with PEAP). The billing of QoS is in the home network. A Mobile Station roaming from
one access point to another often interrupts the flow of data among the Mobile Station and
an application connected to the network. The Mobile Station, for instance, periodically
monitors the presence of alternative access points (ones that will provide a better
connection). At some point, based on proprietary mechanisms, the Mobile Station decides
to re-associate with an access point having a stronger wireless signal. The Mobile Station,
however, may lose a connection with an access point before associating with another
access point. In order to provide reliable connections with applications, the Mobile Station
must generally include software that provides session persistence.
•
External Roaming (2): The MS (client) moves into a WLAN of another Wireless Internet
Service Provider (WISP) and takes their services (Hotspot). The user can independently of
his home network use another foreign network, if this is open for visitors. There must be
special authentication and billing systems for mobile services in a foreign network.
Bluetooth and Personal Area networks.
Bluetooth is a very simple type of wireless networking that can allow up to eight devices to be
connected together in a mini-network. It is very short range in operation, and so is considered to
be for 'personal' networking. With a range typically under 30ft, this allows enough distance to
perhaps communicate across your office, but not any further. This short range is also its major
security feature - anyone wishing to eavesdrop on your Bluetooth communications would not only
need special equipment but would also need to be quite close to you. It is a moderately slow type
of networking, but it can transfer data sufficiently fast enough for most typical applications.
Bluetooth is hoped to be a very low cost type of networking, and, as it becomes more widespread,
the cost of adding Bluetooth to devices should drop down to perhaps no more than an extra $5-10
on the selling price.
Bluetooth is designed to be compatible across a range of very different operating systems
and devices, including things that you would not normally think of as being 'computer' type items
- for example, some types of headset. Bluetooth networking can enable the headset to connect
with other devices such as your phone, your MP3 player, your computer, or your PDA.
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A Bluetooth enabled headset would mean that you can leave your cell phone in your pocket
or briefcase, but still receive incoming phone calls. If your cell phone supports voice recognition
for dialing out, you can even place calls as well as receive them, while never needing to reach for
your phone. The safety benefits of this, if you're driving, are obvious. It is probably better from a
health point of view to have a very low powered headset close to your head than it is to have a
phone that might be generating 100 or even 300 times as much radio energy close to your head.
Bluetooth can also help different devices to communicate with each other. For example, you
might have a phone, a PDA, and a computer. If all three devices have Bluetooth capabilities, then
(with the appropriate software on each device) you can probably share contact information
between all three devices quickly and conveniently. And you can look up a phone number on your
PDA (or laptop) and then place a call direct from the laptop or PDA, without needing to touch
your cell phone.
Data transmission with Bluetooth.
Bluetooth is not a magical solution giving universal connectivity between devices. Each device
also needs to have the appropriate software as well as the basic Bluetooth communication
capability, and so sometimes the promise and theory of what could be possible is not fully
matched by the reality. For best compatibility, devices should support the Bluetooth 1.1 standard.
A new standard - 1.2, was formalized in early November 2003 and this is now the dominant
standard. A newer Bluetooth 2.0 standard, allowing for three to ten times faster network speeds,
and more careful use of battery power, is becoming widely adopted.
Bluetooth has been slow to become accepted in the market, but now is starting to become
increasingly prevalent. Prices are falling and increasing numbers of devices are offering Bluetooth
connectivity.
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Bluetooth devices communicating with each other and your PC. It is very short range in
operation, and so is considered to be for 'personal' networking. With a range typically under 30ft,
this allows enough distance to perhaps communicate across your office, but not any further. This
short range is also its major security feature - anyone wishing to eavesdrop on your Bluetooth
communications would not only need special equipment but would also need to be quite close to
you. It is a moderately slow type of networking, but it can transfer data sufficiently fast enough
for most typical applications. Bluetooth is hoped to be a very low cost type of networking.
Bluetooth is designed to be compatible across a range of very different operating systems and
devices, including things that you would not normally think of as being 'computer' type items - for
example, some types of headset. Bluetooth networking can enable the headset to connect with
other devices such as your phone, your MP3 player, your computer, or your PDA.
A Bluetooth enabled headset would mean that you can leave your cell phone in your pocket or
briefcase, but still receive incoming phone calls. If your cell phone supports voice recognition for
dialing out, you can even place calls as well as receive them, while never needing to reach for
your phone. The safety benefits of this, if you're driving, are obvious.
It is probably better from a health point of view to have a very low powered headset close to your
head than it is to have a phone that might be generating 100 or even 300 times as much radio
energy close to your head.
Bluetooth can also help different devices to communicate with each other. For example, you
might have a phone, a PDA, and a computer. If all three devices have Bluetooth capabilities, then
(with the appropriate software on each device) you can probably share contact information
between all three devices quickly and conveniently. And you can look up a phone number on your
PDA (or laptop) and then place a call direct from the laptop or PDA, without needing to touch
your cell phone.
Bluetooth Range
Bluetooth has three different defined ranges, based on their output power ratings.
Class 1 devices are the most powerful. These can have up to 100 mW of power, and a regular
antenna will give them a range of about 40 m - 100 m (130 - 330 ft).
Class 2 devices are lower power, with up to 2.5 mW of power. A regular antenna will give them
a range of about 15 m - 30 m (50 - 100 ft).
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Class 3 devices use even less power, with up to 1 mW of power. A regular antenna will give
them a range of about 5 m - 10 m (16 - 33 ft).
Most Bluetooth devices will be Class 2 or Class 3.
UNIT 3
INTRODUCTION TO CELLULAR MOBILE SYSTEMS
SPECTRUM ALLOCATION
The spectrum is a limited natural resource. Wireless communication equipment depends on the
appropriate and available frequency bands. The signal propagation characteristics are different
with different frequency bands. Also, the scope of the service
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Structure of TTA standards organization in Korea.
China CCSA organization structure.
for the new system used in that band determines the bandwidth of that band. The higher the data
rate, the wider bandwidth is needed. Manufacturers want to allocate a desired spectrum band and
have an investment leverage of economic scale. The task of authorizing the allocation and
licensing of the available spectrum to different systems (allocation band) and to different service
operators (licenses) falls to different frequency administrative bodies throughout the world. It is a
process of restructuring of frequency band allocations and allows the new systems and services to
migrate toward higher frequency bands. Many issues are surrounded by major controversies
among segments of the communication industry and are brought to Congress to resolve. In the
past, satellite communications needed a global plan of spectrum allocation. The spectrum
allocation of terrestrial communication systems has only come from the regional authorities. As
of today, the global roaming of terrestrial communication systems forces the spectrum allocations
of the systems to be planned globally as well; the GSM system is an example. As of today, there
is a need to have three spectral bands for GSM to roam in most areas of the world: 900 MHz and
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1800 MHz operates in Europe and Asia, 1900 MHz in North and South America. Therefore, a
triple-band handset is a solution today for international roaming.
Spectrum Allocation in the United States
In 1946, the FCC granted AT&T a license for mobile telephone services in St. Louis operating at
450 MHz (VHF) with a channel bandwidth of 120 kHz. In 1947, the service was being offered in
more than 25 cities in the United States. In 1950, the channel bandwidth was reduced to 60 kHz
Diagram of Global Spectrum Allocation
Basic cellular system
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the architecture of most cellular systems can be broken down into the following six
components:
a) Mobile units: A mobile unit is basically a mobile/wireless device that contains a control unit,
a transceiver and an antenna system for data and voice transmission. For example, in GSM
networks, the mobile station will consist of the mobile equipment (ME) and the SIM card.
b) Air Interface Standard: There are three main air interface protocols or standards: frequency
division multiple access (FDMA), time division multiple access (TDMA) and code division
multiple access (CDMA). These standards are basically the medium access control (MAC)
protocols that define the rules for entities to access the communication medium. These air
interface standards allow many mobile users to share simultaneously the finite amount of
radio channels.
c) Mobile Telephone Switching Office (MTSO): An MTSO is a fixed station in a mobile
cellular system used for radio communications with mobile units. The serve as the central
coordinating elements for all cell sites, contain the cellular processor and cellular switch. They
consist of radio channels and transmitter and receiver antenna mounted on a tower.
d) Databases: Another integral component of a cellular system is the databases. Databases are
used to keep track of information like billing, caller location, subscriber data, etc. There are
two main databases called the Home Location Register (HLR) and Visitor Location Register
(VLR). The HLR contains the information of each subscriber who resides in the same city as
the MTSO. The VLR temporarily stores the information for each visiting subscriber in the
coverage area of a MTSO. Thus, the VLR is the database that supports roaming capability.
e) Security Mechanism: The security mechanism is to confirm that a particular subscriber is
allowed to access the network and also to authenticate the billing. There are two databases
used for security mechanism: Equipment Identify Register (EIR) and Authentication Center
(AuC). The EIR identifies stolen or fraudulently altered phones that transmit identity data that
does not match with information contained in either the HLR or VLR. The AuC, on the other
hand, manages the actual encryption and verification of each subscriber.
f)
Gateway: The final basic component of a cellular system is the Gateway. The gateway is the
communication links between two wireless systems or between wireless and wired systems.
There are two logical components inside the Gateway: 1) MTSO and 2) Interworking function
(IWF). The MTSO connects the cellular base stations and the mobile stations to the public
switched telephone network (PSTN) or other MTSO. It contains the EIR database. The IWF
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connects the cellular base stations and the mobile stations to Internet and perform protocol
translation if needed.
PERFORMANCE CRITERIA
There are three categories for specifying performance criteria.
Voice Quality
Voice quality is very hard to judge without subjective tests for users’ opinions. In this technical
area, engineers cannot decide how to build a system without knowing the voice quality that will
satisfy the users. In military communications, the situation differs: armed forces personnel must
use the assigned equipment. CM: For any given commercial communications system, the voice
quality will be based on the following criterion: a set value x at which y percent of customers rate
the system voice quality (from transmitter to receiver) as good or excellent; the top two circuit
merits (CM) of the five listed below.
CM Score Quality Scale
CM5 5 Excellent (speech perfectly understandable)
CM4 4 Good (speech easily understandable, some noise)
CM3 3 Fair (speech understandable with a slight effort, occasional repetitions needed)
CM2 2 Poor (speech understandable only with considerable effort, frequent repetitions needed)
CM1 1 Unsatisfactory (speech not understandable)
MOS: As the percentage of customers choosing CM4 and CM5 increases, the cost of building the
system rises. The average of the CM scores obtained from all the listeners is called mean
opinion score (MOS). Usually, the toll-quality voice is around MOS ≥4. DRT (Diagnostic
Rhyme Test): An ANSI standardized method used for evaluation of intelligibility. It is a
subjective test method. Listeners are required to choose which word of a rhyming pair they
perceived. The words differ only in their leading consonant the word pairs have been chosen
such that six binary attributes of speech intelligibility are measured in their present and absent
states. This attribute profile provides a diagnostic capability to the test.
Data Quality
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There are several ways to measure the data quality such as bit error rate, chip error rate, symbol
error rate, and frame error rate. The chip error rate and symbol error rate are measuring the quality
of data along the transmission path. The frame error rate and the bit error rate are measuring the
quality of data at the throughput.
Picture/Vision Quality
There are color acuity, depth perception, flicker perception, motion perception, noise perception,
and visual acuity. The percentage of pixel (picture element) loss rate can be characterized in
vertical resolution loss and horizontal resolution loss of a pixel.
Service Quality
Three items are required for service quality.
1. Coverage. The system should serve an area as large as possible. With radio coverage, however,
because of irregular terrain configurations, it is usually not practical to cover 100 s
a. The transmitted power would have to be very high to illuminate weak spots with sufficient
reception, a significant added cost factor.
b. The higher the transmitted power, the harder it becomes to control interference. Therefore,
systems usually try to cover 90 percent of an area in flat terrain and 75 percent of an area in hilly
terrain. The combined voice quality and coverage criteria in AMPS cellular systems3 state that 75
percent of users rate the voice quality between good and excellent in 90 percent of the served
area, which is generally flat terrain. The voice quality and coverage criteria would be adjusted as
per decided various terrain conditions. In hilly terrain, 90 percent of users must rate voice quality
good or excellent in 75 percent of the served area. A system operator can lower the percentage
values stated above for a low-performance and low-cost system.
2. Required grade of service. For a normal start-up system, the grade of service is specified for a
blocking probability of .02 for initiating calls at the busy hour. This is an average value. However,
the blocking probability at each cell site will be different. At the busy hour, near freeways,
automobile traffic is usually heavy, so the blocking probability at certain cell sites may be higher
than 2 percent, especially when car accidents occur. To decrease the blocking probability requires
a good system plan and a sufficient number of radio channels.
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3. Number of dropped calls. During Q calls in an hour, if a call is dropped and Q−1 calls are
completed, then the call drop rate is 1/Q. This drop rate must be kept low. A high drop rate could
be caused by either coverage problems or handoff problems related to inadequate channel
availability or weak reception.
OPERATION OF CELLULAR SYSTEMS
Operation Procedures
This section briefly describes the operation of the cellular mobile system from a customer’s
perception without touching on the design parameters the operation can be divided into four parts
and a handoff procedure.
Mobile unit initialization. When a user activates the receiver of the mobile unit, the receiver
scans the set-up channels. It then selects the strongest and locks on for a certain time. Because
each site is assigned a different set-up channel, locking onto the strongest set-up channel usually
means selecting the nearest cell site. This self-location scheme is used in the idle stage and is
user-independent. It has a great advantage because it eliminates the load on the transmission at the
cell site for locating the mobile unit. The disadvantage of the self-location scheme is that no
location information of idle mobile units appears at each cell site. Therefore, when the call
initiates from the land line to a mobile unit, the paging process is longer. For a large percentage of
calls originates at the mobile unit, the use of self-location schemes is justified. After a given
period, the self-location procedure is repeated. When land-line originated calls occur, a feature
called “registration” is used.
Mobile originated call. The user places the called number into an originating register in the
mobile unit, and pushes the “send” button. A request for service is sent on a selected set-up
channel obtained from a self-location scheme. The cell site receives it, and in directional cell sites
(or sectors), selects the best directive antenna for the voice channel to use. At the same time, the
cell site sends a request to the mobile telephone switching office (MTSO) via a high-speed data
link. The MTSO selects an appropriate voice channel for the call, and the cell site acts on it
through the best directive antenna to link the mobile unit. The MTSO also connects the wire-line
party through the telephone company zone office.
