WiMAX: IEEE 802.16 Jorge López Vizcaíno

WiMAX: IEEE 802.16 Jorge López Vizcaíno
TAMPERE POLYTECHNIC
Telecommunications Engineering
Jorge López Vizcaíno
Jorge López Vizcaíno
WiMAX: IEEE 802.16
Supervisor: Senior lecturer Ari Rantala
Instructor: Senior lecturer Ari Rantala
FINAL THESIS
Author: Jorge López Vizcaíno
Supervisor: Senior lecturer Ari Rantala
Name of the thesis: WiMAX: IEEE 802.16
Number of pages: 86
Degree programme: Telecommunications Engineering
Date of presentation: 05.06.2008
ABSTRACT
This thesis is related with the WiMAX (World Interoperability
Microwave Access) technology and the standard elaborated by the Institute
of Electrical and Electronic Engineers (IEEE) under the 802.16 family of
standards.
Nowadays, the demand of broadband access is growing exponentially
every year. Users demand a good-quality connection at anytime in
everywhere, whereas countries look for an economic, fast-to-deploy and high
performance broadband technology to provide access to every region.
WiMAX can be a solution to this problem thanks to its technical advantages
such as large coverage and low-cost, as for the strong support of the industry
through the WiMAX Forum. In addition to, the Mobile WiMAX version can
give the full mobility of cellular services at higher broadband speeds than
other technologies such as Wi-Fi.
In this thesis, after a brief introduction about the broadband wireless
technologies and the different IEEE 802.16 standards, an accurately
explanation of WiMAX and its technical aspects is included. At the end, it is
possible to examine the different applications implemented with WiMAX
and a brief comparison with other wireless technologies.
AKNOWLEDGEMENTS
First of all I would like to express my sincere gratitude and appreciation to
Tampere polytechnic for the opportunity I had to write my thesis in Finland
and especially to my supervisor Ari Rantala whose guidance and
encouragement helped me during these months.
I am also very grateful to all my family (parents, sisters and nephews) for
their enthusiastic support, wise advices, inspiration and love during all
these years, but especially I would like to thank and dedicate this thesis to
my grandmother because she would be deeply glad and proud of me.
I would also like to thank all my friends from Spain and to the ones in here
for their help in the hard moments. “Lapinkaari people” thank you for
sharing unforgettable moments during this year which was the best of my
life.
At last but not least I would like to thank Xana, for all her help in my
English doubts concerning this thesis work, and Cláudia for her support
and help.
Table of Contents
Table of Contents ...................................................................................................... 1
1. INTRODUCTION TO BROADBAND WIRELESS ......................................... 7
1.1-EVOLUTION OF BROADBAND WIRELESS .............................................. 8
1.1.1-Narrowband wireless local-loop systems (WLL)...................................... 9
1.1.2- First-generation line-of-sight (LOS) ........................................................ 9
1.1.3- Second-generation Broadband Systems ................................................... 9
1.1.4- Standard-based technology .................................................................... 10
1.2.-FIXED BROADBAND WIRELESS ............................................................ 10
1.3. - MOBILE BROADBAND WIRELESS ....................................................... 11
1.4.-OTHER BROADBAND TECHNOLOGIES ................................................ 11
2.-WiMAX ............................................................................................................... 13
2.1.-IEEE 802.16 STANDARDS ......................................................................... 13
2.2. - PROTOCOL ARCHITECTURE ................................................................ 18
2.3.-WiMAX FORUM ......................................................................................... 19
2.4.-SPECTRUM OPTIONS ................................................................................ 21
3.-TECHNICAL FOUNDATIONS OF WiMAX ................................................. 23
3.1.-WIRELESS CHANNEL: PATHLOSS AND SHADOWING ..................... 23
3.2.-CELLULAR SYSTEMS ............................................................................... 24
3.3.-FADING ........................................................................................................ 25
4.-OFDM (Orthogonal Frequency Division Multiplexing) ................................. 28
4.1.-INTRODUCTION TO DIGITAL MODULATIONS ................................... 28
4.2.-MULTICARRIER MODULATION ............................................................. 30
4.3.-OFDM BASICS ............................................................................................ 30
4.4.-OFDMA (Orthogonal Frequency Division Multiplexing Access) ................ 34
5.-PHY LAYER ...................................................................................................... 37
5.1- CHANNEL CODING ................................................................................... 37
5.1.1- Randomization ....................................................................................... 38
5.1.2- Forward Error Codes (FEC) ................................................................... 38
5.1.3.-Interleaving............................................................................................. 41
5.1.4.-Repetition ............................................................................................... 41
5.2.-HYBRID-ARQ .............................................................................................. 41
5.3.-TRANSMISSION CONVERGENCE SUBLAYER (TCS) ......................... 42
5.4.-SUBCHANNEL AND SUBCARRIER PERMUTATION .......................... 42
5.4.1.-Downlink Full Usage of Subcarriers (DL FUSC) .................................. 42
5.4.2.-Downlink Partial Usage of Subcarriers (DL PUSC) .............................. 42
5.4.3.-Uplink Partial Usage of Subcarriers (UL PUSC) ................................... 43
5.4.4.-Band Adaptive Modulation and Coding (AMC) .................................... 43
5.5.-RANGING .................................................................................................... 43
5.6.-SLOT AND FRAME STRUCTURE ............................................................ 44
5.6.1.-OFDM PHY Downlink Subframe .......................................................... 44
5.6.2.-OFDM PHY Uplink Subframe ............................................................... 45
5.6.3-OFDMA PHY Frame .............................................................................. 46
5.7.-POWER CONTROL ..................................................................................... 47
5.8.-CHANNEL-QUALITY MEASUREMENTS ............................................... 48
6.-MAC LAYER ..................................................................................................... 49
6.1.-MAC CONVERGENCE SUBLAYER ......................................................... 49
6.2.-MAC PDU OR MAC FRAME ..................................................................... 49
6.3.-QUALITY OF SERVICE (QoS) .................................................................. 51
6.4.-BANDWIDTH REQUEST ........................................................................... 52
6.5.-NETWORK ENTRY..................................................................................... 53
6.6.-CONNECTION MAINTENANCE............................................................... 56
6.7.-PMP vs. MESH MODE ................................................................................ 57
6.8.-MAC FUNCTIONS FOR MESH TOPOLOGY ........................................... 59
7.-MOBILITY ......................................................................................................... 61
7.1.-POWER-SAVING MODES ......................................................................... 61
7.1.1.-Sleep Mode ............................................................................................. 61
7.1.2.-Idle mode ................................................................................................ 61
7.2.-HANDOVER ................................................................................................ 62
8.-WiMAX NETWORK ARCHITECTURE ....................................................... 64
8.1.-NETWORK REFERENCE MODEL ............................................................ 64
8.1.1. - Access Service Network (ASN) ........................................................... 65
8.1.2.-Connectivity Service Network (CSN) .................................................... 65
8.1.3.-Reference Points..................................................................................... 66
8.2.-NETWORK FUNCTIONALITIES .............................................................. 66
8.2.1.-Network Discovery and Selection .......................................................... 66
8.2.2.-Mobility Management ............................................................................ 67
8.2.3.-IP Address Assignment .......................................................................... 67
8.2.4.-AAA Framework .................................................................................... 68
8.2.5.-Quality-of-Service Architecture ............................................................. 68
9.-SECURITY ......................................................................................................... 70
9.1.-AUTHENTICATION AND ACCESS CONTROL ...................................... 71
9.1.1.-Authentication ........................................................................................ 71
9.1.2- Authorization.......................................................................................... 72
9.1.3.-Security in the Network Layer ............................................................... 72
9.2.-DATA ENCRYPTION ................................................................................. 73
10.-APPLICATIONS .............................................................................................. 75
10.1.-WMAN (WIRELESS METROPOLITAN AREA NETWORK)................ 75
10.2.-WiMAX MILITARY APPLICATIONS ..................................................... 76
10.3.-RURAL AREA BROADBAND SERVICES ............................................. 76
10.4.-WIRELESS BACKHAUL .......................................................................... 76
10.5.-LAST-MILE ACCESS TO THE BUILDINGS .......................................... 77
10.6.-PRIVATE NETWORKS ............................................................................. 77
10.7.-SECURITY APPLICATIONS .................................................................... 78
10.8.-MEDICAL APPLICATIONS ..................................................................... 78
10.9.-OTHER APPLICATIONS .......................................................................... 78
11. – COMPARISONS ........................................................................................... 79
11.1.-COMPARISON BETWEEN FIXED AND MOBILE WiMAX ................ 79
11.2.-COMPARISON BETWEEN WiMAX AND Wi-Fi ................................... 79
11.3.-COMPARISON BETWEEEN WiMAX AND 3G ..................................... 80
11.4.-OTHER COMPARABLE SYSTEMS ........................................................ 81
11.5.-COMPARISON TABLE ............................................................................. 81
12.-SUMMARY AND CONCLUSION ................................................................. 82
12.1.-SUMMARY ................................................................................................ 82
12.2.-FINAL CONCLUSION .............................................................................. 82
13.- REFERENCES ................................................................................................ 84
WiMAX: IEEE 802.16
1. INTRODUCTION TO BROADBAND WIRELESS
The demand of broadband services is growing exponentially in the last years. There
are several wired technologies that provide us a high-speed broadband access such as
Digital Subscriber Line (DSL) over twisted-pair telephone or cable over fiber optics.
The main problem of these wired access technologies is the difficulty and high cost of
installation and maintenance, especially in remote and rural areas.
In the last years, Internet has developed from being only an academic tool to having
hundreds of millions of users around the world. Besides, the demand of a high-speed
connection has caused a huge development of the broadband technologies.
As Broadband Access, wireless mobile services have grown considerably in the last
years, from 11 millions of subscribers worldwide in 1990 to more than 2 billion in 2005.
This increase is due to the use of laptops, mobiles and PDAs. There is no doubt that at
the end of the first decade of the 21st century, high-speed wireless data access will be
largely deployed worldwide.
The main reason for the development of WiMAX (“World Interoperability
Microwave Access”) is the demand of higher data rates not only for faster downloading
but also for the use of new applications like voice over Internet Protocol (VoIP), video
streaming, multimedia conferencing, and interactive gaming. WiMAX will
revolutionize broadband communications in developed countries and will allow the
developing countries to be communicated to. With this technology the users will be able
to have access to broadband networks anywhere and anytime. There are some
competitive technologies such as third generation of mobile communications (3G) or
HSPA but nowadays they only can provide high-data rates in small areas of coverage
and under some specific conditions.
Two very different families of WiMAX systems exist and should be treated
separately: Fixed and Mobile WiMAX.
1.1-EVOLUTION OF BROADBAND WIRELESS
Due to the development of telecom industry and the huge growth of Internet, the
carriers were researching to find a new wireless technology to reach the new
requirements. The evolution of WiMAX technology can be structured in four stages:
1) Narrowband wireless local-loop systems (WLL)
2) First-generation line-of-sight (NLOS)
3) Second-generation non-line-of-sight (NLOS)
4) Standards-based broadband wireless systems
8
1. INTRODUCTION TO BROADBAND WIRELESS
1.1.1-Narrowband wireless local-loop systems (WLL)
The first use was obviously voice telephony; it was successful in developing
countries as well as in rural regions in developed countries where the wired technology
is not widely deployed and its deployment can be quite expensive.
The European Telecommunications Standards Institute (ETSI) published a WLL
cordless system in 1992 named DECT (Digital Enhanced Cordless
Telecommunications). The range of DECT equipments is up to a few hundred meters.
DECT works in the 1.9 GHz bandwidth. This system uses digital TDMA (Time
Division Multiple Access) and it has a great success nowadays.
In markets with a robust local-loop infrastructure installed, WLL had to offer more
than voice telephony, so operators found an opportunity with the commercialization of
Internet access services providing high-speed Internet access. In 1997, AT&T
developed a wireless access system for the 1,900 MHz PCS (personal communications
services) offering two voice lines and a 128 kbps data connection. This system was
called “Project Angel”.
At the same time, other companies started to offer wireless Internet access using the
license-exempt 900MHz and 2,4Ghz bands reaching speeds up to a few hundreds of
kilobits. These connections required the installation of antennas on the rooftops.
1.1.2- First-generation line-of-sight (LOS)
The development of DSL and cable modems caused the evolution of wireless
systems for supporting higher speeds to be competitive. Local Multipoint Distribution
System (LMDS) started to be deployed using high frequency bands (24GHz and
39GHz) supporting several hundreds of megabits per second.
At the end of 1990s, multichannel multipoint distribution services (MMDS) began
to be deployed using the 2, 5 GHz band that was used for the cable TV broadcasting in
rural regions where cable TV was not available. Some operators started to offer oneway Internet access using the telephone line as the return path. In 1998, FCC regulated
this band allowing two-ways communication.
This first generation with LOS coverage was deployed using the towers installed for
wireless cable services. It was necessary to install antennas that were high enough and
pointed towards the tower.
1.1.3- Second-generation Broadband Systems
This second generation solved the LOS problem and provided more capacity using a
cellular architecture and techniques as Orthogonal Frequency Division Multiplexing
(OFDM), Code Division Multiple Access (CDMA) and multiantenna processing. It was
possible to reach speeds up to a few megabits per second.
9
WiMAX: IEEE 802.16
1.1.4- Standard-based technology
The Institute of Electrical and Electronic Engineers (IEEE) formed a group in 1998
called 802.16. The aim of this group was develop a standard for the Wireless
Metropolitan Area Network for regulating the 10GHz to 66GHz band.
From the first standard approved in December 2001 until now, several standards and
amendments has been developed. All the standards and its corresponding features will
be analyzed in the chapter 2.
It is important to know that 802.16 is only a collection of standards that includes a
wide range of variations. The IEEE only developed the specifications but it is the
industry and especially an industrial group (WiMAX Forum) who is in charge that has
to convert it into an interoperable standard.
1.2.-FIXED BROADBAND WIRELESS
There are two different network topologies in fixed broadband wireless:
-
Point-to-point applications include interbuilding communications within a
campus and microwave backhaul.
Point-to-multipoint is usually based in a base station mounted in a tower or in a
building that communicates with the subscriber; the most common usages are:
1- Consumers and small business broadband:
The main usage of WiMAX in the near future is broadband services like high-speed
Internet access, telephony over IP (VoIP) and a host of other Internet applications.
WiMAX presents some advantages over wired technologies like lower deployment
costs, lower operational costs for the maintenance, faster realization and independence
of the incumbent’s carriers.
There are two types of deployment models, one of them requires the installation of
an outdoor antenna at the costumer’s building and the other one requires a all-in-one
radio modem installed indoors. Using outdoor antenna improves the coverage and
performance of the system; however it requires a truck-roll with a trained professional
so it implies a higher cost in developed countries but in developing countries turns to be
cheaper.
2- T1 services for business:
The other use of Fixed WiMAX is a solution for competitive T1, fractional T1 and
higher-speed services for the business market. It will be successful due to the fact that
not all the buildings have access to fiber and in business exists a demand of symmetrical
T1 services that cable and DSL cannot reach.
3- Backhaul for Wi-Fi hotspots:
10
1. INTRODUCTION TO BROADBAND WIRELESS
Wi-Fi hotspots are widely deployed in public areas in developed countries. The
traditional solution is using wired broadband connections to connect the hotspots back
to a network point. In this case, WiMAX can be a cheaper and faster alternative for
WiFi backhaul and it can also be used for 3G backhaul.
WiMAX could be very successful in developing countries where a wired network is
not installed. WiMAX will be a cheaper alternative to extend broadband access over the
country.
1.3. - MOBILE BROADBAND WIRELESS
In a context where the users get familiarized with the use of high-speed broadband
services, they will demand same services in nomadic or mobile situations. The first step
is adding nomadic capabilities to fixed broadband connection, thus users can get
connection moving within the service area with pedestrian-speed.
In the market, the cellular spectrum operating licenses are limited and very
expensive so WiMAX could be a good opportunity to offer mobility services for some
operators of fixed lines that do not offer mobile services. However, the existing mobile
operators are more interested in the development of 3G than in adopting WiMAX.
WiMAX presents some important advantages that can be useful for its development
such as the low latency that is fundamental for voice over IP services (VoIP). Other
advantages are the flexible bandwidth and multiple levels of Quality of Services (QoS)
that may allow the use of WiMAX for entertainment applications. Some examples of
these applications could be interactive gaming, IP-TV and streaming audio services for
MP3 players.
The main drawback is that the IEEE 802.16 standard only specifies an air interface
so the core network has to be deployed.
1.4.-OTHER BROADBAND TECHNOLOGIES
There are several broadband wireless technologies which provide broadband
wireless services, some of them are already being used and other are being developed.
In this subchapter, we can see a brief mention of some of them, but in the subchapter 11
we will see a complete comparison between them.
Mobile operators are changing their networks to 3G technologies to deliver
broadband applications to their subscribers. Mobile operators using GSM (global
system for mobile communication) are deploying UMTS (Universal Mobile Telephone
System) and HSDPA (High Speed Download Packet Access). Otherwise, CDMA
operators are upgrading their networks to 1x EV-DO (1x evolution data optimized).
HSDPA is the downlink interface defined in the Third-generation Partnership
Project (3GPP) and is capable of providing a peak user data rate of 14.4 Mbps in a
5MHz channel. The uplink interface defined by 3GPP is HSUPA (high-speed upload
packet access) that supports peak data rates up to 5.8Mbps. HSDPA and HSUPA are
11
WiMAX: IEEE 802.16
defined together as HSPA. 3GPP is developing the long-term evolution (LTE) of the
standard that will be able to support a peak data rate of 100Mbps in the downlink and
50Mbps in the uplink.
In addition to 3G, Wi-Fi is another important providing system of broadband
wireless and it has become the last-feet broadband connectivity at home, offices and
public areas. Current Wi-Fi systems that are based in 802.11a/g standards support a
typical throughput of 23/19 Mbps with an indoor coverage of about 35 meters.
However, a new revision of the standard, 802.11n, support a throughput data-rate of 54
Mbps with coverage of about 70 meters using multiple-antenna spatial multiplexing.
