/smash/get/diva2:830407/FULLTEXT01.pdf

/smash/get/diva2:830407/FULLTEXT01.pdf
MEE09:84
ANALYSIS AND PLANNING MICROWAVE LINK
TO ESTABLISHED EFFICIENT WIRELESS COMMUNICATIONS
MD. RAKIB AL MAHMUD
ZAIGHAM SHABBIR KHAN
This Thesis is presented as part of Degree of
Masters of Science in Electrical Engineering
Blekinge Institute of Technology
September 2009
Blekinge Institute of Technology
School of Engineering
Department of Signal Processing
Examiner: Dr. Jorgen Nordberg
ABSTRACT
Wireless communication is observing a fast development in today’s communication
era. In mobile communication the Base Transceiver Station (BTS) to Base Station
Controller (BSC) or Mobile Switching Centre (MSC) link is based on microwave link.
Therefore, analysis and planning of a microwave link is very much important. The
microwave equipment can be installed after a careful planning and detailed analysis
a microwave radio system. A poorly designed path can result in periodic system
outages, resulting in increased system latency, decreased throughput, or worst case,
a complete failure of the system.
Planning a good, stable and reliable microwave network can be quite
challenging. At the same time, it poses several interesting optimization problems.
The theme of thesis work an iterative technique has been presented to explain the
sequential communication of signal transmission for long and short distance radio
communication through microwave link with better efficiency.
ACKNOWLEDGEMENTS
All praises are to almighty Allah for letting us complete this thesis successfully.
For us this thesis assignment is more than a Masters Degree requirement as our
understanding was appreciably improved throughout the course of its research
work. We are particularly gratified to the Faculty and Staff members of Blekinge
Institute of Technology (BTH), School of Engineering, Karlskrona, Sweden, who
have always been a source of inspiration for us and shored up us enormously
during this thesis work.
Especially we would like to express sincere appreciation to Professor Jorgen
Nordberg for his assistance in the preparation of this manuscript and the work it
represents.
We would also like to honor our parents for delightful support and motivation for
the entire time and allowing us study in the Electrical Engineering department of
Blekinge Institute of Technology (BTH), Karlskrona, Sweden and we would like to
thanks our friends as well for knowledgeable experience of studying together.
2
TABLE OF CONTENTS
List of Figures……………………………………………………………………………...6
List of Tables……………………………………………………………………………….7
Chapter 1: Introduction to Microwaves….……………………………………………..8
1.1
Microwave/Radio Frequency Wave…………………………………………….8
1.2
Wireless Communication…………………………………………………………8
1.3
Historical Background…………………………………………………………….9
1.4
Radio Frequency / Microwave Applications………………………………….10
1.4.1 Communication…………………………………………………………………...10
1.4.2 Television and Radio Broadcasting.…………………………………………….10
1.4.3 Optical Communications…………………………………………………………10
1.4.4 Radar & Navigation……………………………………………………………….10
1.4.5 Remote sensing…………………………………………………………………….11
1.4.6 Domestic and Industrial Application……………………………………………11
1.4.7 Medical applications & Surveillance…………………………………………….11
1.4.8 Astronomy & Space Exploration…………………………………………………11
1.5
Wireless Application & Mobile Cellular Networks……………………………11
1.5.1 Last Mile Access……………………………………………………………………12
1.5.2 Private Networks & Developing Nations………………………………………..12
1.5.3 Disaster Recovery…………………………………………………………………..12
1.5.4 Control and Monitoring…………………………………………………………...13
1.6
Microwave Advantages over cable/fiber based Transmission……………….13
1.7
Microwave Disadvantages………………………………………………………..14
1.8
Objective of this Thesis Work.……………………………………………………14
Chapter 2: Wireless Communication……………………………………………………15
2.1
Communication System’s Basic Structure………………………………………15
2.2
Some Important Terms Used In Communication Systems……………………17
2.2.1 Multiplexing………………………………………………………………………..17
2.2.2 Multiple Accesses………………………………………………………………….17
2.3
Forms of Communication………………………………………………………...18
2.3.1 Point to Point Communication…………………………………………………...18
2.3.2 Point to Multipoint Communication…………………………………………….18
2.3.3 Broadcasting………………………………………………………………………..18
2.3.4 Simplex……………………………………………………………………………...18
3
2.3.5 Half Duplex……………………………………………………………………….19
2.3.6 Full Duplex………………………………………………………………………..19
2.4
Transmission Impairments……………………………………………………....19
2.4.1 Fading………………………………………………………………………………19
2.4.1.1 Rayleigh Fading………………………………………………………………20
2.4.1.2 Ricean Fading…………………………………………………………………20
2.4.1.3 Frequency Selective Fading…………………………………………………20
2.4.1.4 Slow Fading…………………………………………………………………..20
2.4.1.5 Fast Fading……………………………………………………………………20
2.4.1.6 Flat Fading……………………………………………………………………20
2.4.2 Noise……………………………………………………………………………….20
2.4.2.1 AWGN Noise…………………………………………………………………21
2.4.2.2 Inter Symbol Interference (ISI)……………………………………………...21
2.4.2.3 Impulse Noise………………………………………………………………...21
2.4.2.4 Intermodulation………………………………………………………………21
2.4.2.5 Cross Talk……………………………………………………………………..21
2.4.2.6 Thermal Noise………………………………………………………………..21
2.4.3 Delay Distortion…………………………………………………………………..22
2.4.3.1 Scattering……………………………………………………………………..22
2.4.3.2 Reflection……………………………………………………………………..22
2.4.3.3 Diffraction…………………………………………………………………….22
2.4.4 Attenuation………………………………………………………………………..23
2.4.5 Doppler Shift……………………………………………………………………...23
Chapter 3: Microwave Communication & Considerable Parameters……….…….24
3.1
Microwave Communication……………………………………….……………24
3.2
Advantages of Microwave Communication over Fiber Optic………………24
3.3
Considerable Parameters of Microwave……………………………………….25
3.3.1 Microwave Antenna……………………………………………………………..25
3.3.2 The Isotropic Antenna…………………………………………………………...26
3.3.3 Parameters of Antenna………………………………………………………….26
3.3.3.1 Input Impedance…………………………………………………………….26
3.3.3.2 Radiation Pattern……………………………………………………………26
3.3.3.3 Directivity……………………………………………………………………27
3.3.3.4 Polarization………………………………………………………………….27
3.3.3.5 Gain…………………………………………………………………………..27
3.3.3.6 Efficiency…………………………………………………………………….27
4
3.4
Path Loss………………………………………………………………………….29
3.4.1 Path Loss and Distance Calculation……………………………………………30
3.4.1.1 Free Space Pathloss Model (FSPL)…………………………………………30
3.4.1.2 CCIR Path Loss Model (Lccir)……………………………………………...31
3.4.1.3 Hata Path Loss Models (Lhata)……………………………………………..32
3.4.2 Use of Path Loss Model………………………………………………………….33
3.5
Link Budget……………………………………………………………………….35
3.5.1 Calculation of Link Budget……………………………………………………...34
3.5.1.1 Free Space Loss for Link Budget…………………………………………...35
Chapter 4: Adaptive Modulation, Modeling & Simulation……………….………..36
4.1
Adaptive Modulation and Coding Scheme……………………………………36
4.2
Adaptive Modulation in Microwave Link……….……………………………37
4.3
Key Benefits………………………………………….…………………………...37
4.4
Adaptive Modulation …………………………………………………………..38
4.5
Theoretical Performances of Adaptive Modulation ….……………………..40
4.6
Proposed Adaptive Modulation Model……………………………………….43
4.6.1 Performance of Adaptive Modulation………………………………………...44
Chapter 5: Conclusion……………………………….…………………………………..47
5.1
There are several Advantages of microwave radio…………………………..47
5.2
Basic Recommendations…………………………………………………………47
Appendices………………………………………………………………………………...49
A
Abbreviation and Acronyms……………………………………………………50
References………………………………………………………………………………….52
5
LIST OF FIGURES
2.1
Basic Structure of Communication System……………………………………15
2.2
Communication System…………………………………………………………16
2.3
Sketch of three important Propagation Mechanism………………………….22
3.1
Typical RF Transmission System ………………………………………………28
3.2
Physical Environment Path Loss Variables …………………………………..29
3.3
Calculated Path Loss in different Models……………………………………..33
3.4
Link Budget………………………………………………………………………34
4.1
Modulation Schemes for different SNR levels………………………………..39
4.2
SNR versus Spectral Efficiency…………………………………………………41
4.3
BER Performance versus SNR for a Fading Channel………………………...42
4.4
Block Diagram of Simulation Model…………………………………………..43
4.5
SNR vs BER Probability for AWGN Channel…………………………………44
4.6
SNR vs. BER probability for Fading Channel…………………………………45
4.7
SNR vs. Bandwidth Efficiency for Adaptive Modulation……………………46
6
LIST OF TABLES
3.1
Description of Equation……………………………………………………….32
3.2
Calculated Distance Value for Common Example…………………………33
4.1
Modulation Schemes Decision Levels and Bits/Symbol…………………..39
7
Chapter 1
Introduction to Microwaves
T
oday wireless technology is used in many applications well integrated into our
everyday life. Planning a good, stable and reliable microwave network can be
quite challenging.