Network originated call. A land-line party dials a mobile unit number. The telephone company
zone office recognizes that the number is mobile and forwards the call to the MTSO. The MTSO
sends a paging message to certain cell sites based on the mobile unit number and the search
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algorithm. Each cell site transmits the page on its own set-up channel. If the mobile unit is
registered, the registered site pages the mobile. The mobile unit recognizes its own identification
on a strong set-up channel, locks onto it, and responds to the cell site. The mobile unit also
follows the instruction to tune to an assigned voice channel and initiate user alert.
Call termination. When the mobile user turns off the transmitter, a particular signal (signaling
tone) transmits to the cell site, and both sides free the voice channel. The mobile unit resumes
monitoring pages through the strongest set-up channel.
Handoff procedure. During the call, two parties are on a traffic channel. When the mobile unit
moves out of the coverage area of a particular cell site, the reception becomes weak. The current
cell site requests a handoff. The system switches the call to a new frequency channel in a new cell
site without either interrupting the call or alerting the user. The call continues as long as the user
is talking. The user does not notice the handoff occurrences. Handoff was first used by the AMPS
system, and then renamed handover by the European systems because of the different meanings in
British English and American English. Description of handoff will appear in performance
Criteria:
Analog cellular systems
Analog cellular is an industry term given to first generation (1G) cellular systems that transmit
voice information using a form of analog modulation (e.g. FM). Analog cellular systems may
have digital control channels. Analog cellular systems primarily provide voice and low-speed data
communication services over a wide geographic area.
Analog cellular systems use very narrow radio channel (small amount of bandwidth) that varies
from 10 kHz to 30 kHz. Analog systems usually send control information in digital (data) form.
The data signaling rates determine how fast messages can be sent on control channels. The RF
power level of mobile telephones and how the power level is controlled ordinarily determines
how far away the mobile telephone can operate from the base station (radio tower).
Regardless of the size and type of radio channels, all cellular and PCS systems allow for full
duplex operation. Full duplex operation is the ability to have simultaneous communications
between the caller and the called person. This means a mobile telephone must be capable of
simultaneously transmitting and receiving to the radio tower. The radio channel from the mobile
telephone to the radio tower is called the uplink and the radio transmission channel from the base
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station to the mobile telephone is called the downlink. The uplink and downlink radio channels
are normally separated by 45 MHz to 80 MHz.
In early mobile radio systems, a mobile telephone scanned the limited number of available
channels until it found an unused one, which allowed it to initiate a call. Because the analog
cellular systems in use today have hundreds of radio channels, a mobile telephone cannot scan
them all in a reasonable amount of time. To quickly direct a mobile telephone to an available
channel, some of the available radio channels are dedicated as control channels. Most cellular
systems use two types of radio channels, control channels and voice channels. Control channels
carry only digital messages and signals, which allow the mobile telephone to retrieve system
control information and compete for access.
Control channels only carry control information such as paging (alert) and channel assignment
messages. Voice channels are primarily used to transfer voice information. However, voice
channels must also be capable of sending and receiving some digital control messages to allow for
necessary frequency and power changes during a call.
Current analog systems serve only one subscriber at a time on a radio channel so the number of
radio channels available influence system capacity. However, a typical subscriber uses the system
for only a few minutes a day, on a daily basis, and many subscribers share a single channel. As a
rule, 20 - 32 subscribers share each radio channel [ ], depending upon the average talk time per
hour per subscriber. Generally, a cell with 50 channels can support 1000 - 1600 subscribers.
The basic operation of an analog cellular system involves initiation of the phone when it is
powered on, listening for paging messages (idle), attempting access when required and
conversation (or data) mode.
When a mobile telephone is first powered on, it initializes itself by searching (scanning) a
predetermined set of control channels and then tuning to the strongest one. During the
initialization mode, it listens to messages on the control channel to retrieve system identification
and setup information.
After initialization, the mobile telephone enters the idle mode and waits to be paged for an
incoming call and senses if the user has initiated (dialed) a call (access). When a call begins to be
received or initiated, the mobile telephone enters system access mode to try to access the system
via a control channel. When it gains access, the control channel sends an initial voice channel
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designation message indicating an open voice channel. The mobile telephone then tunes to the
designated voice channel and enters the conversation mode. As the mobile telephone operates on
a voice channel, the system uses Frequency Modulation (FM) similar to commercial broadcast
FM radio. To send control messages on the voice channel, the voice information is either
A mobile telephone's attempt to obtain service from a cellular system is referred to as access.
Mobile telephones compete on the control channel to obtain access from a cellular system. Access
is attempted when a command is received by the mobile telephone indicating the system needs to
service that mobile telephone (such as a paging message indicating a call to be received) or as a
result of a request from the user to place a call. The mobile telephone gains access by monitoring
the busy/idle status of the control channel both before and during transmission of the access
attempt message. If the channel is available, the mobile station begins to transmit and the base
station simultaneously monitors the channel's busy status. Transmissions must begin within a
prescribed time limit after the mobile station finds that the control channel access is free, or the
access attempt is stopped on the assumption that another mobile telephone has possibly gained the
attention of the base station control channel receiver.
If the access attempt succeeds, the system sends out a channel assignment message commanding
the mobile telephone to tune to a cellular voice channel. When a subscriber dials the mobile
telephone to initiate a call, it is called "origination". A call origination access attempt message is
sent to the cellular system that contains the dialed digits, identity information along with other
information. If the system allows service, the system will assign a voice channel by sending a
voice channel designator message, if a voice channel is available. If the access attempt fails, the
mobile telephone waits a random amount of time before trying again. The mobile station uses a
random number generating (an internal algorithm) to determine the random time to wait. The
design of the system minimizes the chance of repeated collisions between different mobile
stations, which are both trying to access the control channel since each one waits a different
random time interval before trying again if they have already collided on their first, simultaneous
attempt.
To receive calls, a mobile telephone is notified of an incoming call by a process called paging. A
page is a control channel message that contains the telephone's Mobile Identification Number
(MIN) or telephone number of the desired mobile phone. When the telephone determines it has
been paged, it responds automatically with a system access message that indicates its access
attempt is the result of a page message and the mobile telephone begins to ring to alert the
customer of an incoming telephone call. When the customer answers the call (user presses
"SEND" or "TALK"), the mobile telephone transmits a service request to the system to answer
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the call. It does this by sending the telephone number and an electronic serial number to provide
the users identity.
After a mobile telephone has been commanded to tune to a radio voice channel, it sends mostly
voice or other customer information. Periodically, control messages may be sent between the base
station and the mobile telephone. Control messages may command the mobile telephone to adjust
its power level, change frequencies, or request a special service (such as three way calling).
To conserve battery life, a mobile phone may be permitted by the base station to only transmit
when it senses the mobile telephone's user is talking. When there is silence, the mobile telephone
may stop transmitting for brief periods of time (several seconds). When the mobile telephone user
begins to talk again, the transmitter is turned on again. This is called discontinuous transmission.
A basic analog cellular system. There are two types of radio channels; control channels and voice
channels. Control channels typically use frequency shift keying (FSK) to send control messages
(data) between the mobile phone and the base station. Voice channels typically use FM
modulation with brief bursts of digital information to allow control messages (such as handoff)
during conversation. Base stations typically have two antennas for receiving and one for
transmitting. Dual receiver antennas increases the ability to receive the radio signal from mobile
telephones which typically have a much lower transmitter power level than the transmitters in the
base station. Base stations are connected to a mobile switching center (MSC) typically by a high
speed telephone line or microwave radio system. This interconnection must allow both voice and
control information to be exchanged between the switching system and the base station. The MSC
is connected to the telephone network to allow mobile telephones to be connected to standard
landline telephones.
DIGITAL SYSTEMS
Many digital cellular and cordless phone systems have been developed. The cellular systems are
GSM, NA-TDMA, CDMA, PDC, and 1800-DCS, and the cordless phone systems are DECT and
CT-2 schemes. Although analog cellular systems are limited to using frequency division multipleaccess (FDMA) schemes, digital cellular systems can use FDMA, time division multiple-access
(TDMA), and code-division multiple-access (CDMA). When a multiple-access scheme is chosen
for a particular system, all the functions, protocols, and network are associated with that scheme
Advantages of Digital Systems
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In an analog system, the signals applied to the transmission media are continuous functions of the
message waveform. In the analog system, the amplitude, the phase, or the frequency of a
sinusoidal carrier can be continuously varied in accordance with the voice or the message. In
digital transmission systems, the transmitted signals are discrete in time, amplitude, phase, or
frequency, or in a combination of any two of these parameters. To convert from analog form to
digital form, the quantizing noise due to discrete levels should be controlled by assigning a
sufficient number of digits for each sample, and a sufficient number of samples are needed to
apply the Nyquist rate for sampling an analogwaveform.One advantage of converting message
signals into digital form is the ruggedness of the digital signal. The impairments introduced in the
medium in spite of noise and interference can always be corrected. This process, called
regeneration, provides the primary advantages for digital transmission. However, a disadvantage
of this ruggedness is increased bandwidth relative to that required for the original signal. The
increased bandwidth is used to overcome the impairment introduced into the medium. In addition
to the cost advantage, power consumption is lower and digital equipment is generally lighter in
weight and more compact. In mobile cellular systems, there are more advantages in applying
digital technology
GLOBAL SYSTEM FOR MOBILE (GSM)
CEPT, a European group, began to develop the Global System for Mobile TDMA system in June
1982.17–21 GSM has two objectives: pan-European roaming, which offers compatibility
throughout the European continent, and interaction with the integrated service digital network
(ISDN), which offers the capability to extend the single-subscriber-line system to a multiservice
system with various services currently offered only through diverse telecommunications
networks.
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The external environment of the BSS.
System capacity was not an issue in the initial development of GSM, but due to the unexpected,
rapid growth of cellular service, 35 revisions have been made to GSM since the first issued
specification. The first commercial GSM system, called D2, was implemented in Germany in
1992.
NORTH AMERICAN TDMA
North American TDMA (NA-TDMA) is a digital cellular system22−24 sometimes called
American digital cellular (ADC) or digital AMPS (DAMPS), or North American digital cellular
(NADC) or IS-54 system. This TDMA system was approved and design on it was started in 1987
by a group named TR45-3 after the industry debated between frequency division multiple access
and time-division multiple access. The reason those members voted for TDMA was due to the big
influence of European GSM, which is the TDMA system. However, the requirements of
designing a digital cellular system in Europe and in North America are different. In Europe, there
is a virgin band (935–960 MHz downlink and 890–915 MHz uplink) for the digital cellular
system. In North America, there is no new allocated band for the digital cellular system. The
digital cellular system has to share the same allocated band with the analog system (AMPS,
described in Chap. 3). Also, the digital and the analog systems have to be coexistent. In this
circumstance, the low-risk approach is to use the same signal signature as the analog system (i.e.,
FDMA). Besides, because of the urgent need for large system capacity, the time for designing a
new North American system had to be very short. The North American digital system was needed
to be available in 1990, in only 3 years. To design a digital FDMA system would be a
straightforward task. Since the analog system is a FDMA system, all the physical data gathered
for the analog system in the past 20 years could be used for designing the FDMA digital system,
and design time would be shortened. On the other hand, to design a TDMA digital system in the
same band shared with an FDMA analog system, much more physical parameters would have to
be developed and time would be needed to understand them. Without a good understanding of the
limitation of coexistence between two different signal signatures, FDMA and TDMA, it would be
very difficult to complete a digital system with good performance in a very short time. If GSM
had taken 8 years to develop, NA-TDMA might also need as much time to be revised in order for
it to be mature. Because of the requirement of coexistence, a dual-mode mobile unit was decided
on (i.e., the unit can work on both analog and digital systems). In a dual-mode mobile unit, the 21
call set-up channels for the analog system are available in the unit. Why not share the same call
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set-up channels (analog) for both the analog voice channels and digital voice channels? In this
case, no additional spectrum is needed for the digital set-up channels.25 the spectrum is saved for
adding more digital voice. Furthermore, for the sake of speeding up the completion of North
American digital systems, the call set-up channels of the digital system could be shared with the
analog system to make the call processing the same between the two systems. Thus, the first
phase of the NA-TDMA system could be completed earlier. IS-54 did not perform well. In 1994,
an improved system design was made, and IS-54 was changed to IS-136.
CDMA
CDMA development26–30 started in early 1989 after the NA-TDMA standard (IS-54) was
established. A CDMA demonstration to test its feasibility for digital cellular systems was held in
November 1989. The CDMA “Mobile Station-Base Station Compatibility Standard for Dual
Mode Wideband Spread Spectrum Cellular System “was issued as IS-95 (PN-3118, Dec. 9,
1992). CDMA uses the idea of tolerating inteterference by spread-spectrum modulation. The
power control scheme in a CDMA system is a requirement for digital cellular application
UNIT 4
CELLULAR SYSTEM DESIGN FUNDAMENTALS
Frequency Reuse
Frequency reuse, or, frequency planning, is a technique of reusing frequencies and channels
within a communication system to improve capacity and spectral efficiency. Frequency reuse is
one of the fundamental concepts on which commercial wireless systems are based that involve the
partitioning of an RF radiating area into cells. The increased capacity in a commercial wireless
network, compared with a network with a single transmitter, comes from the fact that the same
radio frequency can be reused in a different area for a completely different transmission.
Frequency reuse in mobile cellular systems means that frequencies allocated to
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Frequency reuse technique of a cellular system.
The service is reused in a regular pattern of cells, each covered by one base station. The repeating
regular pattern of cells is called cluster. Since each cell is designed to use radio frequencies only
within its boundaries, the same frequencies can be reused in other cells not far away without
interference, in another cluster. Such cells are called `co-channel' cells. The reuse of frequencies
enables a cellular system to handle a huge number of calls with a limited number of channels.
Figure shows a frequency planning with cluster size of 7, showing the co-channels cells in
different clusters by the same letter. The closest distance between the co-channel cells (indifferent
clusters) is determined by the choice of the cluster size and the layout of the cell cluster. Consider
a cellular system with S duplex channels available for use and let N be the number of cells in a
cluster. If each cell is allotted K duplex channels with all being allotted unique and disjoint
channel groups we have S = KN under normal circumstances. Now, if the cluster are repeated M
times within the total area, the total number of duplex channels, or, the total number of users in
the system would be T = MS = KMN. Clearly, if K and N remain constant, then
And, if T and K remain constant, then
Hence the capacity gain achieved is directly proportional to the number of times a cluster is
repeated, as shown as well as, for a fixed cell size, small N decreases the size of the cluster with
in turn results in the increase of the number of clusters and hence the capacity. However for small
N, co-channel cells are located much closer and hence more interference. The value of N is
determined by calculating the amount of interference that can be tolerated for a sufficient quality
communication. Hence the smallest N having interference below the tolerated limit is used.