WiMAX is a very flexible and scalable standard that may be adapted to different
frequency bands. The standard is torn in two different goals. On the one hand, if the
frequency and bandwidth are limited, the compatibility and development will be easier.
On the other hand, the frequency and the bandwidth are standardized in different
profiles and this flexibility allows the use of this technology in countries with different
spectrum availability and regulations.
12
2.-WiMAX
2.-WiMAX
2.1.-IEEE 802.16 STANDARDS
As mentioned in 1.1.4, WiMAX is not a standard, it is only a marketing trend
trademarked by WiMAX Forum to describe the IEEE 802.16 based technology.
WiMAX standard refers to a set of capabilities that are likely to experience widespread
implementation.
In its short live, WiMAX has evolved from the market and technological
perspective. The original IEEE 802.16 specification was to provide a high-data rate,
point to point communication and with LOS (Line of Sight) conditions between fixed
locations. This application was created to provide wireless bridging between fixed
locations within the network infrastructure. The typical example of this usage is a tower
that is wirelessly backhauled to a fixed location which is attached to a larger wired
network.
After its first usage, the scope was expanded to offer direct support of end-user
networks interconnecting end-users with network infrastructure. WiMAX can offer
high-data rate over long distances so it is an adequate technology to solve the problem
space of the Internet Service Provider (ISP) in wireless local loop where low-rate wired
infrastructure often limits the capabilities of the connection for the costumers. This
technology is explained in the standard IEEE 802.16a and in the IEEE 802.16d (or also
802.16-2004) which unified the original 802.16 and 802.16a. Although there are already
other technologies in the market for solving this problem space, WiMAX can be very
successful in regions without a good wired infrastructure like in developing regions or
in rural regions in developed countries.
The big evolution of the WiMAX usage was to provide mobility support. WiMAX
is the air-interface for the actual radio interface network, where both fixed and mobile
users can have access to the network. The basis of mobile WiMAX is explained in the
IEEE 802.16e (or 802.16-2005) standard.
In this context, existing incumbent Wireless Service Providers (WSP) in the market
have invested big amounts of money to reach the current level of capabilities so now
they will not adopt easily a new entry technology but it may be a good solution to a
new-entry WSPs to offer wireless services. However, for a new WSP it would not be
easy due to the high cost of the spectrum (to operate in licensed bands), of the
infrastructure and the difficulty to reach the economy of scale required for driving down
the equipment and service costs to a competitive level.
One advantage of this standard is the possibility to offer services in unlicensed
frequency bands but the problem is that WiMAX Forum has no certification profiles for
unlicensed (5.8 GHz) Mobile WiMAX. Other problems for the expansion are the lack of
a kill-app for mobile usage and the evolution of other technologies such as HSDP (High
Speed Download Packet Access).
13
WiMAX: IEEE 802.16
The lack of a “killer app” that gives completely mobile data networking supposes a
drawback for the development. In the market, there is an important demand of nomadic
mobility services that allows moving from one place to another without losing the
connection at pedestrian velocity. However it is not sure that the connection on motion
at vehicular velocity would be successful for the costumers. It would be able to be
useful in military networks and in some public transport scenarios like trains in order to
provide network access to travelers.
The standards only specify the physical layer (PHY) and the media access control
(MAC) of the air interface while the upper layers are not specified and the CN (Core
Network) is not specified and has to be deployed and maintained. Bellow, one can find
explained some characteristics of the different standards:
Most important standards:
IEEE 802.16 (Fixed SSs)
-
-
-
-
Published in April 2002
Network Topology: Point-to- fixed point (PTP) backhaul (dedicated link with only
two nodes: BS and SS)
Frequency bands: 10-66 GHz (licensed band but it is high frequency so there is
less interference and more bandwidth available)
Modulation: Single Carrier
Modulation schemes: It uses Adaptive Modulation so the physical layer (PHY) can
employ the following modulation schemes: QPSK, 16-QAM or 64-QAM
modulation adaptively changing on the basis of channel conditions
Propagation conditions: LOS(Line of Sight) is required in every communication
(radio waves are too short to penetrate buildings)
Channel Bandwidth: 25 MHz in USA and 28 MHz in Europe
Antennas: directional antenna at both sides (outdoor mounting)
Duplexing: it can employ TDD (Time Division Duplexing) or FDD (Frequency
Division Duplexing). TDD requires only one channel that is shared by the uplink
and downlink but separated by different time slots so it is only possible transmitting
or receiving at the same time, it is perfect for data transmission. However, FDD uses
two different channels for the uplink and downlink with the minimum delay, so it is
suitable for voice communication.
Multiplexing: TDM (Time Division Multiplexing) for downlink channel and
TDMA (Time Division Multiple Access) for the uplink channel. In TDM,
subscribers share the same frequency band but they are allocated in different time
slots. In TDMA slots are assigned based on fixed or contention modes.
Data-rate: high data-rate(32-134 Mbps with a channel of 28 MHz) using highly
directional antennas and high power-levels
Cell radius: 2-5 km
Security: Rudimentary, reliance on antenna directivity to mitigate intrusions
Error correction: Red-Solomon block with inner convutional code
14
2.-WiMAX
IEEE 802.16a (Fixed SSs)
-
-
-
-
Published in April 2003
Frequency bands: 2-11 GHz (unlicensed and licensed bands)
Network Topology: Point-to- fixed point (PTP) backhaul and two new modes:
Point-to- multipoint (PMP) and mesh-topology. In PMP, a group of subscribers are
connected to BS separately. However in mesh-topology the SSs are more intelligent
and can perform as transmitter or receiver. Thus, one SSs do not have to connect
only with the BS such as in PMP, so it can transmit to the neighbor and hence
extends the network coverage and reduces the system failures
Propagation conditions: NLOS (non-line of sight) because radio waves with these
frequencies can penetrate in every building. NLOS has worse performance than
LOS owing to attenuation when passing through obstacles and more interference
Antenna: omni-directional antennas (indoor)
DFS (Dynamic Frequency Selection) is used to avoid interference. It consists on
switching the RF (radio frequency) channel on the basis of certain measurement
criteria as SIR(Signal to Interference Ratio)
Flexible channel bandwidth: from 1.25 to 28 MHz (for some devices is difficult to
transmit in a wide bandwidth channel)
Modulation: OFDM(Orthogonal Frequency Division Multiplexing) is used
Modulation scheme: Adaptive modulation is also used
Data rate: Medium data-rate up to 75Mbps
Cell radius: 5-10 km (maximum distance of 50 km)
IEEE 802.16-2004 (Fixed SSs)
-
Published in October 2004
Frequency bands: both of them: 2-11 GHz and 10-66 GHz
Unifies IEEE 802.16, 802.16a, 802.16b and 802.16c
Network Topology: Point-to- fixed point (PTP) backhaul and Point-to- multipoint
(PMP) and mesh topology
Modulation: it provides three different air-interfaces:
a) WirelessMAN-SC2: single carrier modulation
b) WirelessMAN-OFDM: OFDM modulation with a 256-point Fast Fourier
Transform (FFT) with TDMA channel access.
c) WirelessMAN-OFDMA: OFDM is employed with a 2048-point FFT. Multipleaccess is provided assigning a subset of subcarriers to each user.
The physical layer can employ QPSK, 16-QAM or 64-QAM modulation
adaptively changing on the basis of channel conditions
-
Propagation conditions: non LOS propagation and LOS propagation
Channel Bandwidth: 1.25 to 28 MHz
Data rate: Medium data-rate(<75 Mbps with channels of 20MHz)
Cell radius: 5-10 km (maximum coverage of 50 km)
Security: includes two-way authentication
MAC enhancements: supports “multihop” mesh networking to enable to retransmit
one packet from one node to another one and extend the coverage area
Error Correction: FEC and Automatic Retransmission Request(ARQ)
15
WiMAX: IEEE 802.16
-
Antenna techniques: it uses sectored omnidirectional antenna instead of
directional. Thus decrease dependence on a precise antenna pointing and allows
extending the coverage area. Moreover, adaptive antenna beam-forming allows the
improvement of the resistance to interference and scability performance
IEEE 802.16-2005 or 802.16e (Fixed or Mobile SSs)
In this case, 802.16-2005 is not a standalone document. Sometimes it is called
“Mobile WiMAX”. It is strongly influenced by the Korean Standard “Wibro” that is
unified with WiMAX in this standard. It is based on 802.16-2004 and it includes very
important enhancements. The most important are:
-
-
-
-
Published in February 2006
Frequency band: 2- 6 GHz for mobility
Not backward compatible with 802.16-2004 so software and hardware have to be
updated
Modulation: It employs scalable OFDMA (Orthogonal Frequency Division
Multiplexing) Access that is highly robust to network congestion and to
interference. With OFDM only one user can use the channel during one time slot,
whereas in OFDMA multiple users can transmit at the same time
OFDMA supports larger FFT size of 1024 so it a allows more flexible subchannel
allocation
Adaptive: Signal coding, modulation and amplitude are assigned separately to each
subchannel depending on the channel conditions
Mobile Stations appear and with them handover support
Security: secure key exchange during authentication and encryption using
AES(Advanced Encryption Standard) and DES(Data Encryption Standard) during
data transfer
Antenna techniques: includes MIMO(Multiple In Multiple Out) and enhancements
and new implementations of Adaptive Antenna System (AAS)
Error Control: includes advanced FEC coding schemes as turbo codes and lowdensity parity check codes. It also includes Hybrid Automatic Retransmission
Request (HARQ).
Channel bandwidth: 1.25 to 20MHz
Data rates: Low-medium data-rate(<15 Mbps with channels of 5 MHz)
New Performance modes: Power-save, sleep and idle-modes.
Number of users: it supports more number of users than 802.16-2004
Cell radius: 2- 5 km
QoS: Better support for Quality of Service (QoS), a new QoS class appears
Other standards:
IEEE 802.16b (Fixed SSs)
-
Published in October 2002
Frequency bands: 5-6 GHz (license-exempt applications) providing QoS
Replaced by 802.16-2004
16
2.-WiMAX
IEEE 802.16c (Fixed SSs)
-
Published in January 2003
The goal was enable greater levels of interoperability
Frequency bands: 10-66 GHz
Replaced by 802.16-2004
IEEE 802.16.2-2001
-
Published in September 2001
Frequency bands: 10-66 GHz
Replaced by 802.16-2004
IEEE 802.16.2-2004
-
Published in March 2004
Frequency bands: 10-66 GHz and 2-11 GHz
Amendment of 802.16-2004 which includes enhancements to avoid interference
IEEE 802.16f-2005
-
Published in September 2005
Enhanced version of 802.16-2004 that includes Manager Information Base (MIB)
for the MAC and PHY layers and management procedures.
Amendments in progress:
1)
Active amendments
IEEE 802.16f-2005: It includes Management Information Base (MIB) and
associated management procedures. It provides a management reference model for
802.16-2004 networks. The model consists on a Network Management System (NMS),
managed nodes and service flow database. BS and managed nodes collect the
information and
It is sent to NNS via management protocols as SNMP (System Network
Management Protocol)
IEEE 802.16g-2007: It provides Management Plane Procedures and Services to
802.16-2004 and 802.16-2005 to enable interoperable and efficient management of
network resources, mobility and spectrum. Other important goal is to standardize
management plane behavior in 802.16 fixed and mobile devices.
The 802.16 devices can be part of a bigger network; they have to interface with
other entities for management and control processes. Thus, a Network Control
Management System (NCMS) is included that interface with the BS. 802.16g is based
only in the management and control interactions between the NCMS and the PHY and
MAC layers.
17
WiMAX: IEEE 802.16
IEEE 802.16k-2007: It is working in the development of a series of standards as
amendments to 802.16-2004 and 802.16D (IEEE MAC bridges standard) for 802.16
MAC layer bridging.
2)
Amendments under development
IEEE 802.16h: The scope of this standard is improving the coexistence mechanism
for License-Exempt Operation. That means to develop improved MAC mechanisms to
enable coexistence between 802.16-2004 devices and other devices that are using the
license-exempt band.
IEEE 802.16i: It will replace 802.16f providing mobility enhancements to MIB and
associated management procedures
IEEE 802.16j: The goal of this group is to develop amendments to make possible
that 802.16-2005 can support mobile multihop relay operation. It intends to improve the
network’s coverage, throughput and system capacity. It extends the network with three
different types of relay nodes: fixed relays, nomadic relays and mobile relays
IEEE 802.16Rev2: Consolidate 802.16-2004, 802.16e, 802.16g and possibly
802.16i in a new document
3)
Amendment at pre-draft stage
IEEE 802.16m: Advanced Air Interface. Data rates of 100 Mbps for mobile
communications and 1Gbps for fixed applications. It will include cell, macro and micro
cell coverage with no restrictions in the channel bandwidth. It is supposed to be
approved at the end of 2008.
2.2. - PROTOCOL ARCHITECTURE
According to the OSI model, the 802.16 covers the two lowest levels: physical layer
(PHY) and a sublayer of the data link level which is Media-Access Control layer
(MAC). PHY layer provides an electrical, mechanical, and procedural interface to the
transmission medium. MAC sublayer is the responsible of determining which subscriber
stations (SSs) can access to the network and it is divided in three different sublayers:
-
Convergence Sublayer (CS): its task is to map higher data units into proper service
data units and it is also responsible for allowing bandwidth allocation and
preserving/enabling QoS, as well as for header suppression and reconstruction. CS
has two different services: ATM convergence sublayer and packet convergence
sublayer.
-
Common Part Sublayer (CPS): 802.16-2004 and 802.16-2005 is designed to support
PMP (Point-to-multipoint) connection and mesh topology is left as optional. It is
responsible for establishing and maintaining the connection, bandwidth allocation.
18
2.-WiMAX
-
Privacy Sublayer (PS): it provides secure key exchange and encryption. PS has two
different protocols: 1) encapsulation to encrypt data across the network and
authentication, 2) Privacy Key Management (PKM) to facilitate secure distribution
of the keying data from the BS to SS.
PHY layer is responsible for data transmission and reception. It is specified
depending on the frequency band, the propagation conditions (LOS or NLOS) and it
also depends on the channel bandwidth. It supports adaptive modulation (BPSK, QPSK,
16-QAM and 64-QAM).
LLC is the upper sublayer of the level 2 of OSI (data link) responsible for
multiplexing protocols transmitted over MAC (when transmitting) and de-multiplexing
(when is receiving).
PHY and MAC will be explained accurately in the chapters 7 and 8 respectively.
In the next figure, we can see the PHY and MAC layer and their respective sublayers.
LOGICAL LINK CONTROL
(LLC)
Data
link
layer
CONVERGENCE SUBLAYER
(CS)
COMMON PART LAYER
(CPS)
PRIVACY SUBLAYER (PS)
PHY
layer
TRANSMISSION CONVERGENCE
SUBLAYER (TCS)
QPSK
16QAM
64QAM
256QAM
QA
Figure 1: Protocol Stack M
2.3.-WiMAX FORUM
WiMAX Forum was formed in April 2001 and it is an organization dedicated to
certifying the interoperability of WiMAX products. It is composed by more than 522
(Intel, AT&T, Samsung, Nokia, Motorola, etc) members comprising the majority of
operators, component and equipment companies in the communications ecosystem.
WiMAX is not a standard, it is a term trademarked by the WiMAX Forum to
describe
802.16-based
technology
and
ETSI
HyperMAN
(European
Telecommunications Standard’s Institute high performance radio MAN). The IEEE
19
WiMAX: IEEE 802.16
elaborates the specifications and leaves to the industry the task of converting them into
an interoperable standard that can be certified.
WiMAX Forum has eight different working groups: application, certification, global
roaming, regulatory, networking, marketing, service provider and technical.
Certification Working Group (CWG): is responsible of certifying the product in a
lab. This process includes two different tests:
- Conformance test to ensure that the products implement correctly the 802.16 and
HyperMAN specifications
- Interoperability test to check if the products of the different vendors work correctly
within the same network
WiMAX Forum defines two types of profiles to address different classes of products
that use the same technology: system profiles and certification profiles.
One system profile is based on 802.16-2004 and it is optimized for fixed and
nomadic access. On the other hand, the second system is based on 802.16-2005 and it is
optimized for mobile access.
The certification profiles are defined by three characteristics: spectrum band,
channel width and duplexing type. The WiMAX Forum has defined five fixed profiles
and fourteen mobile profiles.
If the product passes the interoperability and conformance test, it will achieve the
“WiMAX Forum Certified” designation. Some suppliers include in their products the
“WiMAX-ready”, “WiMAX-Compliant” or “PreWiMAX” but they are not officially
certified.
In the following tables we can see the different WiMAX certification profiles for
fixed and mobile WiMAX:
Fixed WiMAX
Frequency band
3.5 GHz
5.8 GHz
Frequency band
2.3-2.4 GHz
2.305-2.320 GHz,
2.345-2.360 GHz
2.496-2.69 GHz
Channel
Bandwidth(MHz)
3.5
3.5
7
7
10
Mobile WiMAX
Channel
Bandwidth(MHz)
5
8.75
10
3.5
5
10
5
10
Duplexing
TDD
FDD
TDD
FDM
TDD
Duplexing
TDD
TDD
TDD
TDD
TDD
TDD
TDD
TDD
20
2.-WiMAX
3.3-3.4 GHz
3.4-3.8 GHz
3.4-3.6 GHz
3.6-3.8 GHz
5
7
10
5
7
10
TDD
TDD
TDD
TDD
TDD
TDD
Networking Working Group (NWG): Only with the PHY and MAC
specifications is not enough to build an interoperable broadband wireless network and it
is necessary to specify the end-to-end aspects of the network. NWG is in charge of
elaborating these end-to-end aspects within the 802.16 specifications to support fixed,
nomadic and mobile WiMAX systems.
Application Working Group (AWG): The Application Working Group promotes
WiMAX by analyzing WiMAX applications with an engineering focus, developing
recommendations at the application-network interface, and publishing the results.
Marketing Working Group (MWG): Promotes the WiMAX Forum, its brands
and the standards.
Regulatory Working Group (RWG): RWG is the central authority within the
WiMAX Forum on spectrum and regulatory matters. It influences worldwide regulatory
agencies to promote WiMAX-friendly, globally harmonized spectrum allocations.