Careful planning and detailed analysis is required for a
microwave radio system before the equipment can be installed. A poorly designed
path can result in periodic system outages, resulting in increased system latency,
decreased throughput, or worst case, a complete failure of the system.
1.1
Microwave/Radio Frequency Wave
The term microwave refers to alternate current signals with frequencies between 300
MHz and 300 GHz with a corresponding electrical wavelength between λ = c/f = 1
m and λ = 1 mm respectively. Signals with wavelengths on the order of millimeters
are called millimeters waves. The relation between the frequency f and wavelength λ
being f λ = c, where c is velocity of propagation of the radio wave, which is equal to
that of light waves in frees pace 3x108 m/sec.
Any frequency within the electromagnetic spectrum associated with radio wave
propagation is referred as Radio Frequency (RF). When an RF current is supplied to
an antenna, an electromagnetic field is created that then is able to propagate through
space. Many wireless technologies are based on RF field propagation.
1.2
Wireless Communication
The basic motto of communication system is to ensure the exchange of information
in between the people. When this communications without wired then it’s refereed
to wireless communications. Now a day this wireless communications gets more
attention from the Communication industry and provide better quality information
transfer between portable devices.
Autonomous sensor networks, Multimedia,
Videoconferencing, Distance learning and Internet enabled cell phone are the
Valuable Applications of this technology.
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1.3
Historical Background
“In 1888 historically microwave signal was first introduced by Henrich Hertz at 66 cm
wavelength (454.5 MHz) while millimeter wave signal was first generated by Sir J. C. Bose
in 1895 at 5 mm wavelength (60 GHz). Subsequently in 1890s Sir J.C. Bose also generated
microwave signal at wavelengths up to 2.5 cm (12 GHz) wavelength. Besides Sir J. C. Bose
also developed the world’s first solid state point contact detector working at millimeter wave,
infrared and optical wavelengths use Galena crystal as the detector material. Bose also
developed world’s first horn antenna and waveguide radiator for microwave and millimeter
wave bands.
In 1899 Sir J. C. Bose developed a highly sensitive iron-mercury detector in which a U-tube,
made of glass, filled up with mercury, was used for a fine control of mercury contact pressure
to optimize the sensitivity of the detector. Subsequently in 1901 Marconi employed Sir J. C.
Bose’s technique of the highly sensitive iron-mercury detector.
Sir J. C. Bose also worked for the first time on the response of living and nonliving materials
to microwave and millimeter wave bands, a subject which has now grown to a high level for
the studies of microwave hazards to living beings animals and vegetation and also for
microwaves diathermy therapy of tumor and cancer. From the above history of Science about
Sir J. C. Bose who worked pioneering on a wide range of fields in Radio Science it seems that
Sir J. C. Bose may be called the father of Radio Science.
Investigations of the millimeter waves were continued and stimulated by scientific and
military developments. In 1930 the observation by L. A. Hyland of the Naval Research
Laboratory (NRL) about the reflection of radio signals from over flying aircrafts, stimulated
the investigation of radar techniques for detecting flying aircrafts in NRL.
During World War 2 J. H. Van Vleck predicated theoretically the Oxygen absorption band at
60 GHz. In 1950 Hughes Aircraft Company successfully extended the frequency range of
coupled cavity traveling wave tubes to millimeters waves.
In1960 intensive work in millimeter wave technology was done at BTL on the development of
solid state components required in the development of underground repeaters using low loss
over modes waveguides which was also first developed by Sir J. C. Bose.
9
In 1970s, Hughes began manufacturer a solid state sweep generator which apparently
triggered a chain of development in Hughes in the Millimeter wave Technology area of a full
range of device” [1].
1.4
Radio Frequency / Microwave Applications
The main part of radio frequency microwave signals can be classified in following
terms.
1.4.1 Communication
The communication part concludes satellite and space systems, extensive distances,
wired telephone, naval, mobiles telephone, airbus, roads vehicle, personal, and
WLAN other than that there are also two significant subcategories of
communications should be considered which are optical communication and
television and radio broadcasting.
1.4.2 Television and Radio Broadcasting
In this category of communication radio frequency or microwaves are utilized as the
carriers for audio and video signals. Direct Broadcast System (DBS) is an example
which is deliberately designed to connect or link satellites systems directly to home
users.
1.4.3 Optical Communications
In optical communication microwave modulator is used in the broadcasting part of a
low pass optical fiber and microwave demodulator on the other side. The
microwave signal operates as a transforming signal with the carrier optical signal. In
case of larger frequency channels, optical communication is useful.
1.4.4 Radar and Navigation
This part comprises of air defense, airbus, ships direction, elegant weapons, police
and weather and collision avoidance. The navigation system is used for the direction
and supervision of airbus, ship and road vehicles. The typical applications are:
1. Microwaves Landing System - MLS used to direct airbus to land securely at
airports.
10
2. Global Positioning System - GPS used to find exact positioning or spot on
the globe.
1.4.5 Remote Sensing
In remote sensing many satellite systems monitor globe consistently for weather
situations meteorology, ozone layer, soil, agriculture, crop protection from frost,
forests, thickness of snow, sea icebergs and other parts such as examining and
discovery of resources.
1.4.6 Domestic and Industrial Application
This area deals with microwave ovens, clothes dryers (microwaves), liquefied
heating systems, humidity sensors, tank gauges, automatic doors opener, automatic
toll tax detection, control and monitoring of motorway traffic, chip fault recognition,
power transmission, food protection, bug control etc.
1.4.7 Medical applications and Surveillance
This deals with breeding of cats, heart functional reaction, bleeding control,
sterilization. On the other hand surveillance concludes security alarm systems,
burglar detection and Electronics Warfare (EW) receivers for monitoring of traffic
signals.
1.4.8 Astronomy and Space Exploration
The astronomy and space deals with enormous dish antennas which are used for
monitoring, collecting and record incoming microwave signals from external space,
giving critical information about other planets, stars, other objects and galaxies.