However, the cluster size N cannot take on any value and is given only by the following equation
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where i and j are integer numbers.
Channel Assignment Strategies
With the rapid increase in number of mobile users, the mobile service providers had to follow
strategies which ensure the effective utilization of the limited radio spectrum. With increased
capacity and low interference being the prime objectives, a frequency reuse scheme was helpful in
achieving these objectives. A variety of channel assignment strategies have been followed to aid
these objectives. Channel
Assignment strategies are classified into two types: fixed and dynamic, as discussed below.
Fixed Channel Assignment (FCA)
In fixed channel assignment strategy each cell is allocated a fixed number of voice channels. Any
communication within the cell can only be made with the designated unused channels of that
particular cell. Suppose if all the channels are occupied, then the call is blocked and subscriber
has to wait. This is simplest of the channel assignment strategies as it requires very simple
circuitry but provides worst channel utilization. Later there was another approach in which the
channels were borrowed from adjacent cell if all of its own designated channels were occupied.
This was named as borrowing strategy. In such cases the MSC supervises the borrowing process
and ensures that none of the calls in progress are interrupted.
Dynamic Channel Assignment (DCA)
In dynamic channel assignment strategy channels are temporarily assigned for use in cells for the
duration of the call. Each time a call attempt is made from a cell the corresponding BS requests a
channel from MSC. The MSC then allocates a channel to the requesting the BS. After the call is
over the channel is returned and kept in a central pool. To avoid co-channel interference any
channel that in use in one cell can only be reassigned simultaneously to another cell in the system
if the distance between the two cells is larger than minimum reuse distance. When compared to
the FCA, DCA has reduced the likelihood of blocking and even increased the trunking capacity of
the network as all of the channels are available to all cells, i.e., good quality of service. But this
type of assignment strategy results in heavy load on switching centre at heavy traffic condition.
Ex.: A total of 33 MHz bandwidth is allocated to a FDD cellular system with 25 KHz simplex
channels to provide full duplex voice and control channels. Compute the number of channels
available per cell if the system uses (i) 4 cell, (ii) 7 cell, and (iii) 8 cell reuse technique.
Assume 1 MHz of spectrum is allocated to control channels. Give a distribution of voice and
control channels
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Solution: One duplex channel = 2 x 25 = 50 kHz of spectrum. Hence the total available duplex
channels are = 33 MHz / 50 kHz = 660 in number. Among these channels, 1 MHz / 50 kHz = 20
channels are kept as control channels.
(a) For N = 4, total channels per cell = 660/4 = 165.
Among these, voice channels are 160 and control channels are 5 in number.
(b) For N = 7, total channels per cell are 660/7 _ 94. Therefore, we have to go for a more exact
solution. We know that for this system, a total of 20 control channels and a total of 640 voice
channels are kept. Here, 6 cells can use 3 control channels and the rest two can use 2 control
channels each. On the other hand, 5 cells can use 92 voice channels and the rest two can use 90
voice channels each. Thus the total
solution for this case is:
6 x 3 + 1 x 2 = 20 control channels, and,
5 x 92 + 2 x 90 = 640 voice channels.
This is one solution, there might exist other solutions too.
(c) The option N = 8 is not a valid option since it cannot satisfy equation by two integers i and j.
Handoff Process
When a user moves from one cell to the other, to keep the communication between the user pair,
the user channel has to be shifted from one BS to the other without interrupting the call, i.e., when
a MS moves into another cell, while the conversation is still in progress, the MSC automatically
transfers the call to a new FDD channel without disturbing the conversation. This process is
called as handoff. A schematic diagram of handoff is given in Figure. Processing of handoff is an
important task in any cellular system. Handoffs must be performed successfully and be
imperceptible to the users. Once a signal
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Handoff scenario at two adjacent cell boundaries.
Level is set as the minimum acceptable for good voice quality (Prmin), and then a slightly
stronger level is chosen as the threshold (PrH) at which handoff has to be made, as shown in
Figure. A parameter, called power margin, defined as
is quite an important parameter during the handoff process since this margin can neither be too
large nor too small. If∆ is too small, then there may not be enough time to complete the handoff
and the call might be lost even if the user crosses the cell boundary. If ∆ is too high on the other
hand, then MSC has to be burdened with unnecessary handoffs. This is because MS may not
intend to enter the other cell. Therefore ∆ should be judiciously chosen to ensure imperceptible
handoffs and to meet other objectives.
Factors Influencing Handoffs
The following factors influence the entire handoff process:
(a) Transmitted power: as we know that the transmission power is different for different cells, the
handoff threshold or the power margin varies from cell to cell.
(b) Received power: the received power mostly depends on the Line of Sight (LoS) path between
the user and the BS. Especially when the user is on the boundary of
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Handoff process associated with power levels, when the user is going from i-th cell to j-th cell.
The two cells, the LoS path plays a critical role in handoffs and therefore the power margin _
depends on the minimum received power value from cell to cell.
(c) Area and shape of the cell: Apart from the power levels, the cell structure also a plays an
important role in the handoff process.
(d) Mobility of users: The number of mobile users entering or going out of a particular cell also
fixes the handoff strategy of a cell.
To illustrate the reasons (c) and (d), let us consider a rectangular cell with sides R1 and R2
inclined at an angle teta with horizon, as shown in the Figure. Assume N1 users are having
handoff in horizontal direction and N2 in vertical direction per unit length.
The number of crossings along R1 side is: R1 and the number of crossings along R2 side are:
(N1sina+ N2 cosa)R2.
Then the handoff rate λH can be written as
λH = (N1cosa + N2sina)R1 + (N1sina + N2cosa)R2:
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Figure
: Handoff process with a rectangular cell inclined at an angle .
Now, given the fixed area A = R1R2, we need to find λhmin for a given teta. Replacing
R1 by A/R2 and equating dH/dR1 to zero, we get
Similarly, for R2, we get
From the above equations, we have
Which means it it minimized at teta = 0o. Hence λhmin =
. Putting the value of teta in eq, we have
. This has two implications: (i) that handoff is minimized if rectangular cell is aligned with X-Y
axis, i.e., teta = 0o, and, (ii) that the number of users crossing the cell boundary is inversely
proportional to the dimensionof the other side of the cell. The above analysis has been carried out
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for a simple square cell and it changes in more complicated way when we consider a hexagonal
cell.
Handoff in Different Generations
In 1G analog cellular systems, the signal strength measurements were made by the BS and in turn
supervised by the MSC. The handoffs in this generation can be termed as Network Controlled
Handoff (NCHO). The BS monitors the signal strengths of voice channels to determine the
relative positions of the subscriber. The special receivers located on the BS are controlled by the
MSC to monitor the signal strengths of the users in the neighbouring cells which appear to be in
need of handoff. Based on the information received from the special receivers the MSC decides
whether a handoff is required or not. The approximate time needed to make a handoff successful
was about 5-10 s. This requires the value of _ to be in the order of 6dB to 12dB. In the 2G
systems, the MSC was relieved from the entire operation. In this generation, which started using
the digital technology, handoff decisions were mobile assisted and therefore it is called Mobile
Assisted Handoff (MAHO). In MAHO, the mobile centre measures the power changes received
from nearby base stations and notifies the two BS. Accordingly the two BS communicate and
channel transfer occurs. As compared to 1G, the circuit complexity was increased here whereas
the delay in handoff_ was reduced to 1-5 s. The value of _ was in the order of 0-5 dB. However,
even this amount of delay could create a communication pause. In the current 3G systems, the MS
measures the power from adjacent BS and automatically upgrades the channels to its nearer BS.
Hence this can be termed as Mobile Controlled Handoff (MCHO). When compared to the other
generations, delay during handoff_ is only 100 ms and the value of around 20 dBm. The Quality
Of Service (QoS) has improved a lot although the complexity of the circuitry has further
increased which is inevitable. All these types of handoffs are usually termed as hard handoff as
there is a shift in the channels involved. There is also another kind of handoff, called soft handoff,
as discussed below
Handoff_ in CDMA:
In spread spectrum cellular systems, the mobiles share the same channels in every cell. The MSC
evaluates the signal strengths received from different BS for a single user and then shifts the user
from one BS to the other without actually changing the channel. These types of handoffs are
called as soft handoff_ as there is no change in the channel.
Handoff Priority
While assigning channels using either FCA or DCA strategy, a guard channel concept must be
followed to facilitate the handoffs. This means, a fraction of total available channels must be kept
for handoff_ requests. But this would reduce the carried traffic and only fewer channels can be
assigned for the residual users of a cell. A good solution to avoid such a dead-lock is to use DCA
with handoff priority (demand based allocation)
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Interference & System Capacity
Susceptibility and interference problems associated with mobile communications equipment are
because of the problem of time congestion within the electromagnetic spectrum. It is the limiting
factor in the performance of cellular systems. This interference can occur from clash with another
mobile in the same cell or because of a call in the adjacent cell. There can be interference between
the base stations operating at same frequency band or any other non-cellular system's energy
leaking inadvertently into the frequency band of the cellular system. If there is an interference in
the voice channels, cross talk is heard will appear as noise between the users. The interference in
the control channels leads to missed and error calls because of digital signaling. Interference is
more severe in urban areas because of the greater RF noise and greater density of mobiles and
base stations. The interference can be divided into 2 parts: co-channel interference and adjacent
channel interference..
Co-channel interference (CCI)
For the efficient use of available spectrum, it is necessary to reuse frequency bandwidth over
relatively small geographical areas. However, increasing frequency reuse also increases
interference, which decreases system capacity and service quality. The cells where the same set of
frequencies is used are call co-channel cells. Co-channel interference is the cross talk between
two different radio transmitters using the same radio frequency as is the case with the co-channel
cells. The reasons of CCI can be because of either adverse weather conditions or poor frequency
planning or overly crowded radio spectrum. If the cell size and the power transmitted at the base
stations are same then CCI will become independent of the transmitted power and will depend on
radius of the cell (R) and the distance between the interfering co-channel cells (D). If D/R ratio is
increased, then the effective distance between the co-channel cells will increase and interference
will decrease. The parameter Q is called the frequency reuse ratio and is related to the cluster size.
For hexagonal geometry
From the above equation, small of `Q' means small value of cluster size `N' and increase in
cellular capacity. But large `Q' leads to decrease in system capacity but increase in transmission
quality. Choosing the options is very careful for the selection of `N', the proof of which is given in
the first section. The Signal to Interference Ratio (SIR) for a mobile receiver which monitors the
forward channel can be calculated as
Where i0 is the number of co-channel interfering cells, S is the desired signal power from the
baseband station and Ii is the interference power caused by the i-th interfering co-channel base
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station. In order to solve this equation from power calculations, we need to look into the signal
power characteristics. The average power in the mobile radio channel decays as a power law of
the distance of separation between transmitter and receiver. The expression for the received power
Pr at a distance d can be approximately calculated as
and in the dB expression as
where P0 is the power received at a close-in reference point in the far field region at a small
distance do from the transmitting antenna, and `n' is the path loss exponent. Let us calculate the
SIR for this system. If Di is the distance of the i-th interferer from the mobile, the received power
at a given mobile due to i-th interfering cell is proportional to (Di) n (the value of 'n' varies
between 2 and 4 in urban cellular systems).
Let us take that the path loss exponent is same throughout the coverage area and the transmitted
power be same, then SIR can be approximated as
where the mobile is assumed to be located at R distance from the cell centre. If we consider only
the first layer of interfering cells and we assume that the interfering base stations are equidistant
from the reference base station and the distance between the cell centers is 'D' then the above
equation can be converted as
which is an approximate measure of the SIR. Subjective tests performed on AMPS cellular
system which uses FM and 30 kHz channels show that sufficient voice quality can be obtained by
SIR being greater than or equal to 18 dB. If we take n=4 , the value of 'N' can be calculated .
Therefore minimum N is 7. The above equations are based on hexagonal geometry and the
distances from the closest interfering cells can vary if different frequency reuse plans are used.
We can go for a more approximate calculation for co-channel SIR. This is the example of a 7 cell
reuse case. The mobile is at a distance of D-R from 2 closest interfering cells and approximately
D+R/2, D, D-R/2 and D+R distance from other
interfering cells in the first tier. Taking n = 4 in the above equation, SIR can be approximately
calculated as
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which can be rewritten in terms frequency reuse ratio Q as
assignments of the base stations and their transmit powers. Tilting the base-station antenna to
limit the Using the value of N equal to 7 (this means Q = 4.6), the above expression yields that
worst case SIR is 53.70 (17.3 dB). This shows that for a 7 cell reuse case the worst case SIR is
slightly less than 18 dB. The worst case is when the mobile is at the corner of the cell i.e., on a
vertex as shown in the Figure Therefore N = 12 cluster size should be used. But this reduces the
capacity by 7/12 times. Therefore, co-channel interference controls link performance, which in a
way controls frequency reuse plan and the overall capacity of the cellular system. The effect of
co-channel interference can be minimized by optimizing the frequency spread of the signals in the
system can also be done.
First tier of co-channel interfering cells
Adjacent Channel Interference (ACI)
This is a different type of interference which is caused by adjacent channels i.e. channels in
adjacent cells. It is the signal impairment which occurs to one frequency due to presence of
another signal on a nearby frequency. This occurs when imperfect receiver filters allow nearby
frequencies to leak into the passband. This problem is enhanced if the adjacent channel user is
transmitting in a close range compared to the subscriber's receiver while the receiver attempts to
receive a base station on the channel. This is called near-far effect. The more adjacent channels
are packed into the channel block, the higher the spectral efficiency, provided that the
performance degradation can be tolerated in the system link budget. This effect can also occur if a
mobile close to a base station transmits on a channel close to one being used by a weak mobile.
This problem might occur if the base station has problem in discriminating the mobile user from
the "bleed over" caused by the close adjacent channel mobile. Adjacent channel interference
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occurs more frequently in small cell clusters and heavily used cells. If the frequency separation
between the channels is kept large this interference can be reduced to some extent. Thus
assignment of channels is given such that they do not form a contiguous band of frequencies
within a particular cell and frequency separation is maximized. Efficient assignment strategies are
very much important in making the interference as less as possible. If the frequency factor is
small then distance between the adjacent channels cannot put the interference level within
tolerance limits. If a mobile is 10 times close to the base station than other mobile and has energy
spill out of its passband, then SIR for weak mobile is approximately
which can be easily found from the earlier SIR expressions. If n = 4, then SIR is 52 dB. Perfect
base station filters are needed when close-in and distant users share the same cell. Practically,
each base station receiver is preceded by a high Q cavity filter in order to remove adjacent
channel interference. Power control is also very much important for the prolonging of the battery
life for the subscriber unit but also reduces reverse channel SIR in the system. Power control is
done such that each mobile transmits the lowest power required to maintain a good quality link on
the reverse channel.