Service Provider Working Group (SPWG): It gives service providers a platform
to influence BWA product and spectrum requirements to ensure that their individual
market needs are fulfilled. SPWG is the single source for coordinated recommendations
and requirements that drive the network and air interface specifications for WiMAX
networks and products.
Technical Working Group (TWG): It develops technical product specifications
and certification test suites for the air interface based on the OFDMA PHY,
complementary to the IEEE 802.16 standards, primarily for the purpose of
interoperability and certification of MS, BS and SS conforming to the IEEE 802.16
standards.
Global Roaming Working Group (GRWG): Assure availability of global roaming
service for WiMAX networks in a timely manner as demanded by the market.
2.4.-SPECTRUM OPTIONS
As analyzed in the previous subchapters, the frequency range of frequencies where
WiMAX can work is quite wide. However not all the frequencies are used, the operators
usually use the licensed 2.3 GHz and 3.5 GHz bands and the unlicensed 5.725-5.825
GHz band, other frequencies within the frequency range of the standard can be used in
the future. For instance, due to the upgrade from analog television to digital division, a
large amount of spectrum could be available in the band UHF below 800MHz and it
could be used by WiMAX.
21
WiMAX: IEEE 802.16
In the next table, one can see the different frequency bands used depending on the
region:
REGION
EUROPE
USA
CENTRAL AND SOUTH AMERICA
SOUTH-EAST ASIA
MIDDLE EAST AND AFRICA
TYPICAL FREQUENCY BANDS
FOR WIMAX
2.5, 3.5 and 5.8 GHz
2.3, 2.5 and 5.8 GHz
2.3, 2.5 and 5.8 GHz
2.3, 2.5, 3.3, 3.5 and 5.8 GHz
3.5 and 5.8 GHz
22
3.-TECHNICAL FOUNDATIONS OF WiMAX
3.-TECHNICAL FOUNDATIONS OF WiMAX
3.1.-WIRELESS CHANNEL: PATHLOSS AND SHADOWING
One of the main topics in a wireless communications system is the channel; and
there are some factors to be considered. It is required to study all these factors of the
channel to decide the amount of power necessary or the suitable modulation for a
successful communication.
Path loss is the reduction of density or attenuation in an electromagnetic wave as it
propagates through the space. It includes the propagation losses (effects caused by the
expansion of the wave in the free space), absorption losses (when signal penetrates
different not transparent media), diffraction losses (when the signal is obstructed by
some object in its way), connection losses and others phenomena. Path loss is also
influenced by environment (urban or rural), terrain relief, propagation medium, distance
between transmitter and receptor and also the height of the antennas.
The study of these factors is always required in every wireless communications
system and sometimes it could be difficult. Nowadays there are several programs that
allow us to calculate easily all the conditions but the most important formulas in this
chapter will be enunciated.
The free-space path loss formula or Friis formula is:
𝜆2 𝐺𝑡 𝐺𝑟
(4𝜋𝑑)2
where 𝑃𝑟 and 𝑃𝑡 are the received and transmitted power respectively, λ is the
wavelength, 𝐺𝑟 and 𝐺𝑡 are the gains of the transmitter and receiver antennas and d is the
distance between them. It is more usual to use this formula in decibels units:
𝑃𝑟 = 𝑃𝑡
𝑃𝐿 = 32.45 + 20 log 𝑑 𝑘𝑚 + 20𝑙𝑜𝑔𝑓 𝑀𝐻𝑧 − 𝐺𝑟 − 𝐺𝑡
However, the terrestrial propagation environment is not free space so other factors
have to be considered due to the reflections that create interference. In wireless
communications path loss can be represented by the path loss exponent n, whose valor
is usually between 2(free space) and 4 (lossy environments or flat-earth model) and a
constant C measured in a fixed distance.
𝑃𝐿 = 10𝑛 log 𝑑 + 𝐶
Calculations of the path loss are usually called predictions. There are two different
methods to predict the path loss. On the one hand, the statistical methods or empirical
that are based in measured and averaged losses in different radio links, some examples
are COST-231 or Okumura-Hata. On the other hand there are deterministic models
based on the physical laws but they can offer more reliable predictions but requiring
more computational effort.
23
WiMAX: IEEE 802.16
There are many other factors than can degrade the signal strength, for example trees
or buildings located between the transmitter and the receiver. Modeling all the locations
and objects in the environment is impossible so the method consists in introducing a
random effect called shadowing or scale-fading. Although shadowing can sometimes be
beneficial, usually it modifies considerably the system performance because it requires a
several dB margin to be built into the system.
The empirical path loss with shadowing is:
𝑃𝑟 = 𝑃𝑡 𝑃𝑜 𝜒
𝑑𝑜
𝑑
𝛼
where 𝛼 is the path loss exponent, 𝑃𝑜 is the measured path loss at a reference distance of
𝑑𝑜 and 𝜒 is a sample of shadowing process which is modeled as a lognormal random
variable
𝑥
𝜒 = 1010 , 𝑤𝑕𝑒𝑟𝑒 𝑥~𝑁(0, 𝜎𝑠 2 )
and 𝑁(0, 𝜎𝑠 2 ) is a Gaussian distribution with mean 0 and variance 𝜎𝑠 2 .This standard
deviation is formulated in dB and its usual values are in the range 6-12 dB.
3.2.-CELLULAR SYSTEMS
Due to the effects of shadowing and pathloss, it is known that with a given transmit
power the distance of coverage is limited. It is only possible to cover a wide area with a
high amount of power and by increasing the height of the antennas. So pathloss and
short-range transmission allow to have isolated transmitters operating on the same
frequency channel at the same time.
WiMAX systems are expected to work in cellular architecture. In this kind of
systems, the service area is divided in smaller areas called cells and each one has its
own base station. The main advantage is that the system capacity can be increased and
the power consumption reduced but it implies also a higher inversion in equipments.
The increased capacity of the network comes to the fact that same frequency
channels can be reused in different areas for a different transmission. When a cellular
system is going to be built, a frequency planning is required to determine the frequencyreuse factor and a geographical-reuse pattern. The frequency-reuse factor (f) is the rate
at which the same frequency can be used in the network. It is defined as 1/K where K is
the numbers of cells that cannot use the same frequency channel for transmission. So if
f=1 the same frequency can be used in all the cells but the most common values are 1/3,
1/4, 1/7, 1/9 and 1/12. A set of cells with different frequency channels is called cluster.
In a cellular distribution network it is simple to increase the overall system capacity
only by making the cells smaller and turning down the power. Cellular systems support
user mobility so that call transfers from one cell to another or “handoff” has to be
provided.
24
3.-TECHNICAL FOUNDATIONS OF WiMAX
The performance of a cellular system is affected by the cochannel interference that
is caused by the users from the same cell and from the other cells. The other cells
interference (OCI) is a function of the radius of the cell(R) and the distances to the
centre of the neighboring cochannel cell to the radius of the cell (D) and it is
independent from the transmitted power if the size of each cell is the same. The spatial
isolation between cochannel cells is regulated by the parameter “cochannel-reuse ratio”
𝐷
(Q) which in hexagonal cells (the most usual pattern) is 𝑄 = 𝑅 = 𝑁 where N is the
size of the cluster.
Since the interference in some parts of the cell is high and therefore the SIR (signal
interference ratio) is low, the performance can be improved with the sectoring technique
that consists of the use of directional antennas instead of an omnidirectional at the base
station. In this way, if the cell is divided in six sectors, the amount of bandwidth used is
six times less but the overall capacity of the cell is increased more than six times. The
main problem is the requirement of more antennas and the increasing of the intersector
handoffs. The structure of a cluster (six cells with different channel frequency in each
one) is represented in the scheme bellow. The structure of a cellular system consists of
several clusters.
Base Station (BS)
BSC: It controls all the BSs
Figure 2: Cellular System
3.3.-FADING
Fading is caused by the reception of multiple versions of the same signal due to
reflections in the path that are referred to as “multipath”. In the receiver several signals
with different attenuation, phase and delay arrive. The interference caused can be
25
WiMAX: IEEE 802.16
constructive or destructive depending on the phase difference of the arriving signals.
This effect can be dramatic even if only moving a very short distance the transmitter or
the receiver. Fading is a very relevant effect in urban areas with high population density
and indoors.
Doppler spread: is caused when a user or some of the reflectors in the path are
moving and this user’s velocity causes a shift in the frequency of the signal. Depending
on the Doppler shift there are two kinds of fading; fast and slow fading. Doppler spread
(𝑓𝑑) is determinate by the following formula dependent on the carrier frequency (𝑓𝑐),
speed of the light(c) and the maximum speed between the transmitter and the receiver
(v):
𝑓 𝑣𝑓𝑐
𝑑=
𝑐
This measurement unit in the frequency domain is the coherence time(𝑇𝑐 = 1/𝑓𝑑 )
which is a measure of the minimum time required for the magnitude change of the
channel to become decorrelated from its previous value. It has to be compared with the
symbol time. The terms slow and fast fading refer to the rate at which the magnitude
and phase change imposed by the channel on the signal changes
-
-
Fast fading ( 𝑓𝑑 ≫ 1 𝑜𝑟 𝑇𝑐 ≤ 𝑇) consists of fast variations of the amplitude,
phase and a Doppler shift while the transmitter or receiver is moving or the
environment is changing. This fading is produced every fraction of wavelength
(λ) of motion. It can be studied statistically with probability functions like
Rayleigh, Rice and Nakagami.
Slow fading (𝑓𝑑 ≤ 1 or 𝑇𝑐 ≫ 𝑇) consists of the small changes in the amplitude
of the signal caused by the motion of a transmitter or receiver when they are
moving a distance of more than ten times the wavelength. It is also known as
shadowing that is determinate by a statistical log-normal as it is explained in 3.1.
Delay spread: The carrier frequency of the signal is varied but also the amplitude
will vary. The delay spread (τ) measures the amount of time that elapses between the
first arriving path and the last arriving path. In the frequency domain the measurement
unit is the coherence bandwidth (𝐵𝑐 = 1/𝑇), which measures the minimum separation
in frequency after which two signals will experience uncorrelated fading.
-
Flat fading (τ>>T): the coherence bandwidth is larger than the original of the
signals. All frequency components will have the same magnitude of fading.
Frequency-selective fading (τ≤ 𝑇): the coherence bandwidth is smaller than the
original of the signal so the frequency components will experience different
magnitudes of fading.
Mitigation: To mitigate the fading in the channels, these are classified in two
groups depending on the type of fading that they present. The frequency-selective
fading is more prominent in wideband channels (the channel’s bandwidth is bigger than
the coherence bandwidth and the delay spread is smaller than the symbol time). These
kind of channels with time dispersion or frequency selectivity are known like
“broadband fading”. On the other hand, the channels with only frequency dispersion or
time selectivity are “narrowband fading”.
-
In narrowband fading the most usual techniques are:
26
3.-TECHNICAL FOUNDATIONS OF WiMAX



-
Time diversity (interleaving and adaptive modulation) that consists in
introducing redundancy (forward error correction code) in the transmitted
signal.
Spatial diversity consists of two receive antenna to select the stronger of the
two signals receive.
Frequency diversity (the signal is transmitted using several frequency
channels or spread over a wide spectrum).
In broadband fading, frequency-selective fading causes dispersion in time or
intersymbol interference (ISI), meaning that one symbol interferes with the
following symbol.
 The modulation OFDM (Orthogonal Frequency-Division Multiplexing) is
the best method to overcome ISI and it is based on the multicarrier concept.
It consists of rather than sending a single signal with data rate R and
bandwidth B, sending L signals with data rate R/L and bandwidth B/L.
OFDM will be explained in the next chapter.

Equalization is the other technique. The first type is linear equalization
which consists of running the received signal through a filter that models the
inverse of the channel. The other one is the nonlinear equalization that uses
previous symbol decisions made by the receiver to cancel their subsequent
interference.
27
WiMAX: IEEE 802.16
4.-OFDM (Orthogonal Frequency Division
Multiplexing)
4.1.-INTRODUCTION TO DIGITAL MODULATIONS
In telecommunications, modulation is the process of varying a periodic waveform in
order to convey a message. It consists of modifying one of the three parameters:
frequency, amplitude or phase.
Nowadays, all the telecommunications systems use digital modulations. It consists
of modulating an analog signal by a digital bit stream in order to transport it through a
given channel like fiber optics, radio link, wire, etc. It brings some advantages to analog
modulations like better resistance to noise, more immunity to interference, better data
rates, more security, etc. A unique pattern of binary bits is assigned to each frequency,
phase or amplitude.
If the alphabet consists of M = 2N alternative symbols, each symbol represents a
message consisting of N bits. The symbol rate or baud rate is the number of symbol sent
per second (𝐵𝑑 ) and it is measured in bauds. The data rate is the number of bits sent per
second or 𝑁𝑓𝑠 (bits/sec). For example with an alphabet based in 64 symbols, each
symbol is represented by 6 bits and the data rate is six times the symbol rate.
Many digital modulations can be used in telecommunications systems but in the
IEEE 802.16 standard only four of them are supported: BPSK, QPSK, 16-QAM and 64QAM.
1. BPSK or 2-PSK (Binary Phase Shift Keying)
BPSK is a digital modulation that conveys data by changing the phase of a reference
signal. It is the simplest digital phase modulation and each symbol are separated by a
phase of 180º, so the value of the modulated signal can be π or -π. BPSK is the most
robust modulation but the major problem is that is only able to modulate at 1 bit/symbol
so it is not suitable for high data rate communications when bandwidth is limited
I
b0=1
b1=1
Q
Figure 3: BPSK Constellation
28
4.-OFDM (Orthogonal Frequency Division Multiplexing)
2. QPSK or 4-PSK (Quadrature Phase Shift Keying)
QPSK is also a digital modulation that changes the phase of a reference signal. It
considers two-bit modulation symbol. Many variations of QPSK can be used but all of
them have 4-points constellation. The decision in the receiver is more difficult than in
BPSK where the decision was between “1” and “0”. Now, the system has to differ
between 4 symbols with different phases (π/4, 3π/4, 5π/4 and 7π/4). It is less resistant to
noise and presents less immunity to interference but it is more spectral efficient.
I
01
00
Q
11
10
Figure 4: QPSK Constellation
3. Quadrature Amplitude Modulation(16-QAM and 64-QAM)
QAM is a digital modulation that conveys data by changing the amplitude of two
carrier waves. These two waves are out of phase with each other by 90º. When a signal
is transmitted with QAM is characterized by the following formula:
𝑆 𝑡 = 𝐼 𝑡 cos 2𝜋𝑓𝑜 𝑡 + 𝑄 𝑡 sin⁡
(2𝜋𝑓𝑜 𝑡)
At the receiver this signal can be demodulated using a coherent demodulator.
It should be mentioned that QPSK and 4-QAM are the same modulation. 16-QAM
and 64-QAM are the two amplitude modulations that can be used according to the IEEE
802.16 standard. The most spectral efficient modulation is 64-QAM (6 bits/symbol are
transmitted).
1011 1001
I 0001 0011
1010 1000
0000 0010
Q
1110 1100
0100 0110
1111 1101
0101 0111
Figure 5: 16QAM Constellation
Finally, one has to consider that having more than one modulation could be
interesting for the systems because it allows link adaptation. It means that depending on
the channel conditions one or another modulation can be used. If the radio link is good a
29
WiMAX: IEEE 802.16
high-level modulation can be used. Otherwise, a low-level and robust modulation can
be used.
4.2.-MULTICARRIER MODULATION
The main goal of a multicarrier modulation is a high data rate communication and
an ISI-free channel because a digital communication system cannot work in its
presence. In order to have a channel without ISI the symbol time (T) has to be larger
than the channel delay spread τ.
In wideband channels the desired symbol time is usually much smaller than the
channel delay spread, so intersymbol interference is considerable. In order to solve this
problem, the high data-rate bit stream is divided in L low-rate bit streams, which are
sent over L different orthogonal-frequency subchannel. Each of these substreams
has𝑇𝑠 /𝐿 >> 𝜏, so in this way the channel is ISI-free.
The number of the subcarriers chosen to ensure that each subchannel have a
bandwidth less than the coherence bandwidth of the channel ( 𝐵/𝐿 ≪ 𝐵𝑐 ) , which
ensures flat fading.
However, multicarrier modulation has some problems such as a large bandwidth
penalty will be imposed due to the fact that the subcarriers cannot have perfectly
rectangular pulse shapes and they will be still time-limited. Moreover, this kind of
modulation requires very high quality low-pass filters to maintain the orthogonality of
the subcarriers at the receiver, as well as L independent RF units and demodulation
paths.
4.3.-OFDM BASICS
OFDM is a frequency-division multiplexing digital multi-carrier modulation
scheme, which popularity comes from the high data rates. It can reach that due to its
efficient and flexible management of the intersymbol interference.
OFDM is based in the principle of transmitting simultaneously many narrow-band
orthogonal frequencies called OFDM subcarriers which are designed not to interfere
with each other and they can be separated using FFT (Fast Fourier Transform)
algorithm. These frequencies are orthogonal with each other, allowing a high spectral
efficiency. Each subcarrier can be modulated with one different conventional
modulation scheme. These subcarriers have smaller bandwidth than a single carrier so
they present a better resistance to multipath propagation.
30
4.-OFDM (Orthogonal Frequency Division Multiplexing)
Figure 6: OFDM Spectrum(www.radio-electronics.com)
The OFDM baseband signal transmitted is determined by:
𝑥 𝑡 =
𝐿−1
𝑖=0 𝑠
𝑖 𝑒 −2𝜋𝑗
Δ𝑓+𝑖𝐵𝑐 𝑡
0 ≤ 𝑡 ≤T
where 𝑠 𝑖 is the symbol carried in the 𝑖th subcarrier, 𝐵𝑐 is the distance between two
adjacent subcarriers or subcarrier bandwidth and Δ𝑓 is the frequency of the first
subcarrier and T is the useful symbol duration.