1.5
Wireless Application and Mobile Cellular Networks
The smaller distance communication in and between buildings in a local area
network (LAN) can be accomplished using RF and microwaves. Connecting
buildings via cables creates serious problems in congested metropolitan areas
because the cable has to be run underground from the upper floors [2]. Cellular
companies frequently use to get hard schedules to make sure of services for clients
to produce abrupt profits. In order to revolve up their networks, companies require
11
replacing the data of cell sites to their mobile switching stations. They must have
taken microwave due to its consistency, speed of employment and cost payback
over fiber or leased wired line. Microwave radio is deeply deployed in the rising
2.5G and 3G mobile infrastructures to maintain data handling. The larger numbers
of cell sites required to support a new generation mobiles.
1.5.1 Last Mile Access
A considerable section of business grounds require broadband connectivity.
Wireless networks give the ideal means for connecting new clients to defeat the last
mile bottleneck. If an operator prefers to use non-licensed or multi-point wireless
tools to fix customers, even then high ability microwave offers the perfect way out
for backhaul of client traffic from contact hubs to the nearby fiber point.
1.5.2 Private Networks and Developing Nations
These days’ companies may have high speed LAN and WAN network necessities.
They need to unite elements of their industry on same ground, city or country.
However, microwave radio communication is capable to offer fast, high capacity
links which are compatible with quick and gigabit Ethernet connections, enabling
LANs to be comprehensive without dependence on fiber.
There are various countries which resist with distantly telephone
communications and require upgrading their systems to the most recent digital
technology. The microwave radio has habitually permitted developing nations the
way of setting up telecommunications rapidly over usual immature and unusable
land such as desert, forest and frozen land where spreading cable would be all but
impracticable. The modern digital microwave radio systems shape the foundation
for several worldwide public networks.
1.5.3 Disaster Recovery
In case of natural disasters, it usually shatter pre established network. Thus,
microwave is often used to reinstate connections when communication means i.e.
equipment has been broken by earthquakes, water floods, hurricanes or human
conflicts such as assault by terrorist or wars etc. These days microwave
communication is used widely for speedily restore infrastructure in countries like
12
Kuwait, Iraq, Serbia, Afghanistan and Kosovo, where existing communications has
been mostly destroyed or damaged.
1.5.4 Control and Monitoring
Railroads and public transport organizations are the most important users of
microwave and play an important role for these companies to control and
monitoring information and switching stations.
1.6
Microwave Advantages over cable/fiber based Transmission
Microwave radio offers a number of compelling advantages over cable/fiber based
transmission. The various advantages pertaining to the wide use of microwaves are
described as bellow:
1. Rapid Deployment – Microwave link can installed less than a day.
2. Reliability - In Radio communication MF/HF band, varies widely with
time, weather condition and giving rise to fading effect. But a microwave
frequencies, there is less fading since the propagation of microwaves from
transmitter to receiver takes place by line of sight propagation.
3. No Right-of-Way Issues – Microwave radio can over come barrier such as
railways, road and ponds as well and avoiding taking any permission to
establish the communications and introduce time delay or cost.
4. Flexibility – At minimal or even no cost microwave link capacity can easily
increased. If network needs any changes we can redeploy radios as a result of
customer demand.
5. Easily Crosses City Terrain – In many cities there enormously restricted
street digging to in install any cable/fiber in this situation Microwave radio is
the best solution.
6. Operator Owned Infrastructure - no dependence on competitors.
7. Required negligible operational costs.
8. Radio infrastructure is already acquired by many networks in existing
radio transmission towers, rooftops, and cellular masts.
9. Microwave radio can be repaired in minutes instead of hours or days
where as the cable systems takes long time to fault diagnosis and fix it.
10. In natural disasters microwave link can give better flexibility
13
1.7
Microwave Disadvantages
Microwave radio offers various disadvantages as well over different circumstances.
Following are the disadvantages:
1. Microwave engages huge capital investment in start if there is no vendor
to finance it.
2. Microwave networks required maintenance because if a healthy
planned, correctly implement network with quality equipment are not
installed by good reputed vendor then maintenance required for longer
run [3].
3. Microwave faces trouble like signals loss.
4. Microwave subject to Radio Interference from different environmental factors
such as:
¾ Thermal Inversion – a setback of the normal reduction of air
temperature above sea level [4].
¾ Passing Airplanes, birds and rain
¾ Stellar Flare and Sunspots – stellar flare or solar flare is known as big
explosion in atmosphere of Sun which affects layers. They generate
electromagnetic radiations from radio waves to gamma rays to the
entire wavelengths. Whereas sunspot is dark sports on sun occurred
by severe magnetic action which earth temperature [5].
5. Microwave required a line-of-sight because signals travel in straight lines.
6. Microwave towers, repeaters and other equipment are very much
expensive as compare to fiber optic [6].
1.8
Objective of This Thesis work
In this thesis work an iterative technique has been presented to explain the
sequential communication of signal transmission for long and short distance radio
communication through microwave link with better efficiency. The objective of our
work is to analysis different path loss model and modulation technique that can help
to build efficient microwave link to established wireless communication.
14
Chapter 2
Wireless Communication
B
efore going into the details of the Microwave link in Wireless communications
one need to understand what is meant by communication. The communication
is when information such as voice, data, image and video is transferred to one place
and received at another place with some distance. The basic aim of a communication
system is to ensure the sharing of data information among people over some
distance.
2.1
Communication System’s Basic Structure
Basically every communication system consists of a transmitter, a transmission
medium and a receiver. Fig. 2.1 shows the basic part of a communication system.
Figure 2.1 Basic Structure of Communication System
The Transmitter converts the source message into an electrical signal. The
Transmitter is basically responsible for encoding the message and then this encoded
message is multiplied by carrier frequency i.e. modulate the signal and then
transmitted over the channel. At the receive end, the receiver demodulate the
received signal and decode it and generate the original message. Minimal distortion
at the receiver end is referred as a good communication property.
15
Figure 2.2 Communication System
The transmission standard from transmitter to the receiver can be categorized as
direct medium and indirect medium. The direct medium is usually wired
communications while the indirect medium is normally relied on wireless
communication. The Public telephone, optical fiber and LAN based networks are
examples of wired transmission whereas GSM, GPRS, 3G W-CDMA/UMTS,
WiMAX, 3.9G LTE, WLAN and TV broadcasting are examples of wireless
communication.
Communication systems could also be categorized as analog and digital
communication. The existence of transmission impairments makes the difficulty in
order to reproduce the analog signals at the receiver. All analog and digital systems
transmit analog signals. These signals are poorly exaggerated by the noise or
distortion. On the other hand digital systems offer fine performance and improved
efficiency which is exempted from obligation to noise. Because of these favorable
circumstances
digital
systems
are
more
appropriate
and
popular.
The
communication system such as digital or analog may depends on modulation
scheme. This scheme is described as the distinction of the carrier waves in
provisions to the frequency, amplitude and phase in a radio signal or
electromagnetic wave which tidy to make it appropriate for communication [7].
16
2.2
Some Important Terms Used In Communication Systems
Communication systems are based on different channels communication through
which data is transferred. There are some important terms used in communication
which are:
2.2.1 Multiplexing
It is used to transmit multiple signals through a single medium. While,
demultiplexing is obtained at the receiver side to detached the multiplex
radio signals. It is an integral part of transmitter.