Improving Capacity And Cell Coverage
The Key Trade-off
Previously, we have seen that the frequency reuse technique in cellular systems allows for almost
boundless expansion of geographical area and the number of mobile system users who could be
accommodated. In designing a cellular layout, the two parameters which are of great significance
are the cell radius R and the cluster size N, and we have also seen that co-channel cell distance D
= √3NR. In the following, a brief description of the design trade-of is given, in which the above
two parameters play a crucial role. The cell radius governs both the geographical area covered by
a cell and also the number of subscribers who can be serviced, given the subscriber density. It is
easy to see that the cell radius must be as large as possible. This is because, every cell requires an
investment in a tower, land on which the tower is placed, and radio transmission equipment and
so a large cell size minimizes the cost per subscriber. Eventually, the cell radius is determined by
the requirement that adequate signal to noise ratio be maintained over the coverage area. The SNR
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is determined by several factors such as the antenna height, transmitter power, receiver noise
figure etc. Given a cell radius R and a cluster size N, the geographic area covered by a cluster is
If the total serviced area is Atonal, then the number of clusters M that could be accommodated is
given by
.
Note that all of the available channels N, are reused in every cluster. Hence, to make the
maximum number of channels available to subscribers, the number of clusters M should be large,
which, by Equation , shows that the cell radius should be small. However, cell radius is
determined by a trade-off: R should be as large as possible to minimize the cost of the installation
per subscriber, but R should be as small as possible to maximize the number of customers that the
system can accommodate. Now, if the cell radius R is fixed, then the number of clusters could be
maximized by minimizing the size of a cluster N. We have seen earlier that the size of a cluster
depends on the frequency reuse ratio Q. Hence, in determining the value of N, another trade-off
is encountered in that N must be small to accommodate s large number of subscribers, but should
be sufficiently large so as to minimize the interference effects.
Now, we focus on the issues regarding system expansion. The history of cellular phones has been
characterized by a rapid growth and expansion in cell subscribers. Though a cellular system can
be expanded by simply adding cells to the geographical area, the way in which user density can
be increased is also important to look at.This is because it is not always possible to counter the
increasing demand for cellular systems just by increasing the geographical coverage area due to
the limitations in obtaining new land with suitable requirements. We discuss here two methods for
dealing with an increasing subscriber density: Cell Splitting and Sectoring. The other method,
microcell zone concept can treated as enhancing the QoS in a cellular system.
CELL SPLITTING
Why Splitting?
The motivation behind implementing a cellular mobile system is to improve the utilization of
spectrum efficiency.19 The frequency reuse scheme is one concept, and cell splitting is another
concept. When traffic density starts to build up and the frequency channels Fi in each cell Ci
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cannot provide enough mobile calls, the original cell can be split into smaller cells. Usually the
new radius is one-half the original radius (see Fig. ). There are two ways of splitting. In Fig. the
original cell site is not used, while in Fig. it is.
Then, based on Eq. , the following equation is true.
Let each new cell carry the same maximum traffic load of the old cell; then, in theory,
How Splitting?
There are two kinds of cell-splitting techniques:
1. Permanent splitting. The installation of every new split cell has to be planned ahead of time;
the number of channels, the transmitted power, the assigned frequencies, the choosing of the cellsite selection, and the traffic load consideration should all be considered. When ready, the actual
service cut-over should be set at the lowest traffic point, usually at midnight on a weekend.
Hopefully, only a few calls will be dropped because of this cut-over, assuming that the downtime
of the system is within 2 h.
2. Dynamic splitting. This scheme is based on using the allocated spectrum efficiency in real
time. The algorithm for dynamically splitting cell sites is a tedious job, as we cannot
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Cell splitting.
Sectoring
Sectoring is basically a technique which can increase the SIR without necessitating an increase in
the cluster size. Till now, it has been assumed that the base station is located in the center of a cell
and radiates uniformly in all the directions behaving as an omni-directional antenna. However it
has been found that the co-channel interference in a cellular system may be decreased by
replacing a single omni-directional antenna at the base station by several directional antennas,
each radiating within a specified sector. In the Figure a cell is shown which has been split into
three 120o sectors. The base station feeds three 120o directional antennas, each of which radiates
into one of the three sectors. The channel set serving this cell has also been divided, so that each
sector is assigned one-third of the available number cell of channels. This technique for reducing
co-channel interference wherein by using suit-
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seven-cell cluster with 60o sectors.
able directional antennas, a given cell would receive interference and transmit with a fraction of
available co-channel cells is called 'sectoring'. In a seven-cell-cluster layout with 120o sectored
cells, it can be easily understood that the mobile units in a particular sector of the center cell will
receive co-channel interference from only two of the first-tier co-channel base stations, rather than
from all six. Likewise, the base station in the center cell will receive co-channel interference from
mobile units in only two of the co-channel cells
Microcell Zone Concept
The increased number of handoffs required when sectoring is employed results in an increased
load on the switching and control link elements of the mobile system. To overcome this problem,
a new microcell zone concept has been proposed. As shown in Figure this scheme has a cell
divided into three microcell zones, with each of the three zone sites connected to the base station
and sharing the same radio equipment. It is necessary to note that all the microcell zones, within a
cell, use the same frequency used by that cell; that is no handovers occur between microcells.
Thus when a mobile user moves between two microcell zones of the cell, the BS simply switches
the channel to a different zone site and no physical re-allotment of channel takes place. Locating
the mobile unit within the cell: An active mobile unit sends a signal to all zone sites, which in turn
send a signal to the BS. A zone selector at the BS uses that signal to select a suitable zone to serve
the mobile unit - choosing the zone with the strongest signal.
Base Station Signals: When a call is made to a cellular phone, the system already knows the cell
location of that phone. The base station of that cell knows in which zone, within that cell, the
cellular phone is located. Therefore when it receives the signal, the base station transmits it to the
suitable zone site. The zone site receives the cellular signal from the base station and transmits
that signal to the mobile phone after amplification. By confining the power transmitted to the
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mobile phone, co-channel interference is reduced between the zones and the capacity of system is
increased.
Benefits of the micro-cell zone concept:
1) Interference is reduced in this case as compared to the scheme in which the cell size is reduced.
2) Handoffs are reduced (also compared to decreasing the cell size) since the microcells within
the cell operate at the same frequency; no handover occurs when the mobile unit moves between
the microcells.
3) Size of the zone apparatus is small. The zone site equipment being small can be mounted on
the side of a building or on poles.
4) System capacity is increased. The new microcell knows where to locate the mobile unit in a
particular zone of the cell and deliver the power to that zone. Since
The micro-cell zone concept
The signal power is reduced, the microcells can be closer and result in an increased system
capacity. However, in a microcellular system, the transmitted power to a mobile phone within a
microcell has to be precise; too much power results in interference between microcells, while with
too little power the signal might not reach the mobile phone.This is a drawback of microcellular
systems, since a change in the surrounding (a new building, say, within a microcell) will require a
change of the transmission power.
Trunked Radio System
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In the previous sections, we have discussed the frequency reuse plan, the design trade-off and
also explored certain capacity expansion techniques like cell-splitting and sectoring. Now, we
look at the relation between the number of radio channels a cell contains and the number of users
a cell can support. Cellular systems use the concept of trunking to accommodate a large number
of users in a limited radio spectrum. It was found that a central office associated with say, 10,000
telephones requires about 50 million connections to connect every possible pair of users.
However, a worst case maximum of 5000 connections need to be made among these telephones at
any given instant of time, as against the possible 50 million connections. In fact, only a few
hundreds of lines are needed owing to the relatively short duration of a call. This indicates that the
resources are shared so that the number of lines is much smaller than the number of possible
connections. A line that connects switching offices and that is shared among users on an asneeded basis is called a trunk. The fact that the number of trunks needed to make connections
between offices is much smaller than the maximum number that could be used suggests that at
times there might not be sufficient facilities to allow a call to be completed. A call that cannot be
completed owing to a lack of resources is said to be blocked. So one important to be answered in
mobile cellular systems is: How many channels per cell are needed in a cellular telephone system
to ensure a reasonably low probability that a call will be blocked?
In a trunked radio system, a channel is allotted on per call basis. The performance of a radio
system can be estimated in a way by looking at how efficiently the calls are getting connected and
also how they are being maintained at handoffs. Trunking mainly exploits the statistical behavior
of users so that a fixed number of channels can be used to accommodate a large, random user
community. As the number of telephone lines decrease, it becomes more likely that all channels
are busy for a particular user. As a result, the call gets rejected and in some systems, a queue may
be used to hold the caller's request until a channel becomes available.
In the telephone system context the term Grade of Service (GoS) is used to mean the probability
that a user's request for service will be blocked because a required facility, such as a trunk or a
cellular channel, is not available. For example, a GoS of 2 % implies that on the average a user
might not be successful in placing a call on 2 out of every 100 attempts. In practice the blocking
frequency varies with time. One would expect far more call attempts during business hours than
during the middle of the night. Telephone operating companies maintain usage records and can
identify a "busy hour", that is, the hour of the day during which there is the greatest demand for
service. Typically, telephone systems are engineered to provide a specified grade of service
during a specified busy hour. User calling can be modeled statistically by two parameters: the
average number of call requests per unit time
parameter
and the average holding time H. The
user is also called the average arrival rate, referring to the rate at which calls from
a single user arrive. The average holding time is the average duration of a call. The Trunking
mainly exploits the statistical behavior of users so that a fixed number of channels can be used to
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accommodate a large, random user community. As the number of telephone lines decrease, it
becomes more likely that all channels are busy for a particular user. As a result, the call gets
rejected and in some systems, a queue may be used to hold the caller's request until a channel
becomes available.. The product:
that is, the product of the average arrival rate and the average holding time{is called
the offered traffic intensity or offered load. This quantity represents the average traffic that a user
provides to the system. Offered traffic intensity is a quantity that is traditionally measured in
Erlangs. One Erlang represents the amount of traffic intensity carried by a channel that is
completely occupied. For example, a channel that is occupied for thirty minutes during an hour
carries 0.5 Erlang of traffic. Call arrivals or requests for service are modeled as a Poisson random
process. It is based on the assumption that there is a large pool of users who do not cooperate in
deciding when to place calls. Holding times are very well predicted using an exponential
probability distribution. This implies that calls of long duration are much less frequent than short
calls. If the traffic intensity offered by a single user is Auser, then the traffic intensity offered by
N users is A = NAuser. The purpose of the statistical model is to relate the offered traffic intensity
A, the grade of service Pb, and the number of channels or trunks C needed to maintain the desired
grade of service.
Two models are widely used in traffic engineering to represent what happens when a call is
blocked. The blocked calls cleared model assumes that when a channel or trunk is not available to
service an arriving call, the call is cleared from the system. The second model is known as
blocked calls delayed. In this model a call that cannot be serviced is placed on a queue and will be
serviced when a channel or
trunk becomes available. Use of the blocked-calls-cleared statistical model leads to the Erlang B
formula that relates offered traffic intensity A, grade of service Pb, and number of channels K.
The Erlang B formula is:
When the blocked-calls-delayed model is used, the "grade of service" refers to the probability that
a call will be delayed. In this case the statistical model leads to the Erlang C formula,
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UNIT 5
MULTIPLE ACCESS TECHNIQUES FOR WIRELESS COMMUNICATION
Introduction
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A wireless communications system uses a certain frequency band that is assigned to this specific
service. Spectrum is thus a scarce resource, and one that cannot be easily extended. For this
reason, a wireless system must make provisions to allow the simultaneous communication of as
many users as possible within that band. The problem of letting multiple users communicate
simultaneously can be divided into two parts:
1. If there is only a single Base Station (BS), how can it communicate with many Mobile Stations
(MSs) simultaneously?
2. If there are multiple BSs, how can we assign spectral resources to them in such a way that the
total number of possible users is maximized? And how should these BSs is placed in a given
geographical area? As for the first question, there are different methods, called Multiple Access
(MA) methods that allow multiple users to talk to a BS simultaneously. In this, we discuss the
following three methods:
• Frequency Division Multiple Access (FDMA), where different frequencies are assigned to
different users.
• Time Division Multiple Access (TDMA), where different timeslots are assigned to different
users.
• Packet Radio can be viewed as a form of TDMA, where the assignment of timeslots to users is
adaptive.
Space Division Multiple Access: (SDMA) is a multiple access format for systems with multiple
antennas; it can be combined with all of the other multiple access methods. The so-called
“duplexing,” which separates transmission and reception at a transceiver. The goal of all these
methods is to maximize spectral efficiency – i.e., to maximize the number of users per unit
bandwidth.
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Frequency Division Multiple Access
FDMA is a familiar method of allocating bandwidth. A band of frequencies is divided up in to
channels, each of a particular size. A transmitter in an FDMA system is given exclusive use of
one or more channels. This is how broadcast radio and television are set up - each "owns" a
portion of the frequency spectrum that is set aside just for them. This is also true of cellular
systems, where a base station is allowed to transmit on a set of forward channels and a mobile
unit transmits on one of a set of reverse channels. No other base station within range of the mobile
will be transmitting on the same forward channel, and no other mobile within range of the base
station should be transmitting on the same reverse channel. Both the base and the mobile usually
transmit continuously during a conversation, and fully occupy their assigned forward and reverse
channels. No other conversation can take place on these channels until the first conversation is
completed (or the call is handed off to another base station).
Multiple Access via Frequency Division Multiple Access
FDMA is the oldest, and conceptually most simple, multiaccess method. Each user is assigned a
frequency (sub) band – i.e., a (usually contiguous) part of the available spectrum. The assignment
of frequency bands is usually done during call setup, and retained during the whole call. FDMA is
usually combined with the Frequency Domain Duplexing (FDD) so that two frequency bands
(with a fixed duplex distance) are assigned to each user: one for downlink (BS-to-MS) and one
for uplink (MS-to-BS) communication.
Principle of frequency division multiple access.
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Pure FDMA is conceptually very simple, and has some advantages for implementation:
• The transmitter (TX) and receiver (RX) require little digital signal processing. However, this is
not so important in practice anymore, as the costs for digital processing are continuously
decreasing.