After a brief explanation of what OFDM is, some of the principles and characteristic
will be explained:
-
Orthogonality
In OFDM, the subcarrier frequencies are chosen in such a way that the interference
between the subchannels or co-channel interference is eliminated. This fact simplifies
the design of the transmitter and the receiver owing to only a filter is needed and not
one for each subchannel.
OFDM requires very accurate frequency synchronization between the transmitter
and the receiver. If there is some frequency deviation the subcarriers will not be
orthogonal anymore. Frequency offsets are usually caused by mismatched transmitter
and receiver oscillators and by Doppler shift due to motion. This fact can be solved at
the receiver but if it is combined with multipath, the reflections will appear and it will
be harder to correct. These problems become worse with vehicular –speed movements.
-
DFT (Discrete Fourier Transform) or FFT(Fast Fourier Transform)
OFDM uses an efficient computational technical known as DFT or FFT (matrix
computation that allows DFT to be computed). It allows us to relate events in time
domain to frequency domain. This technique and its inverse IFFT (Inverse Fast Fourier
Transform) are able to create a multitude of orthogonal subcarriers using only a single
radio.
OFDM symbol can be recovered easily by simply computing. Although the usage
the FFT the ISI has been mitigated, the symbol is imperfect due to cochannel
interference, additive noise and other imperfections
-
Guard interval
An OFDM symbol means a group of L data symbols (all the data symbols
transmitted in parallel) and it lasts T seconds, where 𝑇 = 𝐿𝑇𝑠 . As the spectrum of
OFDM is not band limited (sinc(f) function), linear distortion caused by multipath can
31
WiMAX: IEEE 802.16
cause ISI. In order to avoid this effect, it is important to transmit a guard interval
between OFDM symbols. The duration of each guard interval (𝑇𝑔 ) has to be longer than
the delay spread (τ) of the channel to ensure that each symbol interferes only with itself.
After its introduction the duration of each symbol is 𝑇𝑡𝑜𝑡𝑎𝑙 = 𝑇 + 𝑇𝑔 . Its introduction
also reduces the synchronization problems.
The ratio TG/Td is very often denoted G in WiMAX/802.16 documents. If the
channel conditions are good a lighter value of G has to be used and if the multipath
effect is important and the channel is bad a high value of G is required. For OFDM and
OFDMA PHY layers, 802.16 defines the following values for G: 1/4, 1/8, 1/16 and
1/32. For the mobile WiMAX profiles defined, only the value 1/8 is mandatory.
-
Cyclic prefix
One way to prevent ISI is the introduction of a cyclic prefix for the guard interval. It
is transmitted during the guard interval and consists of a copy of the end of the OFDM
symbol. For instance, if the maximum channel delay spread is v+1 samples, it will be
necessary to add a sequence of v samples between each symbol in order to make
independent from the precedent symbol and the next one.
The use of cyclic prefix supposes a power and bandwidth penalty. Since v redundant
𝑣
symbols are sent, the required bandwidth is increased from 𝐵 to 𝐿 + 𝐿 𝐵 and the
𝐿
power loss is determinate by 𝐿+𝑣.
-
Types of subcarriers
Not all the subcarriers convey useful data; there are different types of subcarriers
depending on their function in the system:
1. Data subcarrier: useful data transmission
2. Pilot subcarrier: used for channel estimation and synchronization
3. Null subcarriers: no transmission and used in guard bands
4. Direct Current (DC) subcarrier: it is another null subcarrier but in OFDM is
the carrier with the same frequency than the centre frequency of the
transmitter.
-
Frequency equalization
The performance of equalization in OFDM is simpler than in a conventional singlecarrier modulation. Fading caused by the multipath propagation can be considered flat if
the sub-channel is sufficiently narrow-banded and hence when the number of subchannels is large.
The complex channel gains (the amplitude and the phase) for each subcarrier must
be known in order to estimate the received symbols. After the FFT, the data symbols are
estimated using a one-tap frequency-domain equalizer or FEQ (Frequency Domain
𝑌
Equalizator) whose function is determinate by 𝑋 = 𝐻1 where 𝑌1 is the received signal
1
and 𝐻1 is the response of the channel. Therefore, the FEQ corrects the phase and
equalizes the amplitude before the decision device.
32
4.-OFDM (Orthogonal Frequency Division Multiplexing)
-
Timing and Frequency Synchronization
The receiver has to perform two important actions to achieve a successful
communication: timing and frequency synchronization. The first one consists on
determining the timing offset of the symbol and the optimal timing instants; however
the OFDM symbol structure allows a degree of error. The frequency synchronization,
which can modify the orthogonality and hencemis more stringent, consists of aligning
the carrier frequency as much as possible with the transmitted carrier frequency.
-
Peak-to-Average Ratio (PAR)
OFDM signals have a higher peak-to-average-ratio than single-carriers signals do.
The sum of all the narrow-band carriers in the time domain can be large sometimes and
others can be small, so the peak value is larger than the average value. A high PAR
represents a hard problem for the devices such as high-power (HPA) amplifiers or
digital-to-analog converter (DAC), so this fact causes the requirement of more accurate
and, hence expensive devices.
If a high peak signal is transmitted through a nonlinear device like HPA or DAC, it
generates in-band distortion and out-of-band energy. One of the solutions for this
problem is transmitting a waveform with high peak power in the linear region of the
HPA in order to decrease the average power in the input signal. It is called input backoff
(IBO) and results in an output backoff (OBO). A high backoff can reduce the battery
𝑃
life and the power efficiency. The IBO is defined as: 10𝑙𝑜𝑔10 𝑖𝑛𝑠𝑎𝑡
where 𝑃𝑖𝑛𝑠𝑎𝑡 is the
𝑃
𝑖𝑛
saturation power and 𝑃𝑖𝑛 is the average input power. The typical value of IBO is similar
to the PAR of the signal.
There are several techniques to reduce the PAR effect:
- Clipping: reduces the input power by an amount equal to the PAR and also
truncates the amplitude of signals that exceed one determinate clipping level. It
can be combined with filtering process.
- Tone reservation: adds power to unused carriers like null carriers
- Active constellation extension: based on extending the corner points of an MQAM.
-
OFDM Symbol parameters used in WiMAX
An OFDM symbol is characterized by four different parameters. In the next table,
one can analyze these parameters and their possible values:
33
WiMAX: IEEE 802.16
Parameter
BW
𝑁𝑠
G
n
Description
Nominal channel
bandwidth
Number of subcarriers,
including the DC, pilot and
guard subcarriers
Ratio of Cyclic prefix (CP)
time
Sampling factor(dependent
on BW)
Possible Values
In MHz: 1.25, 1.75, 3.5, 5,
7, 8.75(WiBro), 10, 14 , 15
OFDM: 256
SOFDMA: 128, 512,
1,024, 2,048
1/4, 1/8, 1/16, 1/32
OFDM:
BW multiples ofn
1.25144/125
1.586/75
1.758/7
2.057/50
2.75316/275
Other8/7
OFDMA:
BW multiples of
1.758/7
1.25,1.5,2 or 2.7528/25
Other8/7
4.4.-OFDMA (Orthogonal Frequency Division Multiplexing
Access)
The problem in the access to the network in WiMAX is that many users in the same
geographic area require high data rates in a finite bandwidth and with low latency. To
solve this problem WiMAX uses OFDMA, a multi-user version of OFDM. It is a
technique that creates independent streams of data assigning subsets of subcarriers to
users depending on the demand of each one. In the next figure, an OFDMA spectrum
with 2 users and their corresponding subcarriers assigne:
Figure 7: OFDMA subcarriers
Another important function of OFDM is the adaptive user-to-subcarrier assignment
based on feedback information of the channel conditions. If the assignment is done fast,
it can improve the resistance of OFDM to narrow-band co-channel interference and fast
fading.
34
4.-OFDM (Orthogonal Frequency Division Multiplexing)
There are multiple-access strategies for OFDM to divide the available dimensions
(frequency, time or code division multiplexing) among all the users. In FDMA
(Frequency Division Multiplexing Access) eight frequency slots will be created, one for
each user. In TDMA (Time Division Multiplexing Access), the user will use the eight
time slot created but it is only possible transmitting one eight of the time. Finally, with
CDMA (Code Division Multiplexing Access) each user will transmit all of the time
over all the frequencies but would use one of the eight available orthogonal codes to
avoid the interference with the other seven users.
The major problem of these strategies is that they only ensure the orthogonality
among the users of the same cell.
OFDMA is a mix of FDMA and TDMA, it presents the same advantages than
single-user OFDM like multipath resistance or frequency diversity but it can serve
many different users with different data rates, QoS (Quality of Service) requirements or
using different user’s applications. However, OFDMA has more advantages with regard
to OFDM like its potential to reduce the transmit power, to reduce the PAR problem
and even better spectral efficiency.
One of the advantages of OFDMA is the multiuser diversity which demonstrates
that as the number of user increases, the probability of getting a large channel gain
improves. In WiMAX this gain will be reduced by effects like spatial diversity and the
need to assign users contiguous blocks of subcarriers.
As it was said, WiMAX systems use adaptive modulation and coding based on
feedback information, which means transmitting as high data rate as possible when the
channel conditions are good and transmit a lower data rate when the channels conditions
are poor. A high data rate can be achieved using large constellations such 64-QAM and
less robust-error codes like 3/4 convolutional or turbo codes. On the opposite to this, to
achieve a low data rate, a small or constellation such QPSK has to be used with a
correction rate code of 1/2 or turbo codes. Feedback is critical to apply adaptive
modulation and coding, in addition to the transmitter has to know the channel SINR
(Signal to Interference plus Noise Ratio) to determine the optimum modulation and
transmit power. In the next table, the different data rates depending on the channel
bandwidth, modulation and code rate are presented assuming that the daa rate is shared
among the users in the sector with a downlink-to-uplink bandwidth ratio of 3:1 in TDD
mode:
35
WiMAX: IEEE 802.16
In WiMAX the feedback channel is protected with error correction but the main
problem to its performance is the loss of data due to mobility with vehicular speeds
(more than 20km/h) or carrier frequencies bigger than 2,100 MHz.
Resource-Allocation Techniques for OFDMA
WiMAX standard does not specify algorithms to determine which users to schedule,
how to assign the subcarriers to them or how to determine the appropriate power level
for each user on each subcarrier.
The resource allocation is usually used for two different optimizations: minimize the
total transmit power with a fixed data rate or maximize the data rate with a forced
transmit power. The different algorithms depending on the data rate are:
-
-
-
Maximum Sum Rate Algorithm (MSR): It is used to maximize the sum rate of
all users given a limited transmit power. It is optimal if the goal is to get as
much data as possible through the system but the main problem is that only can
work with excellent channels so it is only achievable for users close to the base
station.
Maximum Fairness Algorithm (MF): In a cellular system as WiMAX the path
loss varies seriously between users due to the different distances of them from
the BS, so it is known that with MSR algorithm, not all the users can be served.
At least that allows the underserved users to get some throughput. MF’s goal is
to allocate the subcarriers and power such that the minimum user’s data rate is
maximized.
Proportional Rate Constraints Algorithm (PRC): In the MF, the throughput
is determined by the user with worst SINR so most of resources are assigned to
that user. Its goal is to maximize the total throughput but with the additional
36
5.-PHY LAYER
-
-
constraint that each user’s data is proportional to a set of predetermined system
parameters.
Proportional Fairness Scheduling (PFS): The previous algorithms achieve
different objectives such as the total sum throughput (MSR), maximum fairness
(equal data rates among users) or set proportional rates for each user. In
addition, other fact has to be considered: the latency. Latency is unacceptable so
PFS is an algorithm that balances throughput and latency and achieves some
degree of fairness.
Comparison: Depending on the system conditions, one or another algorithm
will be the most appropriated. In the next table there is a comparison of them:
Algorithm
Max Sum Rate
Max Fairness
Proportional
Rate
Constraints
Proportional
Fairness
Sum Capacity
Best
Poor
Good
Fairness
Poor
Best
Most flexible
Complexity
Low
Medium
High
Good
Flexible
Low
5.-PHY LAYER
The physical layer (PHY) is based on the 802.16-2004 and 802.16e-2005 standards
and it is strongly influenced by WiFi technology. In the standards we can distinguish
four different formats of physical layer:
-
-
-
WirelessMAN SC: a single carrier PHY layer to work in frequency bands bigger
than 11GHz b and requires LOS propagation conditions
WirelessMAN SCa: a single carrier PHY layer to work in 2-11 GHz frequency
bands and requires NLOS propagation conditions and point-to-multipoint
communications
WirelessMAN OFDM: a 256-FFT-based OFDM PHY layer for point-tomultipoint communications in NLOS conditions and working at frequencies
between 2 and 11 GHz.
WirelessMAN OFDMA: a 2048-FFT-based OFDMA PHY layer for point-tomultipoint communications in NLOS conditions and working at frequencies
between 2 and 11 GHz. In 802.16-2005 the specifications have been modified to
SOFDMA (scalable OFDMA). Different sizes of FFT can be used: 128, 512,
1024, and 2048.
5.1- CHANNEL CODING
37
WiMAX: IEEE 802.16
Randomisation
FEC encoder
Interleaving
Repetition
Modulation
Figure 9: OFDMA transmission chain
Randomisation
FEC encoder
Interleaving
Modulation
Figure 8: OFDM transmission chain
The radio link suffers a great variation from the interference. In order to prevent and
to correct the errors caused, a good performance of the channel coding is required. The
PHY transmission chains of OFDM and OFDMA are illustrated in the following
figures, the only difference is that ODMA PHY includes a repetition block.
5.1.1- Randomization
It is performed in the uplink and in the downlink to all the blocks, except to the FEC
(Frame Control Header) block, using the output of a length shift-register sequence that
is initialized at the beginning of every FEC block. This shift-register sequence added
with the data sequence creates the randomized data. If the amount of data to transmit
does not fit exactly with the amount of data allocated, padding with only ones (0xFF) is
added at the end of the block.
5.1.2- Forward Error Codes (FEC)
A FEC consisting of the concatenation of Red-Solomon outer code and an inner
code has to be supported in the uplink and the downlink. It will be used always as the
code mode to request access to the network and in the FCH burst.
In OFDM PHY there are three different FEC to be applied: Reed-Solomon
Convolutional Code (RS-CC) which is mandatory, Convolutional Turbo Code (CTC)
and Block Turbo Code (BTC) that are both optional.
In OFMA PHY there are four different FEC to be applied; Tail-biting Convolutional
Code (CC) which is mandatory and three that are optional: Convolutional Code (CTC),
Block Turbo Code (BTC) and Low Density Parity Check (LDPC).
38
5.-PHY LAYER
-
Convolutional Codes
For OFDM PHY layer, Reed-Solomon Convolutional Code (RS-CC) is performed
by first passing the data in block format through the RS encoder and then passing it
through a convolutional encoder. This code consists on adding some redundant bits to
the digital sequence.
Reed-Solomon Code is defined by some parameters: RS (N=256, K=239) where N
is the number of overall bytes after encoding and K is the number of data bytes before
encoding and finally T= (N-K)/2=8 is the maximum number of data bytes with error
that can be corrected. So the code rate of OFDM PHY is 239/256.
The convolution coding has an original rate of 1/2 and a single 0x00 tail byte is
added at the end of the burst after randomization. In the RS encoder, the redundant bits
are sent before the input bits keeping the 0x00 tail byte at the end of the allocation
X output
+
Data in
1
bit
delay
1
bit
delay
1
bit
delay
1
bit
delay
1
bit
delay
1
bit
delay
+
Y output
Figure 10: Convolutional Encoder
For OFDMA PHY the mandatory scheme used is also based on convolutional code
(CC). The convolutional encoder uses an encoder with a constraint length 7 and native
code rate ½. Tail-biting is used to initialize the encoder. The encoder memorizes the last
6 bits from the end of the data block and they are appended to the beginning, to be used
as flush bits. These bits flush out the bits left in the encoder by the precious FEC block.
The first 12 parity bits are generated by the convolutional encoder which depend on the
6 bits left of the previous block.
-
Turbo codes
39
WiMAX: IEEE 802.16
Convolutional Turbo Codes (CTC)
It is mandatory for mobile and optional for fixed WiMAX. WiMAX uses duobinary
turbo codes with an encoder of constraint length 4. In this kind of encoders, two bits
from the uncoded sequence are used simultaneously as input. The duobinary
convolution encoder has two generating polynomials: 1 + 𝐷2 + 𝐷3 and 1 + 𝐷3 . Thus,
the encoder has four possible transitions compared the two transitions of a binary turbo
encoder.
Figure 11: OFDMA PHY Convolutional Turbo Code (CTC) Encoder
The output of the native 1/3 coding rate encoder is first separated in six different
blocks(A,B, Y1, Y2, W1 and W2) where A and B contain the system bits, Y1 and W1
contain the parity bits of the encoded sequence in natural order, and Y2 and W2 contain
the parity bits of the interleaved sequence. Each of the blocks is independently
interleaved and the subblocks that contain the parity bits are punctured (remove some of
the parity bits after encoding) to achieve the target code rate.
Block Turbo Codes and Low-Density Parity Check
Although these codes are defined in the standards as optional channel coding
schemes, they are unlikely to be employed in WiMAX. The reason is that most
equipment manufacturers decided to use Convolutional Turbo Codes (CTC) for their
superior performance and advantages.
40
5.-PHY LAYER
5.1.3.-Interleaving
After channel coding, the next step is interleaving. It is used to protect the
communication against long sequence of consecutive errors. These errors may affect a
lot of bits in a row and therefore disable the communication. The encoded data bits are
interleaved with a block of the size of coded bits. Interleaving is applied independently
on each FEC block and is based on two steps:
1- Ensures that adjacent coded bits are allocated in nonadjacent subcarriers. The
distance between the subcarriers to which two adjacent bits are mapped on,
depends on the subcarrier permutation schemes used.
2- Adjacent bits are alternately mapped onto less and more significant bits of the
modulation constellation.
5.1.4.-Repetition
Repetition was added in the standard 802.16e-2005 for OFDMA PHY. It can be
used to increase the margin more than using only FEC mechanisms.