2.2.2 Multiple Accesses
This term of communication system allows various users to connect who
want to contribute for the similar channel. Following are the most common of
multiple access technologies:
•
Frequency Division Multiple Access - FDMA
•
In FDMA scheme, the frequency band is allocated to each single user
by separating the frequency band into further smaller bands. This
allocation of frequency band is done after guaranteeing lowest
interference among frequency bands.
Time Division Multiple Access - TDMA
•
The TDMA scheme is basically used for shared resources for wireless
networks. It uses the same frequency band for all users in different
time intervals. For utilization of complete frequency band during the
period, a separate time slot is allocated to each user. In 2nd Generation
(2G) Cellular networks TDMA channel access scheme is used and GSM
used the combination of FDMA and TDMA schemes.
Code Division Multiple Access - CDMA
One of the basic ideas of data communication is to allow transmitters
for sending data information over single communication,
simultaneously. In this way various users can share bandwidth of
different frequencies. This idea is called multiplexing. CDMA channel
scheme makes a data transmission to multiple users through assigned
code for each transmitter to permit multiple users to be multiplexed
over the same channel. Frequency Division Multiple Access separates
17
•
access by frequency whereas Time Division Multiple Access separates
access through time, while CDMA deals with both frequency and time.
Orthogonal Frequency Division Multiple Access - OFDMA
In OFDMA, the carrier signal is separated into different smaller
subsets. For a single user this separated carrier signal subset is used to
send data. To get quality of service OFDMA may be used beside
OFDM.
2.3
Forms of Communication
In today's communication era wireless communication has taken on an entirely
depth understanding. However, it can generally be categorized into following
forms:
2.3.1 Point to Point Communication
The point to point communication describes that transmission take place
among two points which are too much away from themselves. The common
understanding of point to point communication is voice call among two
communicators.
2.3.2 Point to Multipoint Communication
The point to multipoint communication describes that only one transmitter is
present at the transmitter side and various receivers are present at the
receiver side. For example video conferencing
2.3.3 Broadcasting
The Broadcasting communication describes that all users receive the
conveyed signal. In this situation, the transmitter is generally at the middle
position and it throws information to each receiver. Examples of broadcasting
are TV and radio.
2.3.4 Simplex
When information is sent only in one direction then it’s called a simplex
mode of communication.
18
2.3.5 Half Duplex
In half duplex the data is transferred in both directions but at different time
break such as from sender side to receiver side at once then from receiver part
to sender part at other time then it’s known as half duplex communications.
Examples are walkie talkie sets.
2.3.6 Full Duplex
When the information is send to both directions simultaneously such as in
half duplex then the transmitter and receiver can correspond concurrently.
Mobile Global System for Mobiles – GSM, Hands Free for Voice over IP, 3G
video networks etc are examples of full duplex communication.
2.4
Transmission Impairments
Electrical signals contains data or information with different voltage level which
represents the data information steams. If the transmission medium is perfect then
the receiver will get the exact signal which was send by transmitter during
transmission, but generally communication means are not up to the mark so, the
received signal will not be same as it transmitted. This impairment of signals leads
to errors in signal information [8, 9]. Communication lines suffer from different
problems.
1. Fading
2. Noise
3. Delay Distortion
4. Attenuation
5. Doppler shift
2.4.1
Fading
Fading is define as the noise or distortion gained by a carrier-modulated signal
during transmission over certain propagation media such as multipath fading
[10, 11]. It can be characterized as follows:
•
Rayleigh Fading
•
Ricean Fading
19
•
Frequency selective Fading
•
Slow Fading
•
Fast Fading
•
Flat Fading
2.4.1.1 Rayleigh Fading: In Rayleigh fading when there is no Line Of
Sight (LOS) path exists between transmitter and receiver which have
only indirect path than in result the received signal contains sum of all
scattered and reflected waves [9].
2.4.1.2 Ricean Fading: This type of fading present in a condition when
there exist a LOS and non LOS path between receiver and transmitter
i.e. received signal consist of scattered and direct multipath waves [9].
2.4.1.3 Frequency
Selective
Fading:
Take
place
when
signal
bandwidth and delay spread is larger than bandwidth of a channel
and symbol period respectively [9].
2.4.1.4 Slow Fading: exist in a condition when Doppler Spread
Spectrum is lower and coherence time is more than symbol period in
channel [9].
2.4.1.5 Fast Fading: It takes place in condition when Doppler Spread
Spectrum is higher and coherence time is smaller than symbol period
in channel [9].
Coherence Time: duration of time when channel impulse
response is invariant.
Symbol Time: time required to complete one symbol.
2.4.1.6 Flat Fading: Is a type of fading in which ratio of rising and
falling of all parts of the received radio signal is same [9].
2.4.2 Noise
The data which is not used to transfer or transmit signal other than source is
known as noise. It can be classified into following sub categories:
•
AWGN
•
Inter Symbol Interference
•
Impulse Noise
•
Intermodulation
20
•
Cross Talk
•
Thermal Noise
2.4.2.1 AWGN Noise: AWGN contains uniform continuous spectrum
frequency over specified frequency band which affect the signal
transmitted signal.
2.4.2.2 Inter Symbol Interference (ISI): Is a form of noise or distortion
of signal interference for one symbol with frequent symbols.
ISI is
generally caused by channel’s multipath propagation and the essential
response of non-linear frequency. ISI communication is less reliable.
2.4.2.3 Impulse Noise: Occurs due to frequent disturbance caused by
lighting and voltage spikes in equipment and result generate errors in
transmission.
2.4.2.4 Intermodulation:
Is
a
noise
occurs
due
to
non-linear
characteristics of medium, when two signals sent through the medium
with interferences and different frequencies. When these signals
interfere with each other new frequencies occurs and they create
redundant signals. These signals need to be filter out.
2.4.2.5 Cross Talk: Is a redundant noise occurred due to paths mixture
of two signals which are near to each other.
2.4.2.6 Thermal Noise: Thermal noise occurs due to the “thermal
interruption of electrons in a conductor also know as White Noise” [12]. It
can be calculated for given bandwidth using the following equation:
where:
N = Noise power in watts
k = Boltzmann’s constant. 1.3803 × 10–23 J/K
T = Temperature in Kelvin
B = Bandwidth in Hz
21
2.4.3 Delay Distortion
When different components of frequency arrive at different times, which
deform the signal’s amplitude and delay the signals at receiver end then
delay distortion occurs. Following are the circumstances or factors involve in
channel’s delay distortion:
•
Scattering
•
Reflection
•
Diffraction
Figure 2.4 Sketch of three important Propagation Mechanisms
Reflection (R), Scattering (S), Diffraction (D) [9].
2.4.3.1 Scattering - In scattering a signal can be scatter in different
direction when it hits outsized abrasive surface with same or less
length of the signal wavelength.
2.4.3.2 Reflection – when radio waves ram with flat surface which
have the larger wavelength length of radios then the waves return
backside by having the same angle at which they rammed to the
surface.
2.4.3.3 Diffraction - occurs when a signal is prevented by the rim or
edge of a dense body in which length of dense body is more as
compare to wavelength of signal.
22
2.4.4 Attenuation
Attenuation is defined as power loss of the propagated signal with respect to
time and distance. These Signals required to be strong sufficiently in a way
that receiver can distinguish and detect the required signals. There is
possibility that receiver may not be able to detect the signal at all if the
attenuation level is high. Therefore, for unswerving communication, delay
and attenuation must be stable [11]. Attenuation can be derived as signal
power Ps at transmitter and signal power Pd at receiver, then Ps > Pd. Power
attenuation Ap in dBs is:
A
10 Log
P
P
2.4.5 Doppler Shift
Various copies of same signals can be received at receiver end because of
transmission of multipath radio’s. The Doppler shift is known as when object
is moving with little speed (velocity) then there will be a shift of frequencies
in each received signals. It can be derived as:
f
v f ⁄λ cos α
where,
fd = Doppler shift frequency
v = velocity of moving object,
λ = wavelength of signal
α = angle w.r.t reference point.