• (Temporal) synchronization is simple. Once synchronization has been established during the
call setup, it is easy to maintain it by means of a simple tracking algorithm, as transmission occurs
continuously.
However, pure FDMA also has significant disadvantages, especially when used for speech
communications. These problems arise from spectral efficiency considerations, as well as from
sensitivity to multipath effects:
• Frequency synchronization and stability are difficult: for speech communications, each
frequency
Sensitivity to fading: since each user is assigned a distinct frequency band, these bands are
narrower than for other multi access methods (compare TDMA, CDMA) – i.e., 5–30 kHz. For
such narrow sub bands, fading is flat in practically all environments. This has the advantage that
no equalization is required; the drawback is that there is no frequency diversity. Remember that
frequency diversity is mainly provided by signal components that are more than one channel
coherence bandwidth apart.
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• Sensitivity to random Frequency Modulation (FM): due to the narrow bandwidth, the system is
sensitive to random FM: the Bit Error Rate (BER) due to random FM is proportional to (νmaxTS
)2
Thus, it is inversely proportional to the square of the bandwidth. On the positive side, appropriate
signal-processing schemes can not only mitigate these effects but even exploit them to obtain time
diversity. Note that the situation here is dual to wideband systems, where delay dispersion can be
a drawback, but equalizers can turn them into an asset by exploiting frequency diversity.
Intermodulation: the BS needs to transmit multiple speech channels, each of which is active the
whole time. Typically, a BS uses 20–100 frequency channels. If these signals are amplified by the
same power amplifier, third-order modulation products can be created, which lie at undesirable
frequencies – i.e., within the transmit band. We thus need either a separate amplifier for each
speech channel, or a highly linear amplifier for the composite signal – each of these solutions
makes a BS more expensive.
It is for these reasons that FDMA is mostly used for the following applications:
• Analog communications systems: here, FDMA is the only practicable multiple access method.
• Combination of FDMA with other multiple access methods: the spectrum allocated for a service
(or a network operator) is divided into larger sub bands, each of which is used for serving a group
of users. Within this group, multiple access is done by means of another multiple access method –
e.g., TDMA or CDMA.
• High-data-rate systems: the disadvantages of FDMA are mostly relevant if each user requires
only a small bandwidth – e.g., 20 kHz. The situation can be different for wireless Local Area
Networks (LANs), where a single user requires a bandwidth on the order of 20 MHz, and only a
few frequency channels are available.
Time Division Multiple Access
TDMA is a more efficient, but more complicated way of using FDMA channels. In a TDMA
system each channel is split up into time segments, and a transmitter is given exclusive use of one
or more channels only during a particular time period. For example, in North American TDMA
(also known as Interim Standard 54) each channel is essentially divided into three timeslots. A
maximum of three transmitters take turns sending in their assigned timeslots. A conversation,
then, takes place during the time slots to which each transmitter (base and mobile) is assigned.
TDMA requires a master time reference to synchronize all transmitters and receivers.
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TDMA was the first digital standard to be proposed, and is attractive to current cellular operators
because it allows existing analog customers to continue using the network as digital capability is
added. Under IS-54, channel bandwidth remains 30 kHz and the data format on the control
channels remains identical to those under AMPS. Voice channels are designated as either analog
or digital, and analog cell phones continue to operate as they do under AMPS since they will
always be assigned to analog voice channels. Dual-mode and digital-only phones are assigned
digital voice channels where they are available. Base stations can be converted to digital
capability as demand and funding allow.
In TDMA the system dimensions are divided along the time axis into non overlapping channels,
and each user is assigned a different cyclically-repeating timeslot, as shown in Figure . These
TDMA channels occupy the entire system bandwidth, which is typically wideband, so some form
of ISI mitigation is required. The cyclically repeating timeslots imply that transmission is not
continuous for any user. Therefore, digital transmission techniques which allow for buffering are
required. The fact that transmission is not continuous simplifies overhead functions such as
channel estimation, since these functions can be done during the timeslots occupied by other
users. TDMA also has the advantage that it is simple to assign multiple channels to a single user
by simply assigning him multiple timeslots.
A major difficulty of TDMA, at least for uplink channels, is the requirement for synchronization
among the different users. Specifically, in a downlink channel all signals originate from the same
transmitter and pass through the same channel to any given receiver. Thus, for flat-fading
channels, if users transmit on orthogonal timeslots the received signal will maintain this
orthogonality. However, in the uplink channel the users transmit over different channels with
different respective delays. To maintain orthogonal timeslots in the received signals, the different
uplink transmitters must synchronize such that after transmission through their respective
channels, the received signals are orthogonal in time. This synchronization is typically
coordinated by the base station or access point, and can entail significant overhead. Multipath can
also destroy time-division orthogonality in both uplinks and downlinks if the multipath delays are
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a significant fraction of a timeslot. TDMA channels therefore often have guard bands between
them to compensate for synchronization errors and multipath. Another difficulty of TDMA is that
with cyclically repeating timeslots the channel characteristics change on each cycle. Thus,
receiver functions that require channel estimates, like equalization, must re-estimate the channel
on each cycle. When transmission is continuous, the channel can be tracked, which is more
efficient. TDMA is used in the GSM, PDC, IS-54, and IS-136 digital cellular phone standards.
TDMA is a more efficient, but more complicated way of using FDMA channels. In a TDMA
system each channel is split up into time segments, and a transmitter is given exclusive use of one
or more channels only during a particular time period. For example, in North American TDMA
(also known as Interim Standard 54) each channel is essentially divided into three timeslots. A
maximum of three transmitters take turns sending in their assigned timeslots. A conversation,
then, takes place during the time slots to which each transmitter (base and mobile) is assigned.
TDMA requires a master time reference to synchronize all transmitters and receivers.TDMA was
the first digital standard to be proposed, and is attractive to current cellular operators because it
allows existing analog customers to continue using the network as digital capability is added.
Under IS-54, channel bandwidth remains 30 kHz and the data format on the control channels
remains identical to those under AMPS. Voice channels are designated as either analog or digital,
and analog cell phones continue to operate as they do under AMPS since they will always be
assigned to analog voice channels. Dual-mode and digital-only phones are assigned digital voice
channels where they are available. Base stations can be converted to digital capability as demand
and funding allow.
Space division multiple access
Space-division multiple access (SDMA) uses direction (angle) as another dimension in signal
space, which can be channelized and assigned to different users. This is generally done with
directional antennas, as shown in Orthogonal channels can only be assigned if the angular
separation between users exceeds the angular resolution of the directional antenna. If
directionality is obtained using an antenna array, precise angular resolution requires a very large
array, which may be impractical for the base station or access point and is certainly infeasible in
small user terminals. In practice SDMA is often implemented using sectorized antenna arrays,
discussed in. In these arrays the 360o angular range is divided into N sectors. There is high
directional gain in each sector and little interference between sectors. TDMA or FDMA is used to
channelize users within a sector. For mobile users SDMA must adapt as user angles change or, if
directionality is achieved via sectorized antennas,then a user must be handed off to a new sector
when it moves out of its original sector.
(SDMA) is a multiple access format for systems with multiple antennas; it can be combined with
all of the other multiple access methods. The so-called duplexing,”which separates transmission
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and reception at a transceiver. The goal of all these methods is to maximize spectral efficiency –
i.e., to maximize the number of users per unit bandwidth. As mentioned above, there is also a
different (though related) question:
How can we design a system so that the number of users per unit bandwidth and unit area is
maximized? This goal obviously requires multiple BSs, and the assignment of spectral resources
to them. All this leads us to the cellular principle.
Space-Division Multiple Access (SDMA) is a channel access method based on creating parallel
spatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by which
it is able to offer superior performance in radio multiple access communication systems. In
traditional mobile cellular network systems, the base station has no information on the position of
the mobile units within the cell and radiates the signal in all directions within the cell in order to
provide radio coverage. These results in wasting power on transmissions when there are no
mobile units to reach, in addition to causing interference for adjacent cells using the same
frequency, so called co-channel cells. Likewise, in reception, the antenna receives signals coming
from all directions including noise and interference signals. By using smart antenna technology
and differing spatial locations of mobile units within the cell, space-division multiple access
techniques offer attractive performance enhancements. The radiation pattern of the base station,
both in transmission and reception, is adapted to each user to obtain highest gain in the direction
of that user. This is often done using phased array techniques.In GSM cellular networks, the base
station is aware of the mobile phone's position by use of a technique called "timing advance"
(TA). The Base Transceiver Station (BTS) can determine how distant the Mobile Station (MS) is
by interpreting the reported TA. This information, along with other parameters, can then be used
to power down the BTS or MS, if a power control feature is implemented in the network. The
power control in either BTS or MS is implemented in most modern networks, especially on the
MS, as this ensures a better battery life for the MS and thus a better user experience (in that the
need to charge the battery becomes less frequent). This is why it may actually be safer to have a
BTS close to you as your MS will be powered down as much as possible. For example, there is
more power being transmitted from the MS than what you would receive from the BTS even if
you are 6 m away from a mast. However, this estimation might not consider all the MS's that a
particular BTS is supporting with EM radiation at any given time.
Features
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In a mobile cellular communication network, the SDMA leverages the spatial location of mobile
terminals, equipments and devices within that cell, thereby enhancing the efficiency in network
bandwith utilization.
Unlike traditional mobile cellular network systems, where the base station is tied up, radiating
radio signals in all directions within the cell, with no knowledge of the location of mobile devices,
SDMA architecture enables the channeling of radio signals based on the mobile devices'
locations.
In this way, SDMA architecture not only protects the quality of radio signals, safeguarding
against interference causing noise and signal degradation coming from adjacent cells, but also
saves on redundant signal transmission in areas where mobile devices are not currently active or
unavailable.
Packet Radio
Packet radio access schemes break data down into packets, and each of the packets is transmitted
over the medium independently. In other words, each packet is like a new user that has to fight for
its “own” resources. This allows the transport medium to be exploited much more efficiently
when the data traffic from each user is bursty, as is the case for Web browsing, file downloads,
and similar data applications.
Packet radio shows two main differences from TDMA and FDMA:
Each packet has to fight for its own resources, as described above. The most common methods
for resource allocation are ALOHA systems, Carrier Sense Multiple Access (CSMA), and packet
reservation (polling). 2. Each packet can be routed to the RX in different ways – i.e., via different
relay stations. This aspect does not play a major role in cellular systems, where connection can
only be to the closest BS,5 but it does play an important role in wireless ad hoc and sensor
networks, where each wireless device can act as a relay for information originating from another
wireless device.Appropriate routing is thus a very important aspect of sensor networks.This
method has better spectral efficiency; the drawback is that the tables can become quite large,
especially at nodes in the middle of a network. A related topic is “route discovery” – i.e.,
determination of which route a packet should take from the transmitter to the receiver. Route
discovery is typically done by means of special packets that are broadcast in the network, and
record the quality of the links between different nodes. In order to achieve optimum performance,
routing has to be changed whenever the channel between nodes changes significantly.
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If a very low packet error rate is required, each node that acts as a relay stores the packets in a
buffer and deletes them only after receiving an acknowledgment of successful transmission from
the node it forwarded the packet to.
In packet radio (PR) access techniques, many subscribers attempt to access a single channel in an
uncoordinated (or minimally coordinated) manner. Transmission is done by using bursts of data.
Collisions from the simultaneous transmissions of multiple transmitters are detected at the base
station receiver, in which case an ACK or NACK signal is broadcast by the base station to alert
the desired user (and all other users) of received transmission. The ACK signal indicates an
acknowledgment of a received burst from a particular user by the base station, and a NACK
(negative acknowledgment) indicates that the previous burst was not received correctly by the
base station. By using ACK and NACK signals, a PR system employs perfect feedback, even
though traffic delay due to collisions may be high.
Packet radio multiple access is very easy to implement, but has low spectral efficiency and may
induce delays. The subscribers use a contention technique to transmit on a common channel.
AlOHA protocols, developed for early satellite systems, are the best examples of contention
techniques. ALOHA allows each subscriber to transmit whenever they have data to send. The
transmitting subscribers listen to the acknowledgment feedback to determine if transmission has
been successful or not. If a collision occurs, the subscriber waits a random amount of time, and
then retransmits the packet.
The advantage of packet contention techniques is the ability to serve a large number of
subscribers with virtually no overhead. The performance of contention techniques can be
evaluated by the throughput (T), which is defined as the average number of messages successfully
transmitted per unit time, and the average delay (D) experienced by a typical message burst.
Spread Spectrum Multiple Access
Spread spectrum techniques spread information over a very large bandwidth – specifically, a
bandwidth that is much larger than the inverse of the data rate. In this, we discuss various ways of
providing multiple access by spreading the spectrum. We start out with the conceptually most
simple approach, Frequency Hopping (FH). We then proceed to the most popular form of spread
spectrum, Direct Sequence–Code Division Multiple Access (DS-CDMA). Finally, we elaborate
on time-hopping impulse radio, a relatively new scheme that has gathered interest in recent years
because of its application to ultra wideband systems.
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We have stressed how important spectral efficiency is: we want to transmit as much information
per available bandwidth as possible. Thus, it might seem like a strange idea to spread information
over a large bandwidth in a commercial wireless system. After all, the term
“spread spectrum” comes from the military area, where the main interest lies
communications stealthy, safe from intercept, and safe from jamming efforts
transmitters – issues that do not top the list of concerns of cellular operators.1 It
astonishing that spread spectrum approaches have attained such an important role
communications.
in keeping
by hostile
thus seems
in wireless
This seeming paradox can be res
resolved
olved when we recognize that different users can be spread
across the spectrum in different ways. This allows multiple users to transmit in the same
frequency band simultaneously; the receiver can determine which part of the total contribution
comes from a specific user by looking only at signals with a specific spreading pattern. Thus,
capacity (per unit bandwidth) is not necessarily decreased by using spread spectrum techniques,
and can even be increased by exploiting its special features.
In a spread spectrum
ctrum communication system users employ signals which occupy a significantly
larger bandwidth than the symbol rate. Such a signalling scheme provides some advantages which
are primarily of interest in secure communication systems, e.g., low probability of intercept or
robustness to jamming. In this problem we explore the inherent multiple access capability of
spread spectrum signalling, i.e., the ability to support simultaneous transmissions in the same
frequency band.
In the sequel, assume that the communication
communi
channel is ann additive white Gaussian noise channel
with spectral height
.
One user employs the following signal set to transmit equally likely binary symbols
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Draw a block diagram of the receiver which minimizes the probability of a bit error for this signal
set.