This process is characterized by the repetition factor (R) that can be 2, 4 or 6. Thus,
for the uplink, the number of allocated slots Ns will be a whole multiple of the
repetition factor. However, for the downlink, the number of allocated slots Ns will be in
the range of [R x K, R x K + (R-1)] where K is the number of required slots before the
repetition. For instance, if R=4 the number of the allocated slots (Ns) will be in the
range of 40 and 43 for the burst.
The binary data that can fit in a region that is repetition coded is reduced by a factor
R compared to non repeated region with the same size and same FEC technique used.
The repetition scheme can be used only with QPSK modulation with every type of
coding schemes except applying HARQ with CTC.
5.2.-HYBRID-ARQ
The Hybrid-ARQ uses an additional error code to ensure a more reliable transmission of
data. There are two different types of HARQ that are supported by WiMAX.
In HARQ Type I or “chase combining”, the redundancy bits are not changed from one
transmission to the next. When the coded data block is received, the receiver first checks the
error-correction code, if the channel quality is good enough all the errors should be detected and
it will be possible to obtain the correct data block. However, if the channel quality is not good,
the receiver requests a retransmission of the data block. The current data block is combined with
the previous discarded data blocks stored at the receiver to decode correctly the information.
This process continues until either the block is decoded without error or the maximum number
of allowed transmissions is reached.
In HARQ Type II or also referred to as “incremental redundancy”, the redundancy version
of the encoded bits is changed from one transmission to the next. Thus, it consists on increasing
the redundancy bits in each transmission. The first transmission contains only data and error
detection and if the reception is error free, the data block is decoded. However, if data is
41
WiMAX: IEEE 802.16
received in error, the second transmission will contain FEC parities and error detection, so the
code rate is reduced. If the data block is received with error, error correction can be attempted
by combining the information received from both transmissions. Type II allows a lower bit error
rate (BER) and block error rate (BLER) than Type I.
5.3.-TRANSMISSION CONVERGENCE SUBLAYER (TCS)
It is an option in WiMAX that is located between MAC and PHY layer. If TCS is
enabled, it converts the variable-length MAC PDUs into fixed-length FEC blocks,
called TC PDU. A pointer is added at the beginning of each TC PDU to indicate the
header of the first MAC PDU.
5.4.-SUBCHANNEL AND SUBCARRIER PERMUTATION
In OFDMA, a subchannel is defined as a subset of subcarriers that are assigned to
the users. Subcarriers of a subchannel can be adjacent to each other or distributed
through the entire frequency band depending on the subcarrier permutation mode. A
distributed subcarrier permutation provides better frequency diversity, while an adjacent
subcarrier distribution is better for beamforming and for the exploitation of the
multiuser diversity. The number of subchannel allocated to transmit a data block
depends on the size of the data block, coding rate used and modulation format.
The contiguous sets of subchannels assigned to a single user or group of users is
defined as “data region” and is always transmitted with the same burst profile (one of
the 52 different combinations of modulation format, code rate and type of FEC allowed
in WiMAX). The various permutations schemes allowed are:
5.4.1.-Downlink Full Usage of Subcarriers (DL FUSC)
All the data subcarriers are used to create various subchannels. Each subchannel is
made up of 48 data subcarriers, which are distributed evenly throughout the entire
frequency band to counter the effects of fading channels. The pilot subcarriers are
allocated first and the rest of the subcarriers are mapped on the various subchannels.
The set of the pilot subcarriers is divided in two constant sets and two variable sets. The
variable sets are used to estimate the channel response across the entire frequency band
and the constant sets are just based on the OFDM symbol duration and subcarriers
spacing.
5.4.2.-Downlink Partial Usage of Subcarriers (DL PUSC)
The main difference between FUSC and PUSC is that in PUSC all the subcarriers
are divided in six groups. The first step is arranging all the subcarriers except null
subcarriers in a cluster which is formed by 14 adjacent subcarriers over 2 OFDM
symbols. In each cluster, subcarriers are divided in 24 data subcarriers and 4 pilot
subcarriers. After that, the different clusters are renumbered with a pseudorandom
42
5.-PHY LAYER
scheme and divided in 6 groups. A subchannel will be formed of two clusters of the
same group.
It is possible to allocate all or only one subset of the six groups to a transmitter but it
is useful to allocate separated subsets to neighboring transmitters in order to separate
their signals in the subcarrier space. Thus, it is possible to use a segmentation scheme
and all the sectors in a BS can use the same RF channel maintaining orthogonality
among the subcarriers.
5.4.3.-Uplink Partial Usage of Subcarriers (UL PUSC)
The subcarriers are first divided in tiles and each tile consists of 4 subcarriers over
three OFDM symbols. The subcarriers of a tile are divided in eight data subcarrier and
four pilot subcarriers (the ones of the corners).
There is another optional UL PUSC where each tile is composed by three
subcarriers over three OFDM symbols. Each tile is formed by eight data subcarrier and
one pilot subcarrier.
After that, the tiles are renumbered using a pseudorandom numbering sequence and
they are divided in six groups. Each subchannel is created using six tiles from a single
group.
5.4.4.-Band Adaptive Modulation and Coding (AMC)
All subcarriers constituting a subchannel are adjacent to each other. In this way, it is
easier the exploitation of multiuser diversity, although frequency diversity is lost.
In this permutation (the same for the uplink and downlink), nine adjacent subcarriers
with eight data subcarriers and one pilot subcarrier are used to form a bin. A group of
four rows of bins is called a physical band. An AMC subchannel consists of six
contiguous bins within the same logical band (group of physical bands). Thus, an AMC
subchannel can be formed of one bin over six consecutive symbols, two consecutive
bins over three consecutive symbols or three consecutive bins over two consecutive
symbols.
5.5.-RANGING
In 802.16e-2005, ranging is an uplink physical layer procedure to maintain the
quality of the radio-link communication between the MS and BS. The BS receives the
ranging information from the SS and it processes the signal to determine some
parameters such SINR and time of arrival which allows the BS to indicate the MS
adjustments in the transmit power level or the timing offset.
The ranging procedure involves the transmission of the ranging code repeated over
two OFDM symbols using the ranging channel. This ranging code is a PN sequence of
length 144 and chosen from a set of 256 codes. There are four different types of code
and each one has a determined function. The group N is used for initial ranging, M for
43
WiMAX: IEEE 802.16
periodic ranging, O for bandwidth request and S for handover ranging. This sequence is
modulated in BPSK.
5.6.-SLOT AND FRAME STRUCTURE
The smallest unit of PHY layer resource that can be allocated to a single user in time
or frequency domain is a slot.
In 802.16, both FDD (frequency division duplexing) and TDD (time division
duplexing) are allowed. In FDD, the uplink and downlink subframes are transmitted
simultaneously on different carrier frequencies and a fixed duration of the frame is
established. For mobile stations, there is an additional duplexing mode called H-FDD
(Half-duplex FDD) with the restriction that the MS cannot transmit and receive at the
same time. In full duplex the SS is continuously listening the channel downlink, while
in half-duplex only can listen when it is not transmitting. However, in TDD the uplink
and downlink subframes are transmitted on the same carrier frequencies but at different
times. A frame contains the uplink and downlink subframe and has a fixed duration but
the bandwidth does not have to be divided in two equal parts.
Comparing the two techniques, FDD has a fixed duration for the uplink and
downlink, while TDD is adaptive. Thus it is more suitable for asymmetrical traffic like
Internet. The frame structure of TDD and FDD is the same, except that in FDD, the UL
and DL subframes are multiplexed on different carrier frequencies.
5.6.1.-OFDM PHY Downlink Subframe
It consists of only one PHY PDU that can be shared by several SSs. It starts with
one preamble that allows timing and frequency synchronization to listen to the SSs and
also initial channel estimation, noise estimation and interference estimation. It is
modulated in BPSK (the most robust modulation allowed).
The downlink preamble is followed by a frame control header (FCH) that contains
the Downlink Frame Prefix (DLFP), which provides frame configuration information,
such as the MAP message length, the modulation, and coding scheme, and the utilizable
subcarriers. It is coded with BPSK and a code rate of 1/2.
If DL-MAP or UL-MAP is transmitted (a case where they are not necessary: the
DLFP indicates all the burst profiles of the downlink subframe), it will be allocated in
the first MAC PDU after the FCH and it will indicate the data regions of the various
users in the DL and UL. MAP messages include the burst profile for each user, which
defines the modulation and coding scheme used in that link. As MAP contains critical
information that needs to reach all users, it is often sent over a very reliable link, such as
BPSK with rate 1/2 coding and repetition coding. Using these messages, the SS can
identify which subchannels are for its use.
Periodically, the downlink channel descriptor (DCD) and the uplink channel
descriptor (UCD) are transmitted following the UL-MAP or DL-MAP message with
44
5.-PHY LAYER
additional control information to the description of channel structure and the burst
profiles allowed by the BS.
FCH is followed by one or more downlink burst. The same burst profile can be used
one or more times but these burst profiles are transmitted in order of decreasing
robustness of their burst profiles.
Figure 12: OFDM PHY DL subframe
5.6.2.-OFDM PHY Uplink Subframe
An OFDM Uplink Subframe consists of three different parts, with this order:
-
Contention slots allowing initial ranging: the BS specifies an interval in which
new stations can join the network
Contention slots allowing bandwidth requests: the BS specifies an uplink
interval in which requesting bandwidth for uplink data transmission is possible
One or many uplink PHY PDU and each one transmitted on a burst. Each PDU
is an uplink subframe transmitted from a different SS
45
WiMAX: IEEE 802.16
Figure 13: OFDM PHY UL subframe
5.6.3-OFDMA PHY Frame
In OFDMA, the frame structure is different due to the subcarriers distribution. There
are some non-mandatory elements present in the frame structure, so it is possible to find
different distributions.
Each DL and UL is divided into various zones, each of these using different
subcarrier permutation modes. Some of the zones as DL PUSC are mandatory and the
rest are optional. The size of the slot is dependent on the permutation scheme used:
-
FUSC: Each slot is 48 subcarriers by one OFDM symbols
DL PUSC: Each slot is 24 subcarriers by two OFDM symbols
UL PUSC: Each slot is 16 subcarriers by three OFDM symbols
Band AMC: Each slot is 8,16,24 subcarriers by 6,3, or 2 OFDM symbols
The first OFDM symbol in the DL is used as preamble for timing and frequency
synchronization, initial channel estimation and noise and interference estimation. BPSK
is used to create the preamble in the frequency domain.
The interval between two consecutive DL frame preambles is defined as frame
length; it is variable between 2msec and 20 msec.
After the DL frame preamble, the initial subchannels are used to allocate the frame
control header (FCH). It contains the DL_Frame_Prefix which contains information
about the DL-MAP duration, ranging subchannels and system control information. FCH
is transmitted with QPSK rate 1/2 and with four repetitions. DL-MAP and UL-MAP are
transmitted after FCH to specify the data regions and the different transitions between
zones of the various users in the DL and UL subframes of the current frame.
Periodically DCD and UCD are transmitted like in OFDM PHY uplink.
46
5.-PHY LAYER
The uplink subframe includes:
-
-
Allocation for ranging
Allocation for data transmission
Fast feedback slot and it also contents some performance information such as
handover operation. BS can include a Channel Quality Information Channel for
periodic CINR (Carrier Interference Noise Ratio) reports.
Other optional signaling data allocation and subchannels
In the next figure the TDD frame structure for Mobile WiMAX is represented:
Figure 14: TDD frame structure for Mobile WiMAX
5.7.-POWER CONTROL
The BS uses the ranging channel transmissions of various MSs to estimate the initial
and periodic adjustments for power control in order to maintain the quality of the
communication between BS and MS and to reduce the interference. The BS uses the
MAC management messages to indicate the correction in the power-level to the MS.
The requirements of this mechanism are:
-
Supporting power attenuation due to distance loss or power fluctuations at rates
of 30dB/s with depths of at least 10 dB
Taking into account the interaction of the power amplifier and the different burst
profile. PAR depends on modulation.
The SS should maintain the same transmitted power density unless the
maximum power level is reached. If the subchannels allocated to a SS are
47
WiMAX: IEEE 802.16
increased or decreased, the power level has to be increased or decreased in the
same proportion.
The MS reports to the BS the maximum available power and the transmitted
power and the BS has to adjust to these parameters for an optimal assignment of the
subchannels and an optimal burst profile.
5.8.-CHANNEL-QUALITY MEASUREMENTS
The downlink power-control process, adaptive modulation and adaptive code rate
are based on the measurements of the channel quality. These measurements are RSSI
(Received Signal Strength Indicator) and SINR (Signal to Interference plus Noise Ratio)
and they are transmitted to the BS by the SS. Based on this information provided, the
BS can change the burst profile or change the power level of the DL transmissions.
48
6.-MAC LAYER
6.-MAC LAYER
MAC (Media Access Control) is a part of the layer 2 (Data Link) of OSI (Open
System Interconnect) stack. It is responsible for controlling and multiplexing various
such links over the same physical medium. As mentioned in section 2.2, MAC is
composed by three different sublayers: MAC Convergence Sublayer (CS), MAC
Common Part Sublayer (CPS) and MAC Security Sublayer.
6.1.-MAC CONVERGENCE SUBLAYER
MAC Convergence Sublayer or often referred as CS, is the top sublayer of the MAC
layer and the interface with the layer 3. It provides mapping of external data, received
from the higher layers through the CS service access point (SAP), into MAC Service
Data Unit (SDU). These SDUs are transmitted to the MAC CPS where the MAC
procedures are applied. It is also responsible for packet header suppression.
WiMAX MAC Layer is connection oriented and the logical connection between the
BS and MS is identified by CID (unidirectional connection identifier). SDUs of a
specific destination address can be carried over different connections, depending on the
QoS requirements, and the CS has to determine the corresponding CID.
One of the functions of CS is removing the repetitive part of each SDU to improve
the efficiency of the network. For instance, if the SDUs are IP packets, the address
contained will not change from one packet to next one so, they can be deleted. There are
different packet header suppression rules depending on the service provided. Thus, the
CS determines the part of the header to be suppressed (PHS field) using the PHS mask.
Moreover, there is another function known as PHS verify, if it is enabled the CS
compares the bits of the PHS field with the ones expected. After that, if the new PHS
field matched with the PHS field cached, the header is deleted, whereas if PHS is not
used, CS suppresses all the SDU’s header.
The PHS rules have to be known by the transmitter and the receiver so, the BS sends
a dynamic service allocation (DSA) or a dynamic service change (DSC) with all of the
parameters required in the PHS rule in order to synchronize.
6.2.-MAC PDU OR MAC FRAME
Each MAC PDU or MAC frame consists of one fixed-length header followed by a
payload and a cyclic redundancy check (CRC) which is calculated in all the PDU
(header and payload). The payload can be formed by: zero or more subheaders included
in the payload, zero or more SDUs or a fragment of a SDU.
Regarding to the MAC PDU header, in WiMAX there are two different types of
PDUs:
49
WiMAX: IEEE 802.16
- Generic PDU: used to carry data and MAC-layer signaling messages. It consists of
a generic header followed by payload and CRC. It is the only one used in the
downlink.
- Bandwidth request PDU: used by the MS to indicate to the BS that more
bandwidth is required in the uplink. It consists only of a bandwidth-request
header, with no payload or CRC.
MSB
LSB
MAC Header
ytr98iJ
MAC Payload (optional)
(6 bytes)
CRC (4
bytes)
Zero or
more
subheaders
Figure 15: MAC Frame Structure
Generic Header: is composed of the following fields with its corresponding length:
Field
Length
Header type(HT) set to 0
Encryption Control(EC) 1=encrypted
Type
Extended
1=present
subheader
field
CRC indicator(CI) 1=enabled
(ESF)
Field
Length
1
Encryption subheader field (EKS)
2
1
Reserverd (Rsv)
1
6
Length of MAC PDU in bytes with
the header included
Connection identifier on which the
payload is to be sent (CID)
11
Header check sequence (HCS)
8
1
1
16
Bandwidth request header: has a different frame structure and it is transmitted without
payload:
Field
Length
Header type(HT) set to 1
1
Encryption Control(EC) set to 0
1
Type
3
Field
Bandwidth request (BR) number of
bytes requested by SS for a given CID
Connection identifier on which the
payload is to be sent (CID)
Header check sequence (HCS)
Length
19
16
8
50
6.-MAC LAYER
In a generic MAC PDU, apart from the previous headers, there are five different
types of subheaders which can be included in the payload. It is indicated in the type
field (6 bits) of the header where each bit position indicates:
0. (LSB) Fast-Feedback allocation subheader: contains feedback from the MS
about DL channel state information
1. Packing subheader: indicates that multiple SDUs are packed into a single PDU
2. Fragmentation subheader: indicates that the SDU is fragmented over multiple
MAC PDUs
3. Extended type: indicates if the packing or fragmentation subheader is extended.
If its value is 1
4. ARQ feedback payload: indicates that it is enable with value 1
5. (MSB) Grant-management subheader: management bandwidth messages such as
polling or additional-bandwidth request
When the channel conditions are bad, it could be interesting to fragment one SDU in
more than one PDU in order to have a more efficient use of the bandwidth. On the
contrary, when the channel is good, it is more interesting to pack more than one SDU in
only one MAC PDU to have a better use of the available resources.
Another technique included in 802.16 is concatenation which consists of
concatenating more than one PDU in a single transmission.
6.3.-QUALITY OF SERVICE (QoS)
Quality of service is the guarantee of the service-level performance for a data stream
from a source to destination. It determines the mechanism used by the network to
allocate UL and DL transmission opportunities for the PDUs. Thus, a better sharing of
the bandwidth available among the users is possible. The principal purpose is to define a
transmission ordering and scheduling in the air interface. For instance, a user sending a
mail does not need real-time transmission
The 802.16 standard provides five different QoS classes or scheduling services for
the different applications that might work over WiMAX networks:
1. The unsolicited grant service (UGS): supports real-time data streams and it
consists of fixed-size data packets issued at periodic intervals. It guarantees the
throughput and latency for the system. It is suitable for VoIP and T1/E1.
2. The real-time polling services (rtPS): supports real-time services that generate
variable-size data packets issued at periodic intervals. It is the case of MPEG
transmission. The BS provides unicast polling opportunities to the SS at a fixed
interval (in order to ensure latency) to request bandwidth.