23
Chapter 3
Microwave Communication and Considerable Parameters
icrowaves describes the contemporary current signals between 300 MHz to 300
MGHz
frequency ranges, microwaves have a resultant wavelength among λ =
c/f =
1m and λ = 1mm respectively. These are ideal for transmission of data from
one place to other because microwave power can infiltrate smog, rainfall, snow and
clouds.
3.1
Microwave Communication
Microwave communication broadcast signals through radio using a progression of
microwave towers. Microwave is a form of line of sight communication, because it
requires the obstruction less transmission between the receiving and transmitting
towers for signals to be communicated properly at both ends. After the successful
effort of transmitting microwave message in 1940 from New York to Philadelphia,
microwave communication is the most commonly used transmission technique for
telecommunication services era.
With the continuous growth in cellular and satellite technologies, now
microwave is less broadly used in telecom era. Communication is dominating
towards the fiber optic data transmission. However, at various remote sites where
economically it is not possible to install fiber optic cabling, microwave equipment is
still in. Data communication through microwave occurs in both analog and digital
formats. Whereas, digital format is the most advance type of microwave data
communication.
3.2
Advantages of Microwave Communication over Fiber Optic
There are many convincing advantages of microwave radio over fiber optic cabling
based transmission.
o Microwave link is possible to deploy in a day.
24
o Microwave link is flexible in the capacity that can be increase
effortlessly at negligible or even no cost. Moreover, microwave radio
link can be reinstalled depending on the customer requirement or if
network demands changes. Therefore, loosing clients does not make a
sense that assets are lost as in case of fiber optic.
o Microwave is easily crossable in terrain areas. Whereas, in various
metropolitan cities and authorities, road digging is totally banned to
deploy fiber optic or prohibited or even expensive.
o Microwave radio infrastructure is owned by operator therefore, no
dependence on competitors.
o Microwave radio infrastructure is already available for various
networks in the shape of rooftops, cellular poles and residing towers of
microwave radio transmission.
o Microwave radio systems are not inclined to common disastrous
breakdown of fiber cable systems occurred by cable cuts, it may be
fixed in no time rather than waiting for hours or days.
o It is controllable in the time of natural disasters for example flood,
earthquakes.
o Operational cost is minimal recurring.
3.3
Considerable Parameters of Microwave
This section describes the most considerable parameters of microwaves antennas.
3.3.1 Microwave Antenna
Any conductor that can intercept an RF field can be an antenna. The Basic Principle
of Microwave antennas are similar to those of antenna used at lower frequencies.
Basically an antenna converts RF power into Electromagnetic radiation.
More
briefly an antenna is transducer which is specially designed to transmit and receive
electromagnetic wave. A good transmitting antenna is often a good receiving
antenna. For designing wireless systems, engineers must select an antenna that
fulfils the system's requirements to firmly close the link between the remote points
of the communications system.
25
3.3.2 The Isotropic Antenna
Isotropic Antenna means is an antenna that transmits equally in all directions. It is
hard to achieve isotropic antenna in real life. Actually isotropic antennas do from a
very important functions are used as a standards by which can determine how
directional some other real life antennas are and what their antenna gain might be.
All antennas are therefore compared to the theoretical workings of an isotropic
antenna [13].
3.3.3 Parameters of Antenna
There are various considerable vital parameters that influence an antenna's
performance and it can be synchronization during the designing procedure [14].
Following are the main considerable parameters for antenna:
1. Input Impedance
2. Radiation Pattern
3. Directivity
4. Polarization
5. Gain
6. Efficiency
3.3.3.1 Input Impedance - Input Impedance is the most important parameter
that’s related to the antenna and its transmission line. It is used to determine
the transferring power from the antenna to transmission line and vice versa.
Between antenna and transmission line the Impedance match is expressed by
the term Standing wave ration (SWR) or reflection coefficient and is
expressed in decibels. [14]
3.3.3.2 Radiation Pattern - The radiation pattern is the geometric pattern of
the comparative field strengths of the field discharged by the antenna. It
would be sphere in case of perfect isotropic antenna and a dipole antenna
would be a toroid. Antenna radiation pattern is usually shown by a graph of
three dimensions, or for vertical and horizontal cross sections may be
26
represented by polar plots. The graph must illustrate back and side lobes,
where the gain of the antenna is at maximum or minimum. [14]
3.3.3.3 Directivity - Antenna Directivity means that maximum antenna gain
compared with its gain that is averaged in all direction.
Directivity go
antenna always independent of its radiation efficiency. [14]
3.3.3.4 Polarization - The antenna polarization describes the electromagnetic
wave polarization emitted by the antenna beside a vector initiating at the
antenna and pointed along the principal direction of transmission.
The
polarization position of the wave is defined by the shape and direction of an
ellipse produced by tracing the boundary of the electromagnetic field vector
against time.
However each antenna is elliptically polarized, mainly
antennas are specified by the best polarization circumstances of spherical or
linear polarization. [14]
3.3.3.5 Gain - The hypothetical isotropic antenna radiates power equally
in all direction and measures any real type antenna gain with compare
to the isotropic antenna. Actually the antenna gain means the amount
of energy radiate in the direction compared to the isotropic antenna
radiate the amount of energy in same direction. The maximum gain is
that the direction antenna radiates most power.
The antenna gain may be calculated as:
10
10
(3.1)
where, η is the efficiency of an antenna.
3.3.3.6 Efficiency - The efficiency of an antenna is generally determined on its
capability to emit energy into the air. Antenna can be efficient when during
the radiation procedure it dissipate very less energy. On the other hand
antenna efficiency is generally referred to the power gain as measured with a
regular reference antenna. Whereas, power gain of an antenna is a proportion
to the radiated power of reference antenna which is basically dipole antenna.
27
When the energy is radiated both antenna should supply radio frequency
energy in same behavior and position. [14]
3.4
Path Loss
Generally radio transmission systems consist of transmitter, antennas and receiver.
In radio transmission the most important questions are: how far apart can the
transmitter and receiver be in distance while maintaining acceptable performance,
and what can be changed to increase this separation distance?
Ag/2
Transmitted
Power (Pt)
Ag/2
Path Loss (L)
Distance (D)
Received
Power (Rt)
Figure 3.1 Typical RF Transmission System
The simple answer is: Use the Free Space Path Loss model in determining
transmitter and receiver separation, and change the transmitter power to increase
separation distance.
The following definitions are referring to the equations (3.2) and (3.3):
Pt
= Transmitter power in dBm
Gtot
= (Ag - Cl) Total gain in dB
L
= Transmission path loss in dB
R
= Receiver sensitivity in dBm
d
= Distance between transmitter and receiver in meters
Figure 3.2 shows the typical RF Transmission system. The received signal strength R
is equal to:
(3.2)
28
For a known receiver sensitivity value, the maximum path loss can be derived as:
(3.3)
Base Antenna
d
Buildings
hb
hB
w
hm
b
Street Level
∆ hb = hb - hB
Mobile Antenna
∆ hm = hB – hm
Mobile Station
Incident wave
ø
Directional
Figure 3.2 Physical Environment Path Loss Variables [15]
Figure 3.2 shows the path loss variables, where the following definitions are being
used.
d = Distance in meters
hb = Base antenna height over street level in meters
hm = Mobile station antenna height in meters
hB = Nominal height of building roofs in meters
∆hb = hb-hB = Height of base antenna above rooftops in meters
29
∆hm = hB-hm = Height of mobile antenna below rooftops in meters
b = Building separation in meters (20 to 50m if no data given)
w = Width of street (b/2 if no data given)
Ø = Angle of incident wave with respect to street (use 90º if no data)
3.4.1 Path Loss and Distance Calculation
Path Loss depends on many factors such as frequency, antenna height, receive
terminal location relative to obstacles and reflectors, and link distance. It is the
largest and most variable quantity in the link budget. Usually a statistical path loss
model or prediction program is used to estimate the median propagation loss in dB.