Compute the probability of error achieved by your receiver.
Now, a second users transmits one of the following signals with equal probability
Both signals are transmitted simultaneously, such that the received signal is given by
(1)
Where
is the noise process and
indicate which symbol each of the users is
transmitting. We are interested in receiving the first user's signal in the presence of the second
(interfering) user.
Find the probability of error of
between
and
your receiver from part (a) for distinguishing
if the received signal is given by (1). Which value does the
probability of error approach if the amplitude
of the interfering user approaches
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?
Find the minimum probability of error receiver for distinguishing
distinguishin between
and
in
the presence of the interfering signal
, i.e., if the received signal is given by (1). Note: You
do not need to find the probability of error for this receiver.
Indicate the locations of the relevant signals and the decision regions for
fo your receiver in a
suitably chosen and accurately labeled signal space. Indicate also the decision boundary formed.
Capacity of cellular systems
Capacity is a concern in any wireless communications system. High demand for cellular service,
especially in large urban markets, has created a need to serve a greater number of users in a
limited amount of frequency space. Cellular system operators are looking for new ways to fit
more users into their increasingly crowded network, and many are choosing to move ffrom the
existing analog transmission technology to one of the competing digital standards. These
standards are also being selected by the new PCS providers as they begin to build out their own
networks. Although digital systems provide a variety of benefits,
benefits, this month we'll focus on two
main digital access methods and their effect on system capacity.
Almost all current and proposed digital standards are based on either Time Division Multiple
Access (TDMA) or Code Division Multiple Access (CDMA). These two methods are
fundamentally different and incompatible with each other, but each claim to able to support
anywhere from three to more than twenty times the number of simultaneous callers than the
current AMPS cellular standard, which uses Frequency Division Multiple Access (FDMA).
We derive an expression for the system capacity of a cellular network in which power
equalization (PE) is applied only to the mobile stations (MS) that have stronger channel, while
there is no power equalization for those with weaker
weaker channel. Power equalization means
adjustment of the transmit power of each MS in order to have the sanle received power at the
base station (BS). Systems that use matched filter receiver need power equalization. In general,
however, if another reception strategy is used, the system capacity is higher if there is no power
equalization. Although this strategy is optimal for the sum rate capacity, it will give an advantage
to users staying very close to the BS, and is not fair. In this paper we consider a combined
co
model
in which the power equalization is done partially, i.e., stations that are closer to the BS equalize
their transmit powers, while the others do not. For that reason, we define a cut-off
cut
rate beyond
which no MS can transmit. This corresponds to an equivalent cut-off
off radius in the cell within
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which the transmit power of the mobile stations has to be scaled down such that all of them have
the same receive power.
Outside the cut-off radius all users can transmit with their maximal power (i.e. no PE is applied to
them), such that the overall system capacity is increased. We derive a closed form expression in
terms of the hyper geometric functions for the system capacity when partial power equalization
Is applied and the channel is Gaussian. The main purpose of this paper is like to study the
possibility of using the rate splitting multiple access (RSMA) on cellular communications. From
theoretical point of view maximum achievable rates of multiple access channels have been well
understood for long time the transmit power is affected by large scale path loss, shadowing and
small scale path loss (fast fading). Power equalization (PE) is an operation that adjusts the
transmit power of mobile units in such a way that the mean received power is the same for all
units. It should not be mixed with power control (PC), that is an operation that allocates transmit
power to the transmit antennae according to the channel state information obtained from the
training sequence, in order to maximize some objective function such as the throughput of the
resulting channel. In this paper we consider only power equalization.
Note that in both cases, PE and PC, the transmitter must have information about the channel, but
for the PE this information is much smaller (a pilot signal might be used for that) than for the
power control case where real channel estimation is needed (training sequence). For systems that
use nlatch filter receiver, perfect power equalization is necessary. In general, however, if we use
another reception strategy, the system capacity is higher if there is no power equalization at all.
In authors analyze a cellular system by comparing its spectral efficiency of a spread spectrum
multiple access (SSMA) scheme and of the ideal interference cancellation multiple access
scheme. The latter gives the theoretical upper bound on the maximal sum-rate. Authors conclude
that there is a big gap between the two schemes. They also observe that another huge
improvement could be obtained if power equalization is not used. Although this strategy is
optimal for the sunl rate, it will give an advantage to users staying very close to the BS, and is not
fair, and therefore not applicable in practice. To that end we propose a combined model in which
users that are close to the BS are equalized but those who are further are not. We define a cut-off
rate beyond which no user can transmit. This corresponds to an equivalent (since there is a
random fading) cut-off radius in the cell within which the transmit power of users has to be scaled
such that all of them have the same receive power. Outside the cut-off radius all users can
transmit with their maximal power, such that the overall system capacity is maximized. A pure
case without power equalization, although theoretically more efficient, does not have practical
importance.
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UNIT 6
WIRELESS NETWORKING
Difference between wireless and fixed telephone networks
Transfer of information in the public switched telephone network (PSTN) takes place over landline trunked lines (called trunks) comprised of fiber optic cables, copper cables, microwave links,
and satellite links. The network configurations in the PSTN are virtually static, since the network
connections may only be changed when a subscriber changes residence and requires
reprogramming at the local central office (CO) of the subscriber. Wireless networks, on the other
hand, are highly dynamic, with the network configuration being rearranged every time a subscriber moves into the coverage region of a different base station or a new market. While fixed
networks are difficult to change, wireless networks must reconfigure themselves for users within
small intervals of time (on the order of seconds) to provide roaming and imperceptible handoffs
between calls as a mobile moves about. The available channel bandwidth for fixed networks can
be increased by installing high capacity cables (fiber optic or coaxial cable), whereas wireless
networks are constrained by the meager RF cellular bandwidth provided for each user’s wireless
and fixed telephone network.
The Public Switched Telephone Network (PSTN)
The PSTN is a highly integrated communications network that connects over 70% of the world’s
inhabitants. In early 2001, the International Telecommunications Union estimated that there were
1 billion public landline telephone numbers, as compared to 600 million cellular telephone
Numbers. While landline telephones are being added at a 3% rate, wireless subscriptions are
interexchange service. In the US, there are about 2000 telephone companies, although the Bell
Operating Companies (BOCs) are the most widely known.. Each local exchange consists of a
central office (CO) which provides PSTN connection to the customer premises equipment (CPE)
which may be an individual phone at a residence or a private branch exchange (PBX) at a place of
business. The CO may handle as many as a million telephone connections. The CO is connected
to a tandem switch which in turn connects the local exchange to the PSTN. The tandem switch
physically connects the local telephone net-work to the point of presence (POP) of trunked long
distance lines provided by one or more IXCs [Pec92]. Sometimes IXCs connect directly to the
CO switch to avoid local transport charges levied by the LEC.
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PBX may be used to provide telephone connections throughout a building or campus. A PBX
allows an organization or entity to provide internal calling and other in-building services (which
do not involve the LEC), as well as private net-working between other organizational sites
(through leased lines from LEC and IXC providers), PBX may be used to provide telephone
connections throughout a building or campus. A PBX allows an organization or entity to provide
internal calling and other in-building services (which do not involve the LEC), as well as private
net-working between other organizational sites (through leased lines from LEC and IXC
providers), in addition to conventional local and long distance services which pass through the
CO. Tele-phone connections within a PBX are maintained by the private owner, whereas
connection of the wireless systems are constrained to operate in a fixed bandwidth to support an
increasing num-ber of users over time. Spectrally efficient modulation techniques, frequency
reuse techniques, and geographically distributed radio access points are vital components of
wireless networks. As wireless systems grow, the necessary addition of base stations increases the
switching burden of the MSC. Because the geographical location of a mobile user changes
constantly, extra overhead is needed by all aspects of a wireless network, particularly at the MSC,
to ensure seamless com-munications, regardless of the location of the user.
Merging Wireless Networks and the PSTN
Throughout the world, first generation wireless systems (analog cellular and cordless tele-phones)
were deployed in the early and mid 1980s. As first generation wireless systems were being
introduced, revolutionary advances were being made in the design of the PSTN by land-line
telephone companies. Until the mid 1980s, most analog landline telephone links throughout the
world sent signaling information along the same trunked lines as voice traffic. That is, a single
physical connection was used to handle both signaling traffic (dialed digits and telephone ringing
commands) and voice traffic for each user. The overhead required in the PSTN to handle
signaling data on the same trunks as voice traffic was inefficient, since this required a voice trunk
to be dedicated during periods of time when no voice traffic was actually being carried.
Merging Wireless Networks and the PSTN
Throughout the world, first generation wireless systems (analog cellular and cordless tele-phones)
were deployed in the early and mid 1980s. As first generation wireless systems were being
introduced, revolutionary advances were being made in the design of the PSTN by land-line
telephone companies. Until the mid 1980s, most analog landline telephone links throughout
the world sent signaling information along the same trunked lines as voice traffic. That is, a single
physical connection was used to handle both signaling traffic (dialed digits and telephone ringing
commands) and voice traffic for each user. The overhead required in the PSTN to handle
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signaling data on the same trunks as voice traffic was inefficient, since this required a voice trunk
to be dedicated during periods of time when no voice traffic was actually being carried.
Fixed
Non portable
Cost of fiber +copper+coaxial cable i
Bandwidth fixed in advance
Static in nature
Difficult to find path break if any
Infrastructure is required
Handoff minimum
wireless
portable
Cost is less
Not so
Dynamic in nature
No path break
Infrastructure is required
More Handoff
Development of wireless networks
First Generation Wireless Networks
First generation cellular and cordless telephone networks are based on analog technology. All first
generation cellular systems use FM modulation, and cordless telephones use a single base station
to communicate with a single portable terminal. A typical example of a first generation cellular
telephone system is the Advanced Mobile Phone Services (AMPS) system used in the United
States .Basically; all first generation systems use the transport architecture.
First generation cellular radio network, which includes the mobile terminals, the base stations,
and MSCs. In first generation cellular networks, the sys-tem control for each market resides in the
MSC, which maintains all mobile related information and controls each mobile handoff. The
MSC also performs all of the network management functions, such as call handling and
processing, billing, and fraud detection within the market. The Until the early 1990s, US cellular
customers that roamed between different cellular systems had to register manually each time they
entered a new market during long distance travel. This required the user to call an operator to
request registration. In the early 1990s, US cellular carriers implemented the network protocol
standard IS-41 to allow different cellular systems to automatically accommodate subscribers who
roam into their coverage region. This is called interoperator roaming. IS-41 allows MSCs of
different service providers to pass information about their subscribers to other MSCs on demand.
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IS-41 relies on a feature of AMPS called autonomous registration. Autonomous registration is a
process by which a mobile notifies a serving MSC of its presence and location. The mobile
accomplishes this by periodically keying up and transmitting its identity information, which
allows the MSC to constantly update its customer list. The registration command is sent in the
overhead message of each control channel at five or ten minute intervals, and includes a timer
value which each mobile uses to determine the precise time at which it should respond to the
serving base station with a registration transmission. Each mobile reports its MIN and ESN during
the brief registration transmission so that the MSC can validate and update the customer this is
called interoperator roaming. IS-41 allows MSCs of different service providers to pass
information about their subscribers to other MSCs on demand.
IS-41 relies on a feature of AMPS called autonomous registration. Autonomous registration is a
process by which a mobile notifies a serving MSC of its presence and location. The mobile
accomplishes this by periodically keying up and transmitting its identity information, which
allows the MSC to constantly update its customer list. The registration command is sent in the
overhead message of each control channel at five or ten minute intervals, and includes a timer
value which each mobile uses to determine the precise time at which it should respond to the
serving base station with a registration transmission. Each mobile reports its MIN and ESN during
the brief registration transmission so that the MSC can validate and update the customer thereby
allowing carriers to use different manufacturers for MSC and BSC components. This trend
In standardization and interoperability is new to second generation wireless networks. Eventually,
wireless network components, such as the MSC and BSC, will be available as off-the-shelf
components, much like their wireline telephone counterparts.
All second generation systems use digital voice coding and digital modulation. The systems
employ dedicated control channels within the air interface for simultaneously exchanging voice
and control information between the subscriber, the base station, and the MSC while a call is in
progress. Second generation systems also provide dedicated voice and signaling trunks between
MSCs, and between each MSC and the PSTN.
In contrast to first generation systems, which were designed primarily for voice, second
generation wireless networks have been specifically designed to provide paging, and other data
services such as facsimile and high-data rate network access. The network controlling structure is
more distributed in second generation wireless systems, since mobile stations assume greater
control functions. In second generation wireless networks, the handoff process is mobilecontrolled and is known as mobile assisted handoff .The mobile units in these networks perform
several other functions not performed by first generation subscriber units, such as received power
reporting, adjacent base station scanning, data encoding, and encryption and will serve both
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stationary users and vehicular users traveling at high speeds. Packet radio communications will
likely be used to distribute network control while providing a reliable information transfer
[Goo90]. The terms 3G Personal Communication System (PCS) and 3G Personal Communication
Network (PCN) are used to imply emerging third generation wireless systems for hand-held
devices. Other names for PCS include Future Public Land Mobile Telecommunication Systems
(FPLMTS) for worldwide use which has more recently been called International Mobile Telecommunication (IMT-2000), and Universal Mobile Telecommunication System (UMTS) for
advanced mobile personal services in Europe.
Fixed network transmission hierarchy
Wireless networks rely heavily on landline connections. For example, the MSC connects to the
PSTN and SS7 networks using fiber optic or copper cable or microwave links. Base stations
within a cellular system are connected to the MSC using line-of-sight (LOS) microwave links, or
copper or fiber optic cables. These connections require high data rate serial transmission schemes
in order to reduce the number of physical circuits between two points of connection.
North America and Japan
DS-0 64.0 kbps
1
DS-1 1.544 Mbps
24
T-1
DS-1C 3.152 Mbps
48
T-1C
DS-2 6.312 Mbps
96
T-2
DS-3 44.736 Mbps 672
T-3
DS-4 274.176 Mbps 4032
T-4
CEPT (Europe and most other PTTs)
0
64.0 kbps
1
1
2.048 Mbps
30
E-1
2
8.448 Mbps
120
E-1C
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Traffic Routing in Wireless Networks
The amount of traffic capacity required in a wireless network is highly dependent upon the type
of traffic carried. For example, a subscriber’s telephone call (voice traffic) requires dedicated
network access to provide real-time communications, whereas control and signaling traffic may
be bursty in nature and may be able to share network resources with other bursty users.