3. The non-real-time polling services (nrtPS): support nonreal-time applications
and it consists of variable size data packets ensuring a minimum data rate. Like
in rtPS, there are unicast polling opportunities but in this case the duration
between them is about few seconds (much larger). All the SSs belonging to the
group can also request resources, so sometimes collisions are produced. It is
suitable for FTP traffic.
51
WiMAX: IEEE 802.16
4. The best-effort service (BE): supports data streams for which no minimum
service guarantees are required. Data is sent whenever resources are available;
the MS only uses contention-based polling to request bandwidth. If request is
not successful, the SS will try again later.
5. The extended real-time polling service (ertPS): supports variable data rate realtime applications. It is based in the efficiency of UGS and rtPS. The BS works in
the same way that UGS; however, whereas in UGS allocations are fixed in time,
ertPS allocations are dynamic. It is suitable for VoIP without silence
suppression.
To provide QoS, a unidirectional flow of packets known as service flow is
transmitted. A service flow is a MAC transport service provided for transmission of
uplink or downlink traffic and it is identified by a 32-bit SFID (identifier). It is defined
by a set of QoS parameters such as latency, jitter (maximum delay variation for the
connection) and throughput assurances. A service flow has the following components:
-
-
Service Flow ID: an identifier for the service flow
Connection ID: an identifier of the logical connection used to carry the service
flow. The primary CID is used to transport the MAC messages.
Provisioned QoS parameter set (initial state): the recommended QoS parameters
to be used for the service flow.
Admitted QoS parameter set (intermediate state): defines a set of QoS
parameters for which the BS is reserving resources. It can be a subset of
provisioned QoS parameter set used when the BS is not able to admit the service
with the provisioned QoS parameter set.
Active QoS parameter set (final state): QoS parameters being provided at a
given time
Authorization module: logical BS function that approves or denies every change
of QoS parameters.
The various service flows admitted by WiMAX network are usually grouped into
service flow classes and each one identified by a unique set of QoS requirements. These
classes are not specified by WiMAX and it is the service provider who has to define
them.
To enable the dynamic setup and configuration of the services flow, the standard
defines a set of MAC management messages known as dynamic service messages (DSx
messages) used to negotiate all the QoS parameters related to a service flow. These
messages are dynamic service addition (DSA), dynamic service change (DSC), dynamic
service deletion (DSS).
6.4.-BANDWIDTH REQUEST
Bandwidth request refers to the mechanism used by the SS to indicate the BS the
necessity of bandwidth allocation.
In the downlink, the allocation of bandwidth to various MS is made by the BS on a
per CID basis without MS’s action. MAC PDUs arrive for each CID, the BS schedules
them for the PHY resources based on QoS requirements. After the allocation of the
PHY resources, the BS indicates it to MS with the transmission of DL-MAP.
52
6.-MAC LAYER
In the uplink, a grant is the right for a SS to transmit during a determinate interval,
so the SS requests resources from the BS. The burst profile changes dynamically, all the
requests are made in terms of number of bytes required to carry the MAC header and
the payload, but not the PHY overhead. These bandwidth requests can be transmitted
during any interval except initial ranging interval.
A request may come as a stand-alone bandwidth request header or piggyback
request.
In a standalone request bandwidth, the MAC PDU is transmitted in a dedicated
MAC having a header format without payload type I. There are two different types of
grant-request indicated by the field type:
-
Incremental: when the BS adds the quantity of bandwidth requested to its
perception of the bandwidth needs for the connection.
Aggregate bandwidth: it is made for a particular CID; the BS replaces its
perception of the bandwidth need with the amount of bandwidth requested.
Piggybacked bandwidth request uses the grant management subheader included in a
generic MAC PDU and it can be only incremental.
The 802.16 standard defines two main grant-request methods:
-
Unicast polling or polling: the BS allocates bandwidth to the SS in order to
make bandwidth requests. If a MS is polled individually, the process is called
unicast polling. The BS indicates the UL allocations to the MS to send
bandwidth-request PDU by the UL-MAP message of the DL subframe.
-
Multicast or contention-based polling: if there is not sufficient bandwidth to poll
a MS individually, multicast or broadcast polling is used. It consists of polling a
group of MSs and it is allocated in the bandwidth request contention slot of the
uplink frame (TDD mode) or subframe (FDD mode). Every SS can have access
to the network by asking to the SS for a UL slot. The BS evaluates the servicelevel agreement, radio network state and the scheduling algorithm and after the
evaluation may assign an UL slot for transmitting data. If a bandwidth allocation
is not assigned to a SS after a number of retries, the MAC PDU is discarded.
6.5.-NETWORK ENTRY
When a new SS wants to connect to the WiMAX network, it has to follow a set of
steps to establish correctly the communication with the BS. As it can be seen in the
figure below, the procedure is composed by different steps:
53
WiMAX: IEEE 802.16
Scanning and synchronization to the DL
SS scans the possible
channels on the DL
Obtain DL and then UL parameters
DL MAP and DCD (Broadcast CID)
UL MAP and UCD (Broadcast CID)
Initial Ranging
RNG-REQ (CID=0)
RNG-RSP (CID=0)-->Primary manag
Negotiate basic capabilities
SBC-REQ (Basic CID)
SBC-RSP(Basic CID)
SS authorization and key exchange
PKM-REQ (Primary CID)
SS
PKM-RSP (Primary CID)
Registration
REG-REQ (Primary CID)
REG-RSP (Primary CID)Secondary
manag
Establish IP connectivity
SS uses DCHP to get IP address
Get time of the day
SS gets current time from a server
Transfer operational parameters
SS downloads configuration using TFTP
Establish provisioned connections
Create new Service flowData transfer
Figure 16: Network Entry procedure
BS
54
6.-MAC LAYER
Following this figure, a brief explanation step by step of the procedure:
1- Scan for downlink channel and establish synchronization with the BS: each SS
stores a nonvolatile list of all operational parameters such as the previous
frequency used in the DL. If the synchronization with the stored frequency fails,
it will try to obtain another downlink channel.
2- Obtain transmit parameters: the SS searches for the DL-MAP message in order
to obtain synchronization. It remains in synchronization state while it receives
periodically DL-MAP and DCD. Once it reaches synchronization, it will wait
for the UCD and UL-MAP in order to obtain the transmit parameters for the UL
channel.
3- Perform ranging: the SS performs the initial ranging with the purpose of
obtaining the timing and power-level requirements to maintain the UL
connection with the BS. The SS sends a RNG-REQ in a contention-based initial
interval. The process continues until the SS receives a RNG-RSP with status
complete. After that, the SS can start the UL transmission.
4- Negotiate basic capabilities: in this step, the SS informs the BS of its capabilities
transmitting SBC-REQ (SS Basic Capabilities Request). This capabilities refers
to:
1. Bandwidth allocation: support for half-duplex and full duplex if FDD is
used.
2. PHY related parameters: maximum transmit power, current transmit
power, FFT size, 64 QAM support, HARQ support, STC and MIMO
support, AAS private MAP support, transmission gap, subcarrier
permutation support, uplink power.-control support.
The BS responds to the SS through a SBC-RSP in which the intersection of
the SS and SS capabilities is transmitted.
5- Authorize SS and perform key exchange: within the authentication mechanism
which will be explained in the security chapter, the SS and BS have to exchange
secure keys. SS sends a PKM-REQ (Private Key Management Request) to the
BS and this responds with a PKM-RSP.
6- Perform registration: registration is the process in which the SS obtains a
secondary CID and therefore access to the network. Although the basic
connections between the BS and the SS are established during the initial
ranging, these are not secure; a secure connection is established after the
authorization and registration process. The SS sends a REG-REQ with a hashed
message authentication code (HMAC) used to validate the authenticity of the
message by the BS.
7- Establish IP connectivity: the established secondary connection is used to
establish the IP connectivity; the IP version is established through REG
messages and if it is omitted, it is interpreted as version 4. The SS uses the
Dynamic Host Configuration Control (DHCP) server in order to obtain the IP
address.
55
WiMAX: IEEE 802.16
8- Establish time of the day: it is required for time-stamping logged events. It is
transmitted by UDP (User Datagram protocol). It receives from the UTC
(Universal Coordinated Time) and it is combined with the offset of the DHCP
server to obtain the local time.
9- Transfer operational parameters: after the DCHP procedure is done, the SS
download its configuration with many useful information from the server using
the Trivial File Transfer Protocol (TFTP)
10- Set up connections: After transfer operational parameters (managed SS) or after
registration (unmanaged SS), the BS sends DSA-REQ messages to the SS to set
up connection for provisioned service flows of the SS and the SS responds with
DSA-RES.
Each SS has a universal and unique 48-bit address which is used to identify the
stations in the servers. They also include some security information to authenticate the
SS to the security server and authenticate the responses from the security and
provisioning servers.
6.6.-CONNECTION MAINTENANCE
Once the communication is established between the SS and the BS, in order to
maintain the quality of this, a periodic ranging is performed.
In downlink, if CINR (Carrier to Interference-plus-Noise Ratio) goes outside an
allowed operating region, the SS requests for a change in the burst profile. If the SS has
been granted an uplink bandwidth, it sends a DBPC-REQ (Downlink Burst Profile
Change Request) message and the BS responds with a DBPC-RSP. Whereas if a grant is
not available and the SS requires a new burst profile (based on link adaptation), it sends
and RNG-REQ messages in the initial ranging interval and follows the same procedure
than in initial ranging. Both messages are sent using the primary CID.
In the uplink, for each uplink burst grant in which signal is detected, the BS
analyzes the quality and compares it with certain limits. If the quality is not good
enough, the BS sends a RNG-RSP indicating some correction for the physical layer and
the status “continue”. After a specified number of repetitions of this process, if the
quality is not good, the BS sends a RNG-RSP with the status “abort” and the
management of the link is stopped.
In the SS, when the status received in the RNG-RSP is “continue”, the RNG-REQ
has to be included in the allocated transmitted burst. If the BS determines that the
quality is good enough and the sent status is “success”, the SS uses the grant to service
for its pending uplink data queues.
56
6.-MAC LAYER
6.7.-PMP vs. MESH MODE
In PMP, a centralized BS with a sectorized antenna is the only transmitter operating
in a given direction and a given frequency channel, so all the stations within the sector
receive the same transmission. Due to be the only transmitter, the BS does not have to
coordinate with the other BSs, only for TDD in which the time has to be divided for the
uplink and downlink. The downlink is usually broadcast; it is usually specified in the
DL-MAP the portion of downlink subframe which is for a specific SS. If it is not
specified in the DL-MAP, the SS checks the CID of the received PDUs and saves those
destined to it. Related to uplink, it is shared by the SSs on a demand basis. Depending
on the service class, the SS can have the right to transmit continuously or granted by the
BS after the reception of a request from the user.
The main difference between PMP mode and mesh mode is that in PMP mode,
traffic only occurs between BS and SSs, whereas in mesh mode the traffic can be routed
through other SSs, so can occur directly between SSs.
In Mesh networks, the stations are known as nodes and the one that has a direct
connection to backhaul services outside the mesh network is known as Mesh BS.
Within a Mesh network, uplink and downlink are defined as traffic in direction of Mesh
BS and traffic away from the Mesh BS, respectively.
Some other important concepts in Mesh mode are: neighbor, neighborhood and
extended neighborhood. The nodes with which a node has direct links are called
neighbors. Neighbors of a node form a neighborhood and all the neighbors of a node
form an extended neighborhood.
In a Mesh system, the SS and even the BS have to coordinate with others to
transmit. Thus, using distributed scheduling, all the SS and Mesh BS shall coordinate
their transmission in the two-hop neighborhood and shall transmit their schedules
(grants, requests and available resources) to all the other neighbors. In this distribution
the Mesh BS, using the request from the Mesh SSs in a certain hop range, has to
determine the amount of granted resources for each link in the downlink and uplink and
after that communicates these grants to all the Mesh SSs within the hop range. Each SS
has a physical neighborhood list.
Nowadays, there is not a WiMAX profile using mesh topology although it appears
in the 802.16 standard.
57
WiMAX: IEEE 802.16
Figure 17: PMP Topology
SS
BS
SS
SS
Traffic between BS and SS
Forwared traffic between SS
Figure 18: Mesh Topology
58
6.-MAC LAYER
6.8.-MAC FUNCTIONS FOR MESH TOPOLOGY
Although most of the MAC functions for Mesh topology are the same already
explained for PMP topology, there are some different MAC functions to be considered:
1. Addressing and connections:
When a node is authorized in a network, it will receive a 16-bit Node ID from
the Mesh BS transmitted in the mesh subheader of the MAC PDU.
In order to address the nodes in the local neighborhood, an 8-bit link identifier
known as link-ID will be used to identify each link the node establish with its
neighbors in distributed scheduling. The link-ID is communicated when a new link
is established transmitted inside the CID, specifically in the generic MAC header in
unicast messages.
Since the messages are broadcasted, the receiver node can determine the
schedule using the node ID of the transmitter in the mesh subheader of the MAC
PDU and the link-ID inside the payload in the in the mesh mode schedule with
MSH-DSCH (Mesh Mode Schedule with Distributed Scheduling) message.
2. Bandwidth allocation:
1. Distributed Scheduling mode:
In this distribution every station shall coordinate their transmission in the twohop extended neighborhood. In this mode, some part of the control portion of the
frame is used by a node to transmit its own schedule and proposed schedule
changes to its neighbors. All the stations within a network shall use the same
frequency channel to transmit schedule information (MSH-DSCH messages)
regularly during the control period of the frame.
A SS that has a direct link to the BS shall synchronize to the BS, while a SS
that is at least two-hop from the BS shall synchronize to the neighbor SSs that are
closer to the BS.
There are two different types: coordinate distributed scheduling which ensures
that transmissions are scheduled without being necessary the action of the BS;
whereas in uncoordinated scheduling transmissions are established by directed
grants and requests between two nodes and it is necessary the scheduling to ensure
that no collision with data and control traffic is produced.
2. Centralized Scheduling mode:
In this mode, the Mesh BS acts in similar manner than a BS in PMP topology
with the difference that not all the SSs have to be directly connected to the BS,
however the BS provides schedule configuration (MSH-CSCF) and assignments
(MSCH-CSCH) to all the SSs of the network.
59
WiMAX: IEEE 802.16
3. Mesh Network Synchronization:
Network configuration (MSH-NCFG) and network entry (MSH-NENT)
packets provide a basic level of communication among nodes of nearby networks.
These packets are used to synchronize with the nearby networks, communication
and coordination of channel usage between nearby networks, and to notify the
entrance of a new node in the network.
60
7.-MOBILITY
7.-MOBILITY
The 802.16-2005 standards introduce new concepts related to mobility management
and power management. Power management enables the MS to preserve its battery
resources, an important factor in mobile devices. On the other hand, mobility
management enables the MS to move from the coverage area of one BS to the next
without losing connection.
7.1.-POWER-SAVING MODES
7.1.1.-Sleep Mode
In WiMAX, sleep mode is optional for the MS and mandatory for the BS. A MS
with active connections negotiates with the BS to interrupt its connection over the air
interface for an established period of time called “sleep window”. During this period the
MS will not transmit but it will listen to the channel to maintain the connectivity. It is
followed by a “listen window”, during which the MS restores its connection. The goals
of the sleep mode are to minimize MS power usage and to minimize the use of the
serving base station air interface resources.
The period of time during which all the MS are in sleep-mode and they cannot
receive any DL transmission or send any UL transmission is known as “unavailability
interval”. During this period the BS does not transmit to the MS, so the MS may power
down or can scan neighbor BS to collect information for handover process. During the
“availability interval”, the MS shall operate in the same way as in the state of normal
operation.
Sleep-mode takes place in one of the three power-saving classes depending on the
procedures of activation and deactivation, parameter sets and policies of MS availability
for data transmission. The MAC messages used to establish a sleep-mode are
MOB_SLP-REQ (Sleep Request Message sent by MS) and MOB_SLP-RSP (Sleep
Response Message sent by MS).
7.1.2.-Idle mode
In mobile WiMAX, idle mode is an optional mechanism that allows a MS to receive
broadcast transmission without being registered in the network. For a MS, it eliminates
the need of handoff when there is not an active data session for a given time. It is also
beneficial for the BS to conserve PHY and MAC resources because it does not need to
perform any hand-off procedures or signaling to the MS that is in idle mode. Idle mode
also includes a fast method (paging) to alert to the MS of the existence of pending
downlink traffic.
The BS coverage area is divided in smaller areas called paging groups. The MS are
continuously monitoring the DL transmission of the network to determine the paging
group of its location. If the MS detects that it is in a new paging group, it performs a
61
WiMAX: IEEE 802.16
“paging group update” to inform of its current paging group to the BS. Thus, with this
mechanism, when a BS needs to establish connection with a MS, it only has to page the
ones belonging to the same paging group instead of paging all the BS of the network.
During its operation, the MS can be in “paging-unavailable interval” or in “paginglisten interval”. When it is in “paging-unavailable interval”, it is not available for paging
and can power down, conduct ranging with a neighbor BS or scan the neighbor BSs for
the CINR (Carrier to Interference-plus-Noise Ratio) and SINR (Signal to Interferenceplus-Noise). In the “paging-listen interval”, MS listens to DCD and DL-MAP message
of the serving BS to determine when the paging message is scheduled. When a MS is
paged, it terminates its idle-mode operation and re-enters to the network. A
MOB_PAG-ADV message is broadcasted during the paging interval to request an
update of its location or a re-entry to the network.
7.2.-HANDOVER
This process known as handover or handoff, is similar in all the cellular systems, it
requires support from layers 1, 2 and 3. The ultimate decision is determined by layer 3
but PHY and MAC layer have to provide information to it.
The BS allocates time for each MS to measure the radio condition of the neighbors
BSs (scanning). During this scanning interval, the MS measures the received signal
strength indicator (RSSI) and the signal-to-interference-plus noise ratio (SINR). This
process is based in the following MAC messages: MOB_SCN-REQ (scanning interval
request), MOB_SCN-RSP (scanning interval allocation) and MOB_SCN-REP
(scanning report).