There are many different path loss models available now based on different
condition such as line of sight (LOS) or non-LOS. Figure 3.2 shows the numerous
physical environment variables used to some degree by each of the above models in
calculating path loss [15].
The National Institute of Standards and Technology (NIST) have done an excellent
job in documenting and comparing several realistic empirical propagation loss
models. Based on the NIST study, the remainder of this document examines the
following loss models [15]:
• Free Space Model
• CCIR Model
• Hata Models
3.4.1.1 Free Space Pathloss Model (FSPL)
FSPL is a fundamental factor for numerous radio frequency calculations and it is
used in various locations for predicting power of radio signals which probably
anticipated in a radio frequency system. FSPL is basically the sort of failure in signal
strengths which happens when an electromagnetic wave communicate over a line of
sight path in free space. In this condition there is no obstacle that may ground the
signal to be refracted of reflected, or that may source of extra attenuation. The signal
in FSPL decreases in a manner which is inversely proportional to square of distance
among the signal source [15].
1
(3.4)
30
FSPL Formula - The equation for free space path loss is pretty easy to
employ. In this case path loss is proportional to the square of distance
among the receiver and transmitter whereas, the signal level is
proportional too for the square of frequency in use. It is describe more
briefly below [15]:
4
4
(3.5)
where:
d is the distance in meters for the receiver from the transmitter
f is the frequency in Hertz
λ is the wavelength in meters
c is the speed of light in meters per second
Decibel Version of FSPL Equation - The majority radio frequency
evaluations and dimensions are achieved in decibels (dB). It provides a
simple and steady method to balance the signal levels formed at
different positions [15].
20
20
32.44
(3.6)
where:
d is the distance in km from receiver to the transmitter
f is the signal frequency in MHz
3.4.1.2 CCIR Path Loss Model (Lccir)
The pragmatic formula for the mutual effects of terrain induced and FSPL was
developed by the CCIR - Comite' Consultative International Radio Communication,
now ITU-R [15].
Lccir 69.55 26.16log 10 fMHz ‐13.82log 10 hb ‐ a hm
44.9‐6.55log 10 hb log 10 dKm
(3.7)
where:
a(hm) = [1.1log10(fMHz)-0.7]hm – [1.56log10(fMHz)-0.8]
B = 30 – 25log10 (% of area covered by buildings)
31
Substituting (3.7) into (3.2) and solving for distance yields the following CCIR
maximum distance equation:
dccir
log
a h
10 Pt
G
R
69.55
B / 44.9 – 6.55
26.16
13.82
h
h
(3.8)
3.4.1.3 Hata Path Loss Models (Lhata)
Hata Model is the most popular model for path loss calculation.
Okumura
published many empirical curves useful for radio system planning and were
subsequently reduced to a convenient set of formulas known as the Hata models
that are widely used in the industry. The CCIR and Hata models differ on area
coverage. There are four Hata models: Open, Suburban, Small City, and Large City
[15].
The basic formula for Hata path loss is:
Lhata 69.55 26.16log 10 fMHz ‐13.82log 10 hb – a hm
44.9‐6.55log 10 hb log 10 dkm – K
(3.9)
Substituting (3.9) into (3.2) and solving for distance yields the following Hata
maximum distance equation:
dhata
log 10 10
Pt Gtot ‐ R‐69.55‐26.16log 10 fMHz 13.82log 10 hb
44.9‐6.55log 10 hb
a hm
K
(3.10)
Table 3.1 describes the working of equation (3.9) for different areas.
TABLE 3.1
Type of Area
Open
Suburban
Small City
Large City
[1.1
[1.56
DESCRIPTION OF EQUATION (3.9)
A(h )
K
(
)]2 – 18.33
4.78[
(
) + 40.94
(
) – 0.7] h –
2[
(
/28)]2 + 5.4
(
) – 0.8]
3.2[log10911.75h )]2 – 4.97
0
0
32
3.4.2 Use of Path Loss Model
This section describes the best path loss model to use. The Following Table 3.2
shows the calculated distance value of different path loss model and from this table
our conclusion is that Hata model is best for different situations [15].
TABLE 3.2
CALCULATED DISTANCE VALUE FOR COMMON EXAMPLE [10]
Path Loss Model
Calculated Distance Value in Meters
Free Space
121,000
WIM Los
16,200
Hata Open
5,300
Hata Suburban
1,600
WIM LOS
820
Hata Small/Large City
740
CCIR
550
Figure 3.3 shows the calculated path loss in different models based on our matlab
simulation result. Hata Model is widely used in Path loss prediction in wireless
systems and Hata present in urban area propagation losses. So, it is good to use this
model instead of other to predicts the total path loss along a link of terrestrial
microwave or other type of cellular communications.
Figure 3.3 Calculated Path Loss in different Models
33
3.5
Link Budget
Shaping and calculating all the power gain and loss in a transmission system is
known as link budget. It identifies the total of power form transmitter that is
required to broadcast a signal with a definite Signal to Noise Ratio - SNR and
satisfactory Bit Error Rate - BER. Path loss, distortion, failure by rain, connectors’
losses, cable losses and antenna gain are the aspects which are obligatory to be taken
into the consideration though estimation of link budget. Figure 3.5 illustrates the
link budgeting procedure [16].
Antenna
Gain
Antenna
Gain
Free Space
Loss
Wave Guide
Losses
Wave Guide
Losses
Transmitter Receiver Receiver Threshold Transmit Output Power Figure 3.4 Link Budget
3.5.1 Calculation of Link Budget
In Figure 3.5 once the heights of the transmitting and receiving towers have been
established the designer is in a position to select the appropriate antenna, or
waveguide, transmitter power and receiver sensitivity to operate the proposed
system. The next step is then to calculate the link budget to verify that the design
will operate satisfactorily. There is a need to check, on both of the link transmitting
frequencies, that sufficient signal arrives at the chosen receiver. This is done by
consulting the appropriate data sheets for the chosen items of equipment to
determine their appropriate gains and losses.
34
The starting point of any Link Budget is the equipment parameters of the intended
microwave equipment to be used and these are; RF output power usually expressed
in dBm or Watts. Receiver sensitivity usually expressed as a Bit Error Rate (BER)
against a given RF signal level, for example BER 10-3 and -86 dBm. It is usually
stated as antenna gain which is for example 45 dBm. It should be noted that this gain
is Isotropic and not indicating any RF amplification.
The system Link Budget is then calculated using the following methodology [17]:
o The free space loss along the radio path is calculated (LFS)
o The effective power produced by the transmitter is calculated (EIRP)
o The effective gain of the receiving antenna is obtained (GRX)
o The losses of all components in the receiving chain is calculated (LRX)
o The signal arriving at the receiver is the algebraic sum of all the above
gains and losses can be calculated as follows:
Received Signal
EIRP
LFS
GRX – LRX dBW
.
o This can be compared with its receiver sensitivity limit (RSL)
Providing the received signal exceeds the RSL with sufficient fade margin then the
system is deemed satisfactory.