Alternatively, some traffic may have an urgent delivery schedule while some may have no need to
be sent in real-time. The type of traffic carried by a network determines the routing services,
proto-cols, and call handling techniques which must be employed.
Two general routing services are provided by networks. These are connection-oriented services
(virtual circuit routing), and connectionless services (datagram services). In connection-oriented
routing, the communications path between the message source and destination is fixed for the
entire duration of the message, and a call set-up procedure is required to dedicate network
resources to both the called and calling parties. Since the path through the network is fixed, the
traffic in connection-oriented routing arrives at the receiver in the exact order it was transmitted.
A connection-oriented service relies heavily on error control coding to provide data protection in
case the network connection becomes noisy. If coding is not sufficient to protect the traffic, the
call is broken, and the entire message must be retransmitted from the beginning.
Connectionless routing, on the other hand, does not establish a firm connection for the traffic, and
instead relies on packet-based transmissions. Several packets form a message, and each individual
packet in a connectionless service is routed separately. Successive packets within the same
message might travel completely different routes and encounter widely varying delays throughout
the network. Packets sent using connectionless routing do not necessarily arrive in the order of
transmission and must to be reordered at the receiver. Because packets take different routes in a
connectionless service, some packets may be lost due to network or link failure; however others
may get through with sufficient redundancy to enable the entire message to be recreated at the
receiver. Thus, connectionless routing often avoids having to retransmit an entire message, but
requires more overhead information for each packet. Typical packet over-head information
includes the packet source address, the destination address, the routing information, and
information needed to properly order packets at the receiver. In a connectionless service, a call
set-up procedure is not required at the beginning of a call, and each message burst is treated
independently by the network.
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Packet Switching
Connectionless services exploit the fact that dedicated resources are not required for message
transmission. Packet switching (also called virtual switching) is the most common technique used
to implement connectionless services and allows a large number of data users to remain virtually
connected to the same physical channel in the network. Since all users may access the network
randomly and at will, call set-up procedures are not needed to dedicate specific circuits when a
particular user needs to send data. Packet switching breaks each message into smaller units for
transmission and recovery . When a message is broken into packets, a certain amount of control
information is added to each packet to provide source and destination identification, as well as
error recovery provisions.
Sequential format of a packet transmission. The packet consists of header information, the user
data, and a trailer. The header specifies the beginning of a new packet and contains the source
address, destination address, packet sequence number, and other routing and billing information.
The user data contains information which is generally protected with error control coding. The
trailer contains a cyclic redundancy checksum which is used for error detection at the receiver.
Structure of a transmitted packet, which typically consists of five fields:
In contrast to circuit switching, packet switching (also called packet radio when used over a
wireless link) provides excellent channel efficiency for bursty data transmissions of short length.
An advantage of packet-switched data is that the channel is utilized only when sending or
receiving bursts of information. This benefit is valuable for the case of mobile services where the
available bandwidth is limited. The packet radio approach supports intelligent protocols for data
flow control and retransmission, which can provide highly reliable transfer in degraded channel
conditions. X.25 is a widely used packet radio protocol that defines a data interface for packet
switching .
The X.25 Protocol
X.25 was developed by CCITT (now ITU-T) to provide standard connectionless network access
(packet switching) protocols for the three lowest layers (layers 1, 2, and 3) of the open systems
interconnection (OSI) model (see Figure for the OSI layer hierarchy). The X.25 protocols provide
a standard network interface between originating and terminating subscriber equipment
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(Called data terminal equipment or DTE), the base stations (called data circuit-terminating
equipment or DCE), and the MSC (called the data switching exchange or DSE). The X.25 protocols are used in many packet radio air-interfaces, as well as in fixed networks
The hierarchy of X.25 protocols in the OSI model. The Layer 1 protocol deals with the electrical,
mechanical, procedural, and functional interface between the sub-scriber (DTE), and the base
station (DCE). The Layer 2 protocol defines the data link on the common air-interface between
the subscriber and the base station. Layer 3 provides connection The X.25 protocol does not
specify particular data rates or how packet-switched networks are implemented. Rather, X.25
provides a series of standard functions and formats which give structure to the design of software
that is used to provide packet data on a generic connectionless network.
Wireless Data Services
As discussed in Section 10.5, circuit switching is inefficient for dedicated mobile data services
such as facsimile (fax), electronic mail (e-mail), and short messaging. First generation cellular
systems that provide data communications using circuit switching have difficulty passing modem
signals through the audio filters of receivers designed for analog, FM, common air-interfaces.
Inevitably, voice filtering must be deactivated when data is transmitted over first generation
cellular networks, and a dedicated data link must be established over the common air-interface.
The demand for packet data services has, until recently, been significantly less than the demand
for voice services, and first generation subscriber equipment design has focused almost solely on
voice-only cellular communications. However, in 1993, the US cellular industry developed the
cellular digital packet data (CDPD) standard to coexist with the conventional voice-only cellular
system. In the 1980s, two other data-only mobile services called ARDIS and RAM Mobile Data
(RMD) were developed to provide packet radio connectivity throughout a network.
Cellular Digital Packet Data (CDPD)
CDPD is a data service for first and second generation US cellular systems and uses a full 30kHz
AMPS channel on a shared basis. CDPD provides mobile packet data connectivity to existing data
networks and other cellular systems without any additional band-width requirements. It also
capitalizes on the unused air time which occurs between successive radio channel assignments by
the MSC (it is estimated that for 30% of the time, a particular cellular radio channel is unused, so
packet data may be transmitted until that channel is selected by the MSC to provide a voice
circuit). CDPD directly overlays with existing cellular infrastructure and uses existing base station
equipment, making it simple and inexpensive to install. Furthermore, CDPD does not use the
MSC, but rather has its own traffic routing capabilities. CDPD occupies voice channels purely on
a secondary,noninterfering basis, and packet channels are dynamically assigned (hopped) to
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different cellular voice channels as they become vacant, so the CDPD radio channel varies with
time. As with conventional, first generation AMPS, each CDPD channel is duplex in nature. The
forward channel serves as a beacon and transmits data from the PSTN side of the network, while
the reverse channel links all mobile users to the CDPD network and serves as the access channel
for each subscriber. Collisions may result when many mobile users attempt to access the network
simultaneously. Each CDPD simplex link occupies a 30 kHz RF channel, and data is sent at
19,200 bps. Since CDPD is packet-switched, a large number of modems are able to access the
same channel on an as needed, packet-by-packet basis. CDPD supports broadcast, dispatch,
electronic mail, and field monitoring applications. GMSK BT = 0.5 modulation is used so that
existing analog FM cellular receivers can easily detect the CDPD format without redesign.
CDPD transmissions are carried out using fixed-length blocks. User data is protected using a
Reed–Solomon block code with 6-bit symbols. For each packet, 282 user bits are coded into 378
bit blocks, which provide correction for up to eight symbols. Two lower layer protocols are used
in CDPD. The mobile data link protocol (MDLP) is used to convey information between data link
layer entities (layer 2 devices) across the CDPD air interface. The MDLP provides logical data
link connections on a radio channel by using an address contained in each packet frame. The
MDLP also provides sequence control to maintain the sequential order of frames across a data
link connection, as well as error detection and flow control. The radio resource management
protocol (RRMP) is a higher, layer 3 protocol used to manage the radio channel resources of the
CDPD system and enables an M-ES to find and utilize a duplex radio channel without interfering
with standard voice services. The RRMP handles base-station identification and configuration
messages for all M-ES stations, and provides information that the M-ES can use to determine
usable CDPD channels without knowledge of the history of channel usage. The RRMP also
handles channel hopping commands, cell handoffs, and M-ES change of power commands.
CDPD version 1.0 uses the X.25 wide area network (WAN) subpro-file and frame relay
capabilities for internal sub networks.
Common Channel Signaling (CCS)
Common channel signaling (CCS) is a digital communications technique that provides
simultaneous transmission of user data, signaling data, and other related traffic throughout a
network. This is accomplished by using out-of-band signaling channels which logically separate
the net-work data from the user information (voice or data) on the same channel. For second
generation wireless communications systems, CCS is used to pass user data and
control/supervisory signals between the subscriber and the base station, between the base station
and the MSC, and between MSCs. Even though the concept of CCS implies dedicated, parallel
channels, it is implemented in a TDM format for serial data transmissions.
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Before the introduction of CCS in the 1980s, signaling traffic between the MSC and a sub-scriber
was carried in the same band as the end-user’s audio. The network control data passed between
MSCs in the PSTN was also carried in-band, requiring that network information be carried within
the same channel as the subscriber’s voice traffic throughout the PSTN. This technique, called inband signaling, reduced the capacity of the PSTN, since the network signaling data rates were
greatly constrained by the limitations of the carried voice channels, and the PSTN was forced to
sequentially (not simultaneously) handle signaling and user data for each call. CCS is an out-ofband signaling technique which allows much faster communications between two nodes within
the PSTN. Instead of being constrained to signaling data rates which are on the order of audio
frequencies, CCS supports signaling data rates from 56 kbps to many megabits per second. Thus,
network signaling data is carried in a seemingly parallel, out-of-band, signaling channel while
only user data is carried on the PSTN. CCS provides a substantial increase in the number of users
which are served by trunked PSTN lines, but requires that a dedicated portion of the trunk time be
used to provide a signaling channel used for network traffic.
In first generation cellular systems, the SS7 family of protocols, as defined by the Integrated
System Digital Network (ISDN) are used to provide CCS. Since network signaling traffic is
bursty and of short duration, the signaling channel may be operated in a connectionless fashion
where packet data transfer techniques are efficiently used. CCS generally uses variable length
packet sizes and a layered protocol structure. The expense of a parallel signaling channel is minor
compared to the capacity improvement offered by CCS throughout the PSTN, and often the same
physical network connection (i.e., a fiber optic cable) carries both the user traffic and the network
signaling data.
The Distributed Central Switching Office for CCS
As more users subscribe to wireless services, backbone networks that link MSCs together will
rely more heavily on network signaling to preserve message integrity, to provide end-to-end
connectivity for each mobile user, and to maintain a robust network that can recover from
failures.CCS forms the foundation of network control and management functions in second and
third generation networks. Out-of-band signaling networks which connect MSCs throughout the
world enable the entire wireless network to update and keep track of specific mobile users,
wherever they happen to be. Figure illustrates how an MSC is connected to both the PSTN and
the signaling network.
The CCS network architecture is composed of geographically distributed central switching
offices; each with embedded switching end points (SEPs), signaling transfer points (STPs), a
service management system (SMS), and a database service management system (DBAS) The
MSC provides subscriber access to the PSTN via the SEP. The SEP implements a storedWorld Institute Of Technology
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program-control switching system known as the service control point (SCP) that uses CCS to set
up calls and to access a network database. The SCP instructs the SEP to create billing records
based on the call information recorded by the SCP.
Network A
Network B
The STP controls the switching of messages between nodes in the CCS network. For higher
reliability of transmission (redundancy), SEPs are required to be connected to the SS7 network
(described in Section 10.8) via at least two STPs. This combination of two STPs in parallel is
known as a mated pair, and provides connectivity to the network in the event one STP fails. The
SMS contains all subscriber records, and also houses toll-free databases which may be accessed
by the subscribers. The DBAS is the administrative database that maintains service records and
investigates fraud throughout the network. The SMS and DBAS work in tandem to provide a
wide range of customer and network provider services, based on SS7.
In telephony, Common Channel Signaling (CCS), in the US also Common Channel Interoffice
Signaling (CCIS), is the transmission of signaling information (control information) on a separate
channel from the data, and, more specifically, where that signaling channel controls multiple data
channels.
For example, in the public switched telephone network (PSTN) one channel of a communications
link is typically used for the sole purpose of carrying signaling for establishment and tear
down of telephone calls. The remaining channels are used entirely for the transmission of voice
data. In most cases, a single 64kbit/s channel is sufficient to handle the call setup and call cleardown traffic for numerous voice and data channels.
The logical alternative to CCS is Channel Associated Signaling (CAS), in which each bearer
channel has a signaling channel dedicated to it.
CCS offers the following advantages over CAS, in the context of the PSTN:
Faster call set-up time
No falsing interference between signaling tones by network and speech frequencies
Greater trunking efficiency due to the quicker set up and clear down, thereby reducing traffic on
the network
No security issues related to the use of in-band signaling with CAS
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Can transfer additional information along with the signaling traffic, providing features such
as caller ID
The most common CCS signaling methods in use today are Integrated Services Digital
Network (ISDN) and Signaling System 7 (SS7).
ISDN signaling is used primarily on trunks connecting end-user private branch exchange (PBX)
systems to a central office. SS7 is primarily used within the PSTN. The two signaling methods are
very similar since they share a common heritage and in some cases, the same signaling messages
are transmitted in both ISDN and SS7.
CCS is distinct from in-band or out-of-band signaling, which are to the data band what CCS and
CAS are to the channel.
Integrated Services Digital Network (ISDN)
Integrated Services Digital Network (ISDN) is a complete network framework designed around
the concept of common channel signaling. While telephone users throughout the world rely on the
PSTN to carry conventional voice traffic, new end-user data and signaling services can be
provided with a parallel, dedicated signaling network. ISDN defines the dedicated signaling network that has been created to complement the PSTN for more flexible and efficient network
access and signaling and may be thought of as a parallel world-wide network for sig-naling
traffic that can be used to either route voice traffic on the PSTN or to provide new data services
between network nodes and the end-users.
ISDN provides two distinct kinds of signaling components to end-users in a telecommunications
network. BRI provides two 64 kbps bearer channels and one 16 kbps signaling channel (2B+D),
whereas the PRI provides twenty-three 64 kbps bearer channels and one 64 kbps signaling
channel
ATM is a packet switching and multiplexing technique which has been specifically designed to
handle both voice users and packet data users in a single physical channel. ATM data rates vary
from low traffic rates (64 kbps) over twisted pair to over 100 Mbps over fiber optic cables for
high traffic rates between network nodes. ATM supports bidirectional transfer of data packets of
fixed length between two end points, while preserving the order of transmission.
ATM data units, called cells, are routed based on header information in each unit (called a label)
that identifies the cell as belonging to a specific ATM virtual connection. The label is determined
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upon virtual connection of a user, and remains the same throughout the transmission for a
particular connection. The ATM header also includes data for congestion control, priority
information for queuing of packets, and a priority which indicates which ATM packets can be
dropped in case of congestion in the network.
ATM cells (packets) have a fixed length of 53bytes, consisting of 48 bytes of data and 5 bytes of
header information. Fixed length packets result in simple implementation of fast packet switches,
since packets arrive synchronously at the switch [Ber92]. A compromise was made in selecting
the length of ATM cells to accommodate-date both voice and data users.