The hand-off process is performed following a set of stages:
1. Cell reselection: MS performs scanning and association with one or more
neighboring BSs in order to determine its suitability as target BS.
2. Handoff decision and initiation: handover is initialized when a MS decides to
migrate its connections from one BS to another. The MS sends a
MOB_MSHO-REQ to a BS indicating one or more BSs as handoff target and
the BS responds indicating the suitable BS to be used for the handoff by a
MOB_MSHO-RSP message. After that, the MS sends a MOB_MSHO-IND
indicating the BS selected for the handoff from the ones specified in the
MOB_MSHO-RSP.
This process can also be initialized by the BS sending a MOB_BSHO-REQ
to the MS indicating one or more BSs for the handoff target. After that, the MS
responds indicating its choice through a MOB_BSHO-IND.
3. Synchronization to the target BS: After the target BS determination, the MS
analyzes the DL frame preamble to obtain time and frequency synchronization.
After that, it analyzes the DL-MAP, DCD and UL-MAP to obtain information
about the ranging channel.
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7.-MOBILITY
4. Ranging with target BS: This process is similar to the initial ranging in network
entry process. The MS synchronizes its UL transmission with the BS.
5. Termination of context with previous BS: after the establishment of the
connection with the target BS, the MS terminates the connection with the
serving BS, sending a MOB_HO-IND message to the BS.
Apart from the conventional handoff process previously explained, WiMAX defines
two optional handoff procedures: macro diversity handover (MDHO) and fast base
station switching (FBSS). In the first case, the MS is allowed to transmit and receive
using the air interface of more than one BS at the same time. The BS of the diversity set
which controls the UL MAP and DL MAP is known as anchor BS.
In the case of FBSS, each BS has a diversity set of all the BSs with which the MS
has an active connection (one or more CID established and periodic ranging with all the
BSs). The difference related to MDHO is that the MS only communicates in the uplink
and downlink with one BS, the anchor BS.
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WiMAX: IEEE 802.16
8.-WiMAX NETWORK ARCHITECTURE
As mentioned, the IEEE 802.16 standards only specify the PHY and MAC of the
radio link and that is not enough to build an interoperable wireless network. WiMAX
Forum’s Network Working Group (NWG) is in charge of developing and standardizing
the end-to-end network aspects such as network architecture that are beyond the IEEE
802.16 standards. On the other hand, WiMAX Forum’s Service Provide Group (SPWG)
helps to define requirements and priorities.
The specifications are collected in three different documents elaborated by WiMAX
Forum:
- Stage 1: scenarios and services requirements (elaborated by SPWG)
- Stage 2: network reference model according to the requirements
- Stage 3: protocols associated with the network architecture
8.1.-NETWORK REFERENCE MODEL
The WiMAX reference model is composed by three different components
interconnected by reference points. These components are: Mobile Station (MS), Access
Service Network (ASN) and Connectivity Service Network (CSN). The figure bellow
illustrates the network reference model with its components and reference points.
Figure 19 : Network Reference Level
The ASN can be composed by one or more BSs and one or more ASN gateways. It
includes the capabilities which will provide radio access connection to the network to
the user with a MS. One ASN or several ASNs (interconnected through R4) which are
owned by a NAP (Network Access Provider) may be used by one or more business
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8.-WiMAX NETWORK ARCHITECTURE
entities called as NSP (Network Service Provider). Each NSP has a Connectivity
Service Network (CSN) which provides IP connectivity and all the IP core functions.
The subscriber may be served by a Home NSP or by a Visited NSP, a NSP with which
the home NSP has a roaming agreement.
8.1.1. - Access Service Network (ASN)
The ASN has the following functions in the network:
-
Layer 2 connectivity with the subscribers (MS)
Radio Resource Management (RRM) mechanism such as handover control and
execution
Mobility functions such paging within the ASN or location management
Network discovery and selection of the favorite CSN/NSP
Relay function to establish connectivity in layer (IP) between the MS and the
CSN
There are three different profiles for ASN and depending on which one; some
functions are performed by BS and others by ASN-Gateway. For instance, profile B
combines BS and ASN-GW in a single entity, whereas profiles A and C divide the
functions between the two entities. The main differences between profile A and C are:
-
In profile C, the handover function is in the ASN-GW and in profile C is in the
BS and the ASN-GW only performs the handover relay function.
In profile A, the radio resource controller (RRC) is located in ASN-GW,
whereas in profile C this function is in the BS.
The BS implements the functions related to PHY and MAC layers described in the
802.16 standards and it is defined by a sector and frequency assignment. In the case of
multiple frequencies assigned, the ASN will have as number of BS as frequencies
assigned. It is responsible for scheduling the uplink and downlink, traffic classification,
signaling messages exchanged with the ASN-GW, relaying authentication messages
between the MS and the ASN-GW, reception and delivery of the traffic encryption key
and key encryption key to the MS, and DCHP proxy functionality.
The ASN-GW is a logical entity which includes control function entities paired with
a corresponding function in the ASN, in the CSN or in another ASN and bearer plane
routing or bridging. Some of its functions are: to provide ASN location management
and paging, to act as a server for network session and mobility management, the
admission control and temporary caching of subscriber profiles and encryption keys,
provides mobility tunnel establishment and management with the BS; to act as a client
for session/mobility management; and to perform routing to the CSN. ASN-GW can be
divided in two groups: enforcement point for bearer plane functions (EP) and decision
point for no-bearer plane functions (DP).
8.1.2.-Connectivity Service Network (CSN)
CSN is the logical entity which provides all the functions that enables IP
connectivity to the WiMAX subscriber. To support all the functions some equipment is
needed such as routers, AAA proxy servers, DCHP servers, firewall, interworking
65
WiMAX: IEEE 802.16
gateways to interoperability and user data-base. Some of the important provided
functions are:
-
Authentication, authorization and accounting (AAA) server
IP address allocation to the MS
Subscriber billing
Inter-CSN tunneling to support roaming between NSPs
Inter-ASN mobility management
Connectivity infrastructure and policy control for services such as IP networks
and Internet
Mobility based on Mobile IP home agent (HA) where the MS’s location is
registered
8.1.3.-Reference Points
-
-
-
-
R1 (MS-ASN): implements air-interface specifications. It needs to include
protocols related to the management plane.
R2 (MS-CSN): includes protocols related to authentication, authorization, IP
host configuration management. It is a logical and not direct protocol interface
between MS and CSN.
R3 (ASN-CSN): transports control plane messages (AAA, mobilitymanagement capabilities and data plane messages tunneling between the ASN
and the CSN).
R4 (ASN-ASN): transports control and data plane messages especially during
the handover of a WiMAX user between two ASNs.
R5 (CSN-CSN): consists of a set of bearer and control plane protocols for
interworking between the home and the visited network.
R6 (BS-ASN Gateway): consists of a set of control plane protocols (mobility
tunnel management based on MS mobility) and bearer plane protocols (intraASN data path or inter-ASN tunnels between the BS and the ASN-GW)
R7 (ASN GW DP-ASN GW EP): coordination between the two groups of
functions
R8 (BS-BS): transports control plane message flow (inter-BS communication
protocol and additional protocols to control the efficient data transfer between
BSs in a handover process) to reach a fast handover between BSs. It also can
transport bearer plane messages (protocols that allow the data transfer between
BSs during HO process).
8.2.-NETWORK FUNCTIONALITIES
8.2.1.-Network Discovery and Selection
It is common that one SS may operate in an environment where more than one
network is available and multiple service providers are offering services over these
networks. WiMAX implements a solution to discover and select the networks,
composed of four stages:
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8.-WiMAX NETWORK ARCHITECTURE
-
-
-
NAP discovery: using this process, the SS can discover all available NAPs
within a coverage area. MS decodes the DCD message in the DL-MAP and
identifies the “operator ID” in the BSID field.
NSP discovery: using this process, the MS will discover all the NSPs that are
providing service over a given ASN. It uses the list NSP ID broadcasted by the
ASN.
NSP enumeration and selection: the MS will select one of the available NSPs
using an algorithm. The selection can be automatic or manual.
ASN attachment: after the NSP selection, the MS indicates its selection and
attaches to an ASN providing its identity and home NSP domain by sending a
NAI (Network Access Identifier) message.
8.2.2.-Mobility Management
The mobility procedures are divided in two different levels:
-
Micromobility or ASN-anchored mobility: it refers to intra-ASN mobility where
a CoA (Care of Address) address update is necessary and the MS maintains the
same anchor foreign agent. The handover is between R6 or R8 reference points.
-
Macromobility or CSN-anchored mobility: it refers to inter-ASN mobility where
the MS changes to a new anchor foreign agent. The handover is produced in the
R3 interface with tunneling over R4 to transport undelivered packets. WiMAX
systems must support one of the following mobile IP schemes: proxy-MIP(MS
is unaware and there is not signaling over the air to communicate the CSN
change) and the other scheme is Client-MIP (client participates in inter-ASN
mobility)
8.2.3.-IP Address Assignment
WiMAX networks support two addressing mechanism: IPv4 and IPv6. The
Dynamic Host Control Protocol (DHCP) is used to allocate a dynamic point of
attachment (PoA) IP address to the MS. The home CSN may allocate IP address to the
ASN via AAA. The DHCP proxy will be allocated in the ASN, whereas the DHCP
server will be in the CSN.
To support IPv6, the ASN includes an IPv6 access router functionality to assign a
globally routable IP address to the MS. In mobile IPv6, the MS obtains the care-of
address (CoA), which is a temporary IP address, from the ASN of the visited network
and the home address (HoA) from the home CSN.
In the next table, there is a classification of the different PoA methods used
depending on the IP version and the type of service:
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WiMAX: IEEE 802.16
Service type
Fixed
Nomadic
Mobile
-
-
PoA IP address
scheme IPv4
Static or dynamic
Dynamic
DHCP for P-Mobile IP
terminals
MIP mode for CMobile IP terminals
PoA IP address
scheme IPv6
Static or stateful
Stateful or stateless
Stateful or stateless
Stateful: Host obtains address from a server that keeps track of which addresses
have been assigned to which hosts.
Stateless: The stateless mechanism allows a host to generate its own addresses
using a combination of locally available information and information advertised
by routers
Static: fixed address assigned by a server
Dynamic: temporary address that changes every connection
8.2.4.-AAA Framework
WiMAX architecture is designed to support all the IEEE 802.16e services based on
IETF (Internet Engineering Task Force)-EAP (Extensible Authentication Protocol)
specifications. The following services are included:
-
Authentication: device authentication in the network
Authorization: user profile information used for mobility or QoS management
Accounting: information for pre-paid or post- paid services
Security will be explained accurately in the chapter 9.
8.2.5.-Quality-of-Service Architecture
WiMAX Forum defines the various functional entities to provision and manage the
service flows. It enables different flexible support of simultaneous use of a diverse set
of IP services. The architecture supports differentiated levels of QoS, bandwidth
management and admission control. It calls for an extensive use of standard IETF
(Internet Engineering Task Force) mechanisms for managing policy decision and policy
enforcements between operators.
The important functional entities are:
-
Policy function (PF): evaluates a service request against policy database located
in the home NSP. Service request to the PF may come from the SFA or from an
AF.
-
AAA server: it stores the user QoS profiles and the associated policy rules. User
QoS is downloaded to a SFA at network entry within the authentication and
authorization process.
68
69
-
Service Flow Management (SFM): it is located in the BS and manages local
resource information and performs administration control that determines, based
on available radio resource and other local information, whether a radio link can
be created.
-
Service Flow Authorization (SFA): this logical entity is located in the ASN and
evaluates the incoming service request against the QoS profile, after that the
SFA will decide whether to allow the flow or not. If the QoS profiles are not
with the SFA, it forwards the service flow to the PF for decision making. For
each MS, one SFA is assigned as anchor SF for a given session and it is
responsible for the communication with the PF. The relay SFA that directly
communicates with the SFM is called serving SFA. The SFAs will perform
ASN-level policy enforcement using a Local Policy Database (LPD)
-
Application function: it can initiate service flow creation on behalf of a user.
WiMAX: IEEE 802.16
9.-SECURITY
A secure wireless architecture should support the following basic requirements:
-
Data integrity: ensure data are protected from being tampered by someone
during the path
Authentication: ensure that the user/device is the one that says to be
Privacy: protection against spy network
Authorization: verify that the user/device is authorized to receive a specific
service of a network
Access control: ensure that only authorized users can access to the network
Security is handled in various layers of the OSI reference model. Thus, the security
sublayer specified in the 802.16 standard deals with the link layer for authentication,
authorization and encryption processes. In this layer the most used techniques are AES,
X.509 and PKI.
RSA-based
authentication
EAP encapsulation/
decapsulation
Authorization/SA
Control
PKM control management
Traffic data
encryption/authentication
processing
Control message processing
Message authentication
processing
PHY-SAP
Figure 20 : Protocol stack of the security sublayer
In the network layer, firewalls and AAA servers are used in order to avoid malicious
attacks. In this case, RADIUS and DIAMETER are the deployed techniques for AAA
interaction.
Transport layer (through Transfer Layer Security (TLS)) and application layer
(through certificates, end-to-end security and digital signatures) present additional
security measures.
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9.-SECURITY
9.1.-AUTHENTICATION AND ACCESS CONTROL
The data-link layer security functions perform most of the functions related to
authentication, authorization and encryption between the user and the base station. An
access control system is composed of three elements: supplicant (entity that wants to
access to the network), authenticator (entity which controls the access) and
authentication server (entity which decides based on some factors if a user can get
access).
9.1.1.-Authentication
Authentication can come in two different ways:
-
Unilateral authentication: where the BS identifies the MS
Mutual authentication: where BS authenticates the MS and the MS authenticates
the BS
Authentication procedure is made using a private key interchange protocol that
allows authentication and also the establishment of the encryption key. In WiMAX, the
Privacy Key Infrastructure (PKI) is built based on symmetric key encryption which is
composed by two different keys: public key and private key. The public key is known
widely, whereas the private key is kept as secret.
PKM establishes an Authentication Key (AK) of 160 bits between the MS and the
SS, after that the Key Encryption Key (KEK) of 128 bits is derived from the AK. KEK
is not used for encryption of traffic data, for this purpose the Traffic Encryption Key
(TEK) is required. TEK is generated in the BS with a TEK encryption algorithm where
KEK is used as encryption key.
The IEEE 802.16e standard defines a Privacy Key Management (PKM) version 2
whose main difference respect to PKMv1 is the usage of mutual authentication, so the
BS is also authenticated in order to prevent connection to a false BS. It allows three
types of authentication options:
-
RSA based authentication: A BS identifies the SS through a X.509 digital
certificate issued by the SS manufacturer which contains the MS’s Public Key
(PK) and MAC address. After that, the BS uses the PK to encrypt an AK which
is then sent to the MS.
-
EAP (Extensible Authentication Protocol) based authentication: specifies a set
of request messages that the supplicant (SS) sends to the authentication server
located on the BS; based on the responses, the access to the network is possible
or not. Some rules to authenticate a user or a device are defined as EAP methods
such as certificates, credentials, passwords and smart cards. In WiMAX, the
choice of the authentication method depends on the operator. Thus, there are
three different methods: EAP-AKA (for SIM cards), EAP-TLS (for X.509
digital certificate) and EAP-TTLS (for MS-Chapv2 (Microsoft –Challenge
Handshake Authentication Protocol)).
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WiMAX: IEEE 802.16
-
RSA based authentication followed by EAP authentication.
Once the authorization is done, a Security Association (SA), which is a set of
security information managed by the BS and shared between the BS and some MSs, is
established to support secure communications. This set of security information consists
of methods for data encryption, data authentication and TEK exchange.
There are three types of SA: primary, dynamic and static. Primary SAs consists of
an initial TEK exchange during the MS initialization phase, whereas static SAs are
performed within the BS. After the SA establishment, the BS periodically will refresh
the keying elements in response to the creation and termination of service flows. This
process is known as dynamic SAs.
9.1.2- Authorization
After authentication, the MS requests authorization (request for an AK and SA
identity (SAID)) to the BS through X.509 certificates, encryption algorithms and
cryptographic IDs. The BS interacts with the AAA server and responses with the AK
encrypted with the MS’s public key and also with a lifetime key (from 30 minutes to 7
days) and a SAID. After the initial authorization, the AAA server reauthorizes
periodically the SS through the BS.
The X.509 digital certificate version 3 used in WiMAX provides a public key
infrastructure used for authentication. Each SS carries a unique X.509 certificate which
has been issued by a Certification Authority and installed by the SS manufacturer. This
certificate includes the SS MAC address and the SS RSA public key.
9.1.3.-Security in the Network Layer
The authentication and authorization methods mentioned previously run over PHY
and MAC layer and between the client and the authentication server. However, there are
other mechanisms that run over network layer between the authenticator and the
authenticator server: RADIUS and DIAMETER.
RADIUS is a client/server UDP application that runs over IP where the
authentication server is the RADIUS server and the authenticator is the RADIUS client.
It supports AAA functions and also provides some other function measuring session
volume and duration. The name of DIAMETER comes from the fact that is twice better
than RADIUS. It corrects some deficiencies of RADIUS protocol.
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9.-SECURITY
Authenticator
SS
EAP
IPN
E
T
W
O
R
K
Authentication
Server
EAP
WiMAX Link Layer
AAA- RADIUS/
AAA- DIAMETER
Figure 21: WiMAX Access-Control Structure
9.2.-DATA ENCRYPTION
Encryption is the process used to protect the confidentiality of data from the
transmitter to the receiver. For this purpose, a block of data to encrypt known as
plaintext is taken and combined with another block of data known as encryption key to
perform a reversible mathematical operation in order to generate the ciphertext. In the
receiver, an inverse operation, decryption, is performed in order to extract the plaintext
of the block of data. If the same code is used for encryption and decryption, the process
is known as symmetric key encryption. If different codes are used it is an asymmetric
encryption.
In the 802.16 standard, two different encryption methods can be used: AES
(Advanced Encryption Standard) and DES (Data Encryption Standard). DES has several
security holes and it is considered insecure so the link-layer encryption method widely
adopted in wireless telecommunications such as WiMAX is AES. It offers strong
encryption and AES is also fast, easy to implement in hardware and software, and
requires less memory than other encryption schemes.