3.5.1.1 Free Space Loss
The other major factor in calculating the Link Budget is the operating
frequency and the "Free Space Loss". Free space loss can be expressed with
the simple calculation below:
LdB
92.44
20 log d
20 log f
.
LdB is the loss in dB
d is the distance or path length) Km.
f is the transmit frequency in GHZ.
35
Chapter 4
Adaptive Modulation, Modeling and Simulation
A
daptive modulation is a way to provide balance between Bit Error Rate (BER)
and spectral efficiency. It is possible to make effective use of adaptive
modulation in a slowly varying fading channel with noise based on SNR estimation.
Phase of high gain of power or lower fading, will improve the SNR, which allow the
higher modulation schemes to be worked with less probability of error. On the other
hand, phase of higher fading, will deteriorate the SNR and force us to work with
lower modulation schemes in order to make transmission more effective.
4.1
Adaptive Modulation and Coding Scheme (AMC)
In wireless communication AMC or Link Adaption indicates the identification of the
coding, modulation and signals and protocol parameters depending on the
circumstances of radio link. For more understandings consider the examples of
pathloss, the intrusion due to transmitted signals from different transmitters,
receiver sensitivity, and power outskirts of existing transmitter. Let’s consider the
example of Enhanced Data rates for GSM Evolution (EDGE) which uses the
adaption algorithm of Modulation and Coding Schemes (MCS). It depends on the
excellence of radio channel, bit rate and more importantly on data transmission
robustness. The link adaption procedure is dynamic but protocol parameters and
signal depends on the radio link circumstances. If circumstances change signals and
protocol parameters change. As an example, High-Speed Downlink Packet Access
(HSDPA) in Universal Mobile Telecommunications System (UMTS) occurs after
every 2ms.
The channel information is frequently required by the adaptive modulation system
at the transmitter. It can be assumed in time division duplex systems that the
channel from the transmitter to receiver and receiver to transmitter are more or less
be same. On the other hand the channel information may also be calculated
deliberately at the receiver and gradually pass back to the transmitter. At the
transmitter, the adaptive modulation improves the Bit Error Rate (BER) or rate of
36
transmission boldly by exploiting the channel information, especially over fading
channels which represents the wireless broadcasting environments. In a result
adaptive modulation reveal enormous performance as compared to system which
does not exploit channel information at transmitter.
4.2
Adaptive Modulation in Microwave Link
In microwave radio systems adaptive modulation is introduced for point to point
digital communication to give more capacity to user over air throughout the period
of good transmission conditions, where the path conditions will adapt dynamically
the modulation level of radio link. [18]
Automatic Transmit Power Control – ATPC is an adaptive technique which
has been used in microwave radio system to lowers the output power when
circumstances are good to lessen power utilization and network interface. If channel
is suffering from fading then the power will be automatically increase in order to
maintain the required level of performance link. ATPC is taken further by adaptive
modulation by scheming of output power and modulation level dynamically, to
regulate the link ability to fit with transmission conditions.
4.3
Key Benefits
Adaptive Modulation (AM) enables the service providers to easily grow the existing
capability of links without increasing the size of antenna, no need of hardware
changes and license conditions. Licensed radio links usually designed to carry
system availability due to transmission give rise to outages of purely 99.9%, which
means that the radio link will not be available for approximately 50 minutes in a
year. For rest of the time the fading margin is essentially unexploited. Therefore, it is
kept in reserve. The unexploited margin comes at elevated price, requiring radio
links to be smaller duration, larger antennas or link capability to be inadequate than
required. Whereas, adaptive modulation permit excessive use of fade margins to
significantly increase the radio link capability for a smaller or no extra cost [18].
37
4.4
Adaptive Modulation
It is important to discuss about how to change the modulation scheme. In which our
system will make a way to decide best suitable modulation scheme for present,
future – delayed feedback conditions depending on different SNR level. Dunlop and
Pons [18] asserted that BER at receiver level can be good enough to decide switching
scheme. In this thesis the rejected metric of Pons and Dunlop is being used in order
to estimate the Link SNR. The adaption rate would be restricted because BER
estimation is complicated over short periods. Now, the question arises: How and
what ranges of SNR can be best to use for which modulation scheme? The answer
would be finding in performance of AWGN for each modulation scheme.
The received signal equation,
(4.1)
where, r(t) is a received signal, c(t) is a fading channel which is multiply with
transmitted signal s(t) with addition of noise n(t). Generally is the signal to noise
ratio decided by the noise since the signal power usually is restricted. To consider
this the transmitted power of the signal is multiplied by fading channel. In result,
the direct received signal power can be compare instantaneously with noise, which
allows us to put BER in fading or AWGN channel. Now, let’s take the BER
performance for three modulation schemes which are QPSK, 16 QAM and 64 QAM.
Modulation scheme 128 QAM is an ideal state which is not used practically [19].
In [22] the formula can find for the probability of error when using 4-QAM system,
this formula is extended for 16-QAM, 64-QAM and 128-QAM. By using, those
calculations for different schemes following graph is plotted.
38
Figure 4.1 Modulation Schemes for different SNR levels
Figure 4.1 illustrates the probability of error for different modulation schemes for
different SNR values. Now, consider that the minimum BER level is to be 10-3, and
dropped the 128-QAM as it is not practically used in normal situations. Now, the
system will try to maintain a BER less than 10-3 with the best possible spectrally
efficient scheme. In this way need to set the spectral efficiency as number of bits on a
fixed transmission symbols.
TABLE 4.1
MODULATION SCHEMES DECISION LEVELS AND BITS/SYMBOL
Modulation
Techniques
SNR
Bits/Symbol
QPSK
SNR<17
2
16-QAM
17≤SNR≤23
4
64-QAM
SNR>23
6
Table 4.1 gives us two things, the level for which the modulation should be
switched, and the number of bits per symbol which will be used to calculate the
spectral efficiency for the adaptive modulation. While operating at BER of 10-3 no
modulation scheme will provide preferred SNR level below 10dB. So, it will be good
39
to select the more robust QPSK which gives us SNR performance between 10dB and
17dB. The 16QAM system can get the better spectral efficiency which includes
among 17dB and 23dB. For SNR greater than 23dB, 64QAM will provide maximum
spectral efficiency for required BER performance.
4.5
Theoretical Performance of Adaptive Modulation
This section will illustrate the theoretical performance of adaptive modulation,
under the light of spectrally efficient and BER. The ways to switch the modulation
schemes is already discussed in section 4.4. Hanzo and Torrance gave the detailed
study reference of adaptive modulation [19].
It is needed to address the received Power Density Function of instantaneous,
Rayleigh amplitude s. The Rayleigh function is given by [20]:
√
,
(4.2)
In equation 4.2 ‘S’, is average signal power. Now, second step is to establish the BER
for selected modulation schemes. It can be logically describe by [20]:
⁄
.
,
(4.3)
is the BER in an AWGN channel. The
In equation 4.3 P is the channel BER, and
adaptive modulation for BER considerations [20]:
⁄
2
|
.
.
,
4
⁄
.
,
6
⁄
.
,
(4.4)
In equation 4.4 ‘B’, is average spectral efficiency. Here l1, l2, l3 & l4 are thresholds of
Signal to Noise Ration among modulation schemes. The values for l1, l2, l3 & l4 can
obtain from Table 4.1. B is calculated as:
2.
,
4
,
6
,
(4.5)
40
Following are the graphical representation of adaptive modulation after obtaining
the mathematical grounds. Figure 4.2, shows the spectral efficiency of adaptive
modulation against SNR in dB [20]. The proposed system classified the spectral
efficiency as per number of bits should be send for each modulation scheme. There
is no condition for the bits sending criteria either correct ones or not. This is because
the BER is implied to a point where system will adapt to maintain the required level
of performance.