Signaling System No. 7 (SS7)
The SS7 signaling protocol is widely used for common channel signaling between interconnected
networks (see Figure, for example). SS7 is used to interconnect most of the cellular MSCs
throughout the US, and is the key factor in enabling autonomous registration and auto-mated
roaming in first generation cellular systems.
ATM is a packet switching and multiplexing technique which has been specifically designed to
handle both voice users and packet data users in a single physical channel. ATM data rates vary
from low traffic rates (64 kbps) over twisted pair to over 100 Mbps over fiber optic cables for
high traffic rates between network nodes. ATM supports bidirectional transfer of data packets of
fixed length between two end points, while preserving the order of transmission.ATM data units,
called cells, are routed based on header information in each unit (called a label) that identifies the
cell as belonging to a specific ATM virtual connection. The label is determined upon virtual
connection of a user, and remains the same throughout the transmission for a particular
connection. The ATM header also includes data for congestion control, priority information for
queuing of packets, and a priority which indicates which ATM packets can be dropped in case of
congestion in the network.
Integrated Services Digital Network (ISDN) is a set of communications standards for
simultaneous digital transmission of voice, video, data, and other network services over the
traditional circuits of the public switched telephone network. It was first defined in 1988 in
the CCITT red book.[1] Prior to ISDN, the telephone system was viewed as a way to transport
voice, with some special services available for data. The key feature of ISDN is that it integrates
speech and data on the same lines, adding features that were not available in the classic telephone
system. There are several kinds of access interfaces to ISDN defined as Basic (BRI), Primary Rate
Interface (PRI) and Broadband ISDN (B-ISDN).
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ISDN is a circuit-switched telephone network system, which also provides access to packet
switched networks, designed to allow digital transmission of voice and data over
ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone
can provide. It offers circuit-switched connections (for either voice or data), and packet-switched
connections (for data), in increments of 64 kilobit/s. A major market application for ISDN in
some countries is Internet access, where ISDN typically provides a maximum of 128 kbit/s in
both upstream and downstream directions. Channel bonding can achieve a greater data rate;
typically the ISDN B-channels of 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.
ISDN should not be mistaken for its use with a specific protocol, whereby ISDN is employed as
the network, data-link and physical layers in the context of the OSI model. In a broad sense ISDN
can be considered a suite of digital services existing on layers 1, 2, and 3 of the OSI model. ISDN
is designed to provide access to voice and data services simultaneously.
However, common use reduced ISDN to be limited to Q.931 and related protocols, which are a
set of protocols for establishing and breaking circuit switched connections, and for
advanced calling features for the user. They were introduced in 1986.
In a videoconference, ISDN provides simultaneous voice, video, and text transmission between
individual desktop videoconferencing systems and group (room) videoconferencing systems.
Advanced intelligent networks
The Advanced Intelligent Network (AIN) is a telephone network architecture that separates
service logic from switching equipment, allowing new services to be added without having to
redesign switches to support new services. It encourages competition among service providers
since it makes it easier for a provider to add services and it offers customers more service choices.
Developed by Bell Communications Research, AIN is recognized as an industry standard in
North America. Its initial version, AIN Release 1, is considered a model toward which services
will evolve. Meanwhile, evolutionary subsets of AIN Release 1 have been developed. These are
shown in the AIN Release Table below. Elsewhere, the International Telecommunications Union
(see ITU-T), endorsing the concepts of AIN, developed an equivalent version of AIN called
Capability Set 1 (CS-1). It comes in evolutionary subsets called the Core INAP capabilities.
The switch - known as the Service Switching Point (SSP) - forwards the call over a Signaling
System 7 (SS7) network to a Service Control Point (SCP) where the service logic is located.
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The Service Control Point identifies the service requested from part of the number that was dialed
and returns information about how to handle the call to the Service Switching Point. Examples of
services that the SCP might provide include area number calling service, disaster recovery
service, do not disturb service, and 5-digit extension dialing service.
In some cases, the call can be handled more quickly by an Intelligent Peripheral (IP) that is
attached to the Service Switching Point over a high-speed connection. For example, a customized
voice announcement can be delivered in response to the dialed number or a voice call can be
analyzed and recognized.
In addition, an "adjunct" facility can be added directly to the Service Switching Point for highspeed connection to additional, undefined services.
One of the services that AIN makes possible is Local Number Portability (LNP).
The AIN Release Table
(Some terms in this table are not yet defined on whatis.com.)
AIN
Capabilities
Release
Release 0 Trigger checkpoints at off-hook, digit collection and analysis, and
routing
points
of
call
Code gapping to check for overload conditions at SCP
75
announcements
at
the
switching
system
Based on ANSI TCAP issue 1
Release Adds a formal call model that distinguishes the originating half of
0.1
the
call
from
the
terminating
half
Additional
triggers
254
announcements
at
the
switching
system
Based on ANSI TCAP issue 2
Release Adds Phase 2 Personal Communication Service (PCS) support
0.2
Voice
Activated
Dialing
(VAD)
ISDN-based
SSP-IP
interface
Busy
and
no-answer
triggers
Next
events
list
processing
at
SCP
Default routing
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Release 1 A full set of capabilities
UNIT 7
INTELLIGENT CELL CONCEPT AND APPLICATION
INTELLIGENT CELL CONCEPT
In the cellular industry, system capacity is a great issue. As demand for cellular service grows,
system operators try to find ways to increase system capacity. Capacity can be increased by
reducing the cell sizes. This is called the conventional microcell approach, but it does not provide
intelligence. When the cell size becomes smaller, the control of interference among the cells
becomes harder. Also, the handoff time from the beginning of the initiation to the action
completion sometimes may take around 15 s. If a mobile station is moving at a speed of 25 km/h
(7 m/s), then the mobile station will travel 105 m in 15 s; at a speed of 50 km/h, the mobile station
travels 205 m in 15 s. Because within a microcell of 0.5-km radius the overlapped region for a
handoff is very small, then the mobile station is in the overlapped region too short a time for the
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handoff action to be complete. As a result, the call drops. In a conventional microcell system,
interference is hard to control and the handoffs may not have enough time to complete.
The intelligent cell can solve the two problems. The intelligent cell concept can be used not only
in microcells but also in regular cells to bring extra capacity to the system.
There are two definitions to describe an intelligent cell. One definition of intelligent cell is that
the cell is able to intelligently monitor...
In the cellular industry, system capacity is a great issue. As demand for cellular service grows,
system operators try to find ways to increase system capacity. Capacity can be increased by
reducing the cell sizes. This is called the conventional microcell approach, but it does not provide
intelligence. When the cell size becomes smaller, the control of interference among the cells
becomes harder. Also, the handoff time from the beginning of the initiation to the action
completion sometimes may take around 15 s. If a mobile station is moving at a speed of 25 km/h
(7 m/s), then the mobile station will travel 105 m in 15 s; at a speed of 50 km/h, the mobile station
travels 205 m in 15 s. Because within a microcell of 0.5-km radius the overlapped region for a
handoff is very small, then the mobile station is in the overlapped region too short a time for the
handoff action to be complete. As a result, the call drops. In a conventional microcell system,
interference is hard to control and the handoffs may not have enough time to complete.
The intelligent cell can solve the two problems. The intelligent cell concept can be used not only
in microcells but also in regular cells to bring extra capacity to the system.
There are two definitions to describe an intelligent cell. One definition of intelligent cell is that
the cell is able to intelligently monitor where the mobile unit or portable unit is and find a way to
deliver confined power to that mobile unit. The other definition of intelligent cell is that signals
coexist comfortably and indestructibly with the interference in the cell. From the first definition,
the intelligent cell is called the power-delivery intelligent cell, and from the second definition, it is
called the processing-gain intelligent cell. The intelligent cell may be a large cell such as a
microcell or a small cell such as a microcell. The intelligent cell increases capacity and improves
performance of voice and data transmission. Because personal communication service (PCS)
needs vast capacity and high quality, the intelligent cell concept is well-suited to it. Actually,
using any means intelligently in a cell to improve the performance of services is what the
intelligent many different wireless versions of an intelligent cell can be used as long as they can
deliver power to the location of the mobile unit. The easiest explanation is the analogy of a person
entering a house in a conventional microcell or microcell, when a mobile unit t cell stands for
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Enters a cell or a sector, the cell site will cover the power to the entire cell or sector. This is
because the cell site does not know where the mobile unit is within the cell or sector. This is just
like a house that turns on all the lights when a person enters it.
Applications of intelligent micro-cell Systems
In the cellular industry, system capacity is a great issue. As demand for cellular service grows,
system operators try to find ways to increase system capacity. Capacity can be increased by
reducing the cell sizes. This is called the conventional microcell approach, but it does not provide
intelligence. When the cell size becomes smaller, the control of interference among the cells
becomes harder. Also, the handoff time from the beginning of the initiation to the action
completion sometimes may take around 15 s. If a mobile station is moving at a speed of 25 km/h
(7 m/s), then the mobile station will travel 105 m in 15 s; at a speed of 50 km/h, the mobile station
travels 205 m in 15 s. Because within a microcell of 0.5-km radius the overlapped region for a
handoff is very small, then the mobile station is in the overlapped region too short a time for the
handoff action to be complete. As a result, the call drops. In a conventional microcell system,
interference is hard to control and the handoffs may not have enough time to complete.
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The intelligent cell can solve the two problems. The intelligent cell concept can be used not only
in microcells but also in regular cells to bring extra capacity to the system. There are two
definitions to describe an intelligent cell. One definition of intelligent cell is that the cell is able to
intelligently monitor...
Advanced intelligent networks have become a buzz word in telecommunications today. Each
carrier has its own version and interpretation of these smart networks, but virtually all use SS7 to
implement their AIN. While each version may differ, the concept of AIN is the same. Before calls
are sent to their final destination, the network queries a database asking, "What should I do with
this phone call?" The response determines how the call is handled. This is a tremendous saving of
resources, since the call does not have to be attempted if the remote end is busy. In addition, if
offers all kinds of advanced features, such as distinctive ring, caller ID, 900 number blocking,
enhanced Toll-Free features, etc.
Advanced intelligent networks operate over SS7 (Signaling System 7). SS7 is an industry
standard for transmitting signaling information in a switched network. It is designed to efficiently
transfer information between network Signaling Points and interconnected networks.
SS7 utilizes out-of-band signaling to improve call processing setup times. Signaling information
is sent ahead of a call, over a separate channel, to establish and control network connections. This
set-up information includes supervisory signals (answer, non-answer), billing information (who
called whom and for how long) and network management signals such as maintenance test signals
and routing information.
In-Building Communication
In an intelligent macro cell or microcell, when a mobile unit enters a cell or a sector, the cell site
covers only a local area, which follows the mobile unit. This is just like a house that turns on only
the light of the first room a person enters. When the person enters the second room, the light of
the first room is turned off and the light of the second room is turned on. Therefore, the light of
only one room is on at a time and not the lights in the whole house. When the lights of the entire
house A and the lights of the entire house B are on, the two houses should be largely separated in
order to avoid the light being seen from one house to the other. If the light of only one room of
house A and house B is on, the light that can be seen from one house to the other house is
relatively weak. Thus, the distance between the two houses can be much closer.
This same analogy can be applied to a cellular system. In a cellular system, the frequency reuse
scheme is implemented for the purpose of increasing spectrum efficiency. If two co channel cells
(cells that use the same frequency) can be placed much closer, then the same frequency channel
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can be used more frequently in a given geographical area. Thus, the finite number of frequency
channels can provide many more traffic channels, and both system capacity and spectrum
efficiency can be further increased. In order to reduce the separation between two co channel
cells, the power of each cell should be reduced to cover merely one of numerous local areas in a
cell if the cell operator is intelligent enough to know in which local area the mobile unit or
handset is. Therefore, there are two required conditions:
1. The cell operator has to know where the mobile unit is located. Different resolution methods
can be used to locate the mobile unit.
2. The cell operator has to be able to deliver power to that mobile unit. If the power transmitted
from the cell site to the mobile unit can be confined in a small area (analogous to the light of a
small room turning on when a person enters it), co channel interference reduces, and the system
capacity increases.
In the cellular industry, system capacity is a great issue. As demand for cellular service grows,
system operators try to find ways to increase system capacity. Capacity can be increased by
reducing the cell sizes. But it does not provide intelligence, because when the cell size becomes
smaller, the control of interference among the cells becomes harder. The intelligent cell is able to
intelligently monitor where the mobile unit or portable unit is and find a way to deliver confined
power to that mobile unit. The intelligent cell increases capacity and improves performance of
voice and data transmission. Because personal communication service (pcs) needs vast capacity
and high quality, the intelligent cell concept is well-suited to it. Actually, using any means
intelligently in a cell to improve the performance of services is what the intelligent cell stands for.
CDMA cellular Radio Networks
A cellular network is a radio network distributed over land areas called cells, each served by at
least one fixed-location transceiver known as a cell site or base station. When joined together
these cells provide radio coverage over a wide geographic area. This enables a large number of
portable transceivers (e.g., mobile phones, pagers, etc.) to communicate with each other and with
fixed transceivers and telephones anywhere in the network, via base stations, even if some of the
transceivers are moving through more than one cell during transmission.
Cellular networks offer a number of advantages over alternative solutions:
Increased capacity
Reduced power use
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Larger coverage area
Reduced interference from other signals
In a cellular radio system, a land area to be supplied with radio service is divided into regular
shaped cells, which can be hexagonal, square, circular or some other irregular shapes, although
hexagonal cells are conventional. Each of these cells is assigned multiple frequencies (f1 - f6)
which have corresponding radio base stations. The group of frequencies can be reused in other
cells, provided that the same frequencies are not reused in adjacent neighboring cells as that
would cause co-channel interference.
The increased capacity in a cellular network, compared with a network with a single transmitter,
comes from the fact that the same radio frequency can be reused in a different area for a
completely different transmission. If there is a single plain transmitter, only one transmission can
be used on any given frequency. Unfortunately, there is inevitably some level of interference from
the signal from the other cells which use the same frequency. This means that, in a standard
FDMA system, there must be at least a one cell gap between cells which reuse the same
frequency.
In the simple case of the taxi company, each radio had a manually operated channel selector knob
to tune to different frequencies. As the drivers moved around, they would change from channel to
channel. The drivers know which frequency covers approximately what area. When they do not
receive a signal from the transmitter, they will try other channels until they find one that works.
The taxi drivers only speak one at a time, when invited by the base station operator.
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