In the 802.16e standard, four different implementations of AES are considered: AES
in CBC (Cipherblock Chaining Code) mode, AES in CBCM mode (CBC-MAC), AES
in Counter (CMC) mode, AES KeyWrap with a 128-bit key. Both Wi-Fi and WiMAX
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WiMAX: IEEE 802.16
systems specify the use of AES in counter mode with cipher-block chaining messageauthentication mode (CBC-MAC or CCM).
In the next figure, the MAC PDU using AES-CCM is represented. First, a 4-bytes
packet data number (PN) is added before the encrypted data. It will be followed by the
plaintext of L-bytes which is encrypted using the current TEK and followed by the
cyphertext Message Authentication Code (MAC) or Integrity Check Value (ICV) of 8bytes. PN is linked to the SA and incremented in each PDU transmitted and this PN is
not encrypted but it is included for the MAC calculation.
L Bytes
PDU payload plaintext
Encryption
L + 12 Bytes
8 Bytes
4 Bytes
PN
Ciphertext Payload
Ciphertext MAC
(Message
Authentication Code)
Figure 22: Encrypted payload format in AES-CCM
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10.-APPLICATIONS
10.-APPLICATIONS
WiMAX has some advantages to wired connections: low-operational cost, cheaper
implementation costs, less maintenance costs, less impact on environment, quicker and
easier setup and more flexibility.
WiMAX can have a range up to 50 km at data rates of 75 Mbps using both
unlicensed and licensed frequency bands. It can be used for large area coverage and also
for last-mile and backhauling. In this chapter, some of the applications will be
explained. In the next figure the main usages of WiMAX are represented:
Figure 23 : Main usages of WiMAX
10.1.-WMAN (WIRELESS METROPOLITAN AREA NETWORK)
WiMAX can provide wireless broadband access to metropolitan areas with the same
result as traditional MAN (Metropolitan Access Network) technologies but without the
necessity of maintaining the physical transmission medium (fiber, copper). Another
drawback of wired connections is the limitation due to distance and the quality of
wiring.
With a WMAN based on WiMAX is possible to provide broadband access for
services as internet or multimedia applications up to several kilometers using PMP
(point-to-multipoint) topology. However, this WMAN based in WiMAX is limited by
frequency ability, transmit power and receiver sensitivity.
Usually a group of users is connected to the network through a BS in NLOS
conditions and each BS is backhauled to the core network via fiber or through a PTP
(point-to-point) microwave link. In the next figure the typical structure is represented:
75
WiMAX: IEEE 802.16
Figure 24: WMAN Network
10.2.-WiMAX MILITARY APPLICATIONS
WiMAX is a suitable technology for military applications because it uses frequency
bands higher than commercial and military frequency bands, so it does not interfere in
the current communications services.
With WiMAX is possible to exchange information from different sources and it is
ideal for tactical defense operations. From the commander center it is possible to
transmit information to the soldiers (through the antenna attached on their vehicles) in a
wide area.
10.3.-RURAL AREA BROADBAND SERVICES
Providing a good quality and high-speed connection to all the areas in a country is
one of the challenges that every government has to overcome. In the big cities is not a
problem due to the existence of a wired infrastructure via fiber or copper, whereas in the
rural areas is not as easy. There are rural areas where the broadband services are limited
by low-speed dial-up connections or without any connection to Internet at all.
For the operators extend the wired infrastructure can result so expensive and without
economic-sense due to the low-density of population of these rural areas and the long
distance from the urban areas. Although satellites can be used to serve these areas, it has
some disadvantages such as limited upstream bandwidth, spectrum unavailability and
high-delay. However, WiMAX can be a perfect solution due to its low cost of
installation and maintenance, its scalability that allows adding a new cell easily and also
it can provide high-speed access.
10.4.-WIRELESS BACKHAUL
The backhaul is the connection from the access point to the provider and also the
connection from the provider to the core network. For instance, for cellular systems, the
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10.-APPLICATIONS
backhaul is done by leasing T1 services for a third-provider and it can be really
expensive. However, WiMAX can provide high-capacity backhaul serving multiple
cells and with the possibility of expansion to more cell with a low cost.
Due to the expansion of Wi-Fi hotspots, WiMAX can be used as backhaul for them.
WiMAX can combine multiple Wi-Fi access points together into a cluster and fill the
gap between their coverage areas. For this implementation, the radio link is established
in LOS conditions and usually with PTP topology, although in some cases PMP is used
to provide a complete solution. In the next figure, the backhaul for a cellular network is
represented:
Figure 25 : Cellular Backhaul
10.5.-LAST-MILE ACCESS TO THE BUILDINGS
Sometimes, the last-mile connection to the buildings can be a problem to provide
high-speed access to subscribers, SOHO (Small Office Home Office) or businesses. The
installation of DSL and cable solutions can result expensive, laborious and also requires
a long-time. WiMAX can be used for this last-mile link with a lower cost and offering a
comparable speed. The topology used is PMP linking a central station to a group of
users.
10.6.-PRIVATE NETWORKS
For some big companies or government departments can be useful to connect the
central office with the remote offices through a high-speed connection. For this purpose,
WiMAX is an alternative to wired connections which can result expensive. Using a
PMP topology it will be possible to link the central office with the others.
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WiMAX: IEEE 802.16
10.7.-SECURITY APPLICATIONS
Wireless video surveillance combines IP technology and WiMAX technology for
the surveillance of public places such as cities or private places such as shops, military
bases or buildings. Using WiMAX is possible to cover a wide area and also hard-toreach locations without the necessity of installing wired infrastructure. Through the use
of IP networks, it is possible to transmit the videos via secure and private IP.
10.8.-MEDICAL APPLICATIONS
In some cases, WiMAX technology can be used as the foundation of a mobile
hospital and a platform for e-health. A doctor can diagnose his patient from the distance
connecting the doctor’s computer with the patient’s computer through WiMAX. For
example a report of blood pressure can be send to the doctor and he can give a
diagnostic.
WiMAX can also be useful to connect the mobile hospital vans and communicate
some data and instructions within a disaster zone where the wired infrastructure is
broken down.
10.9.-OTHER APPLICATIONS
After the explanation of the most common usages of WiMAX, some other
applications will be mentioned in this section:
-
Expansion of ATM to rural and remote areas connecting them to the central
office through WiMAX
WiMAX allows a video conferencing to its subscribers
Real-time monitoring for dangerous works and sensor networks to monitor
temperature, air-quality and other factors
WiMAX can provide vehicular data voice services to allow the logistic
providers to contact their vehicles in real-time
Backup/ redundancy to wired networks
Though WiMAX, public safety agencies can be connected with each other
In disaster zones where the wired infrastructure break down, WiMAX can be a
solution for the communications
Telephone’s services using VoIP technology
Communications and Internet access in the sea
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11. – COMPARISONS
11. – COMPARISONS
11.1.-COMPARISON BETWEEN FIXED AND MOBILE WiMAX
At the moment, Fixed WiMAX is more developed and has more certified products
in the market. It could be a suitable solution to provide broadband wireless technology
in rural places or developing countries where the wired infrastructure is not deployed.
However, in developed countries with a well wired infrastructure deployed, Fixed
WiMAX does not offer any advantage in speed to high data-rate technologies such as
cable or DSL. The main advantage that WiMAX can offer is to provide a cheaper
broadband wireless access allowing nomadicity (connection works everywhere in the
city although can be necessary to restart session) or mobility (connection works
everywhere without the necessity of restarting session).
Another advantage of WiMAX is that Fixed WiMAX can use the same
infrastructure than Mobile WiMAX. Thus, for an operator can be useful to start
covering a small area with Fixed WiMAX and obtaining a leading position before the
final deployment of Mobile WiMAX.
The principal advantages that Mobile WiMAX presents respect to Fixed WiMAX
are:
-
Support for mobility (even at vehicular velocities) and robust support for
nomadicity, non-line of sight (NLOS)
It offers also Mobile VoIP, so it can be also an alternative to cellular phones
Mobile WiMAX can become an alternative to 3G mobile communications for
data applications
It can be a good alternative for new operators to offer mobile services without
the necessity of an expensive infrastructure and with low operational cost
The main drawback is that Mobile WiMAX is a more complex technology, so that
means a higher cost for the network operator, especially if Multiple Input Multiple
Output (MIMO) or Adaptive Antenna System (AAS) are used.
11.2.-COMPARISON BETWEEN WiMAX AND Wi-Fi
The main difference is that WiMAX has much longer coverage distance than WiFi
and can also allow mobility between cells. These two technologies are complementary
and WiMAX can be used as backhaul for Wi-Fi hotspots. Due to the short coverage
range of Wi-Fi hotspots, it is necessary the installation of several access points to cover
an area, however with WiMAX one user can move around a city without disconnection.
Besides, the use of the inefficient CSMA/CA and the interference constraints of
operating in license-exempt band reduce the capacity of outdoor Wi-Fi. Another
additional drawback is that Wi-Fi cannot offer broadband access at vehicular velocity.
Although WiMAX has a better performance than Wi-Fi, the cost and complexity of
the equipment and the high cost of the frequency license, can make difficult the
replacement of Wi-Fi by WiMAX for some applications. The main advantage of Wi-Fi
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WiMAX: IEEE 802.16
is the wide availability of terminal with Wi-Fi interface such as PDAs, laptops, cellular
phones, cameras or media players.
Other important advantage is that WiMAX can serve many user per channels (100
or more), whereas in Wi-Fi, only one user can be served in a channel. Another
advantage is the five QoS that WiMAX offers instead of only one QoS class (based on
best effort) in Wi-Fi.
11.3.-COMPARISON BETWEEEN WiMAX AND 3G
Although 3G is currently deployed and 3G terminals are already in the market, there
is space on the market for both. Thus, in a few years depending on the application,
country and on the market, 3G or WiMAX will be more suitable. In this subchapter the
principal advantages of both will be explained:
Advantages of 3G:
-
-
-
This technology is already in use in many countries, so 3G presents and advance
of three years respect to WiMAX
The spectrum used in WiMAX can change from one country to another, so it is
possible that not all equipments can be used worldwide and it will be needed
multifrequency equipments. On the contrary, most of the 3G terminals can work
in other countries.
WiMAX uses higher frequencies than 3G and usually when frequency increases
received power decreases. It is usual that transmitted power is limited at these
frequencies due to environmental and regulatory requirements.
Operators with license and manufacturer companies developing 3G terminals are
more interested in 3G than in developing a new technology
In roaming and high-speed mobility, WiMAX capabilities are unproven
comparing to 3G
Advantages of WiMAX:
-
-
-
-
High amounts of money were paid in some countries for 3G license, however
WiMAX spectrum should be cheaper
The WiMAX physical layer is based on OFDM, that is high spectral-efficient.
New upgrades of 3G including OFDM and MIMO in it are being developed.
This evolution is called LTE (Long-Term Evolution)
WiMAX is an all-IP technology, whereas 3G systems use some protocols
developed for the first version of 3G that are not all-IP. Using this IP
architecture simplifies the core network whereas 3G has a complex and separate
core network for voice and data. IP also allows a better integration with
application developers and an easier convergence with other networks.
WiMAX has a strong support of important industry companies (WiMAX
Forum)
WiMAX is an open system where many algorithms are left to the vendor so it
allows optimization. On the other hand, it could cause some interoperability
problems
WiMAX offers higher data-rates, greater flexibility, higher average throughputs
and system capacity
80
11. – COMPARISONS
-
WiMAX has the ability of supporting more symmetrical links for T1
replacement and flexible adjustment of uplink-downlink data-rate ratio, whereas
3G has fixed data rate for uplink and downlink
11.4.-OTHER COMPARABLE SYSTEMS
Two other standards can emerge in the future and compete with WiMAX, at the
moment are under development: IEEE 802.20 and IEEE 802.22.
The IEEE 802.20 standard is aimed to provide broadband access at vehicular
velocities up to 250 kmph in frequency lower than 3.5 GHz with downlink data-rate of
4 Mbps and uplink data-rate of 1.2 Mbps.
The IEEE 802.22 standard is aimed to provide wireless broadband access in rural
and remote regions using the frequency bands of unused TV channels that were
operating in VHF and UHF bands. FCC plans to allow the use of this spectrum without
licenses.
11.5.-COMPARISON TABLE
In the next table, the different parameters of Fixed WiMAX, Mobile WiMAX, 3G and WiFi are presented:
Parameter
Standards
Fixed WiMAX
IEEE 802.16-2004
3.5 and 5.8 GHz
initially
3.5MHz
and
Bandwidth
7MHz in 3.5GHz
band ; 10 MHz in
5.8GHz
Peak DL data 9.4Mbps in 3.5
MHz with 3:1 DLrate
to-UL
ratio
TDD;6.1Mbps
with 1:1
Mbps
in
Peak UL data 3.3
3.5Mhz using 3:1
rate
DL-to-UL
ratio;6.5Mbps
with1:1
QPSK,
Modulation
16QAM,64QAM
Frequency
Duplexing
Multiplexing
Cell coverage
TDD, FDD
TDM
Mobile WiMAX
IEEE
802.16e2005
2.3 GHz, 2.5 GHz
and 3.5 GHz
3.5 MHz, 7MHz,
5MHz,
10MHz
and
8.75MHz
initially
46Mbps with 3:1
DL-to-UL
ratio
TDD;
32Mbps
with 1:1
Wi-Fi
IEEE
802.11a/g/n
2.4 GHz, 5 GHz
800/900/1,800
/1,900 MHz
for 5MHz
20MHz
802.11a/g;
20/40MHZ for
802.11n
54 Mbps shared
using 802.11a/g;
more
than
100Mbps peak
layer 2 throughusing
7Mbps in 10 MHz put
using 3:1 DL-to- 802.11n
UL ratio; 4 Mbps
using 1:1
QPSK,
16QAM,64QAM
TDD initially
TDM/OFDMA
5-10 km (up to 50 2-5 km
km)
3G (HSPA)
3GPP Release 6
BPSK,
QPSK,16QAM,
64QAM
TDD
CSMA
14.4
Mbps
using all 15
codes; 7.2Mbps
with 10 codes
1.4
Mbps
initially;
5.8Mbps later
QPSK, 16QAM
FDD
TDM/CDMA
Indoor : 30 m 1.5-5 km
Outdoor: 300 m
81
WiMAX: IEEE 802.16
12.-SUMMARY AND CONCLUSION
12.1.-SUMMARY
During this thesis WiMAX and the IEEE 802.16 standard were explained. In this
section, a brief summary with the main aspects of WiMAX is included:
-
-
-
-
-
WiMAX is based on a flexible and robust air interface defined by the IEEE
802.16 standard (only PHY and MAC layers are specified)
The IEEE elaborates the specifications and leaves to WiMAX Forum (group of
industries) the task of converting them into an interoperable standard that can be
certified
The physical layer is based in OFDM, specifically OFDMA a multi-user version
of OFDM, which allows overcome the multipath distortion and intersymbol
interference. For increasing the reliability of the link layer, some techniques
such as hybrid ARQ or error correction coding are used
WiMAX uses adaptive modulation and coding, multiuser diversity and spatial
multiplexing. These techniques allow to improve the overall capacity of the
system
Flexible MAC layer can accommodate different types of traffic such as video,
voice or multimedia
WiMAX specifies security functions such as strong encryption and
authentication
Two versions of WiMAX are specified: Fixed WiMAX (802.16-2004) and
Mobile WiMAX (802.16-2005). Mobile WiMAX will allow mobility up to
vehicular velocities without losing the connection
To support mobility, WiMAX incorporates mechanisms for location
management and handoff management
WiMAX an all-IP-based network architecture that allows all the advantages of
IP
Advantages: lower deployment and maintenance cost than wired infrastructure,
mobility support, high-data rates, strongly secure communications, possibility of
using unlicensed frequency bands and wide coverage up to 50 km
12.2.-FINAL CONCLUSION
WiMAX has generated a tremendous amount of interest in the wireless community
and it can be one of the most deployed technologies in the future. The main reason for
its success is the strong support of the industry and the strong base of standardization.
IEEE 802.16 has evolved considerably from the first standard approved in 2002 and it
will evolve more due to the new standards under development. At the moment, in 2008,
it is deployed for some applications such as providing broadband access in rural areas,
backhaul for Wi-Fi hotspots or video surveillance in the cities. Moreover, the
combination of Wi-Fi (indoor) at home and WiMAX (outdoor) can be a really
competitive technology.
82
12.-SUMMARY AND CONCLUSION
In the market there are already a wide variety of certified products for Fixed
WiMAX. However, as mentioned, in places with a well developed telecommunications
infrastructure, Fixed WiMAX does not present any advantage to technologies such as
DSL or cable.
In the future, Mobile WiMAX is expected to offer broadband full mobility for all
kind of services such as voice, data and video and therefore offer 4G services. In April
2008, the first eight Mobile WiMAX products for 2.3 GHz were certified by WiMAX
Forum and it is expected that more than 100 products (also for 2.5 GHz profiles) will be
certified at the end of 2008.
WiMAX Forum projects that in 2012, WiMAX will have 133 millions of
worldwide users (70 per cent of them will use mobile and portable devices).
Nowadays, Korea is one of the most important markets for WiMAX, there are
140,000 of subscribers and expects at least 420,000 at the end of 2008 covering the 40
per cent of the country. In Europe, in almost every country there are operators offering
broadband services, especially in rural regions, through WiMAX. For instance, in
Finland, there are 15 WiMAX operators with coverage especially in rural areas of
Lapland. The expansion of WiMAX can be helped by the action of governments which
can be interested in providing a high-speed broadband access to every user in every
area.
It is clear that year after year users demand higher speed access and have the
necessity of mobility and to be connected in everywhere at anytime. No one knows
which technology will be more successful in the future but WiMAX can achieve
success thanks to the IEEE 802.16 group mainly, which is continuously adapting the
standard to the new requirements, and the strong support of the industry.
83
WiMAX: IEEE 802.16
13.- REFERENCES
1.
Jeffrey G