When foremost QPSK is used, system gets 2 bits/symbol where SNR level is low.
Though, SNR level increases, gradually throughput improves, which shows that we
are ready to take spectrally efficient schemes.
Figure 4.2 SNR versus Spectral Efficiency [21]
When required SNR achieve, the system will be able to select the more capable
modulation schemes. When 64 QAM is used the curve reached up to the SNR level
of 30dB, where QPSK is not often used.
41
Figure 4.3 BER Performances versus SNR for a Fading Channel [21]
Figure 4.3 shows that QPSK curve is overlapped by adaptive modulation. It is
comparable to the spectrally efficient curve in Figure 4.2, where QPSK is the prime
scheme used for lower SNR. So, this improves the working of adaptive modulation
as compared to QPSK provided results.
Let’s consider a transmission which is facing intensive fading and also consider
three modulation schemes QPSK, 16 and 64 QAM which are different in robustness
and spectral efficiency. If we are taking the fade effect extremely intensive there may
be a possibility that half of the bits would be in error. It can be advantage to send the
fewer bits because number of errors would be decreased, where capacity BER is
more than total number of bits sent. When the channel is fadeless it is possible to
send many bits, in this way BER level can be low by sending more bits because there
is less probability of errors. This is the amalgamation of two principles that consider
the adaptive modulation system performance for BER which is more useful than
static ones, which concurrently provide spectral efficiency for majority SNR ranges.
42
4.6
Proposed Adaptive Modulation Model
Every wireless communication system has the problem of a fading channel. In order
to reduce this problem different modulation techniques are implemented which
have different transmitted signal power, bandwidth efficiency and error probability.
The objective of our work is to study and evaluate BER along with the spectral
efficiency by using adaptive modulation technique.
Already defined wireless environment suffers from fading due to many reasons,
multipath fading, free space loss and others. However, in this thesis work the
proposed model shows the effect of the Additive White Gaussian Noise (AWGN)
channel and this thesis will study a dynamically changed modulation scheme based
on the channel status, mainly the SNR is to preserve the maximum throughput with
minimum error rate. The adaptive modulation technique uses M-level of Phase Shift
Keying (PSK) as well as M-level of Quadrature Amplitude Modulation (QAM)
where M = {2, 4, 8….}.
SNR, BER and Bandwidth Efficiency are the main factors which are taken into
consideration. In wireless networks, the channel distorted due to the fading effect
and AWGN.
Figure 4.4 Block Diagram of Simulation Model
43
The modulation is build to be varying with the time depending on the channel
condition, thus a feed back is needed. New model figure 4.4 is proposed using
adaptive modulation and simulation is done based on the model.
Figure 4.4 illustrates the block diagram for the analysis, the main two important
components are the decision controller which will decide which modulation scheme
4-QAM, 16-QAM, 64-QAM to be used and the modulator which will change the
modulation based on the information received from the controller. This model will
provide results for the slowly varying fading channel with addition of AWGN noise.
4.6.1 Performance of Adaptive Modulation
Different modulation techniques have been employed that reduces the fading and
noise factors. In adaptive modulation technique, the system adopted different
modulation schemes with respect to the distance of the subscriber from the base
stations [23] and [25].
Figure 4.5 SNR vs BER Probability for AWGN Channel
44
Security and available bandwidth but high date rate is the main issue now for any
wireless communications system. Improving the spectral efficiency over wireless
fading channel adaptive modulation or link adaptation is powerful technique. With
adaptive modulation a high spectral efficiency is attainable at a given bit error rate
in favorable channel conditions [24] and [26].
From Figure 4.5 and figure 4.6 it is easy to conclude that by using AWGN channel
system can keep the threshold level when SNR level is 10dB. When system wants to
keep the same threshold level while fading channel is introduced then system needs
to increase the SNR level.
Figure 4.6 SNR vs. BER probability for Fading Channel
45
However, Figure 4.7 shows the joint advantage of this technique and reflects the
main advantage achieved when adaptive modulation is used, it can be noticed that
as the SNR is increased, the throughput is increased as compared to QPSK which
maintains a constant level although the SNR is increased. Shanon Capacity is the
theoretical capacity for error free system condition which cannot achieved in reality.
Figure 4.7 SNR vs. Bandwidth Efficiency for Adaptive Modulation
46
Chapter 5
Conclusion
M
icrowave communication is playing the role of a key factor in the current time
of wireless communication. The performance and quality of service of all the
Mobile Communication Service Providers broadly depends on the quality and
availability of their Microwave Link. It requires a series of works to establish a
microwave link between two long distance and short distance points. All the steps
are performed by engineer as to establish a Microwave link. For establishing a
microwave link analyzing factors, first factor is Terrestrial factors that are Site
Survey, Condition of Terrain (Flat / Hilly /desert), and Presence of water body like
big river/lake/sea, presence of forest or big trees. Second factor is select the
modulation technique, environmental conditions and Link budget considering
required Fade margin using appropriate antennas, cables, wave-guides and
connectors.
5.1
There Are Several Advantages of Microwave Radio
Today’s the various advantages pertaining to the wide use of microwaves. Most
import ants are:
1. Less affected by natural calamities
2. Less prone to accidental damage
3. Links across mountains and rivers are more economically feasible
4. Single point installation and maintenance
5. Single point security
6. They are quickly deployed
5.2
Basic Recommendations
Some basic recommendations which are important for planning efficient wireless
network through microwave links:
1. To use higher frequency bands for shorter hops and lower frequency
bands for longer hops.
2. Avoid lower frequency bands in urban areas.
47
3. In areas with heavy precipitation, if possible, use frequency bands
below 10 GHz.
4. The activities of microwave path planning and frequency planning
preferably should performed in parallel with line of sight activities and
other network design activities for best efficiency.
5. To use updated maps that is not more than a year old. The terrain
itself can change drastically in a very short time period.
48
APPENDICES
49
APPENDIX A
Abbreviations and Acronyms
RF
Radio Frequency
GHz
Gigahertz
MHz
Megahertz
KHz
Kilohertz
WLAN
Wireless Local Area Network
DBS
Direct Broadcast System
MLS
Microwaves Landing System
GPS
Global Positioning System
EW
Electronics Warfare
MF
Medium Frequency
HF
High Frequency
PSTN
Public Switched Telephone Network
GSM
Global System for Mobile Communication
FDMA
Frequency Division Multiple Access
TDMA
Time Division Multiple Access
CDMA
Code Division Multiple Access
OFDMA
Orthogonal Frequency Division Multiple Access
LOS
Line Of Sight
AWGN
Additive White Gaussian Noise
ISI
Inter Symbol Interference
50
SWR
Standing wave ration
NIST
National Institute of Standards and Technology
FSPL
Free Space Pathloss Model
SNR
Signal to Noise Ratio
BER
Bit Error Rate
EIRP
Equivalent Isotropic Radiated Power
RSL
Receiver Sensitivity Limit
QAM
Quadrature Adaptive Modulation
QPSK
Quadrature Phase Shift Keying
AM
Adaptive Modulation
AMC
Adaptive Modulation and Coding Scheme
EDGE
Enhanced Data rates for GSM Evolution
MCS
Modulation and Coding Schemes
UMTS
Universal Mobile Telecommunications System
HSDPA
High-Speed Downlink Packet Access
PSK
Phase Shift Keying
ATPC
Automatic Transmit Power Control
51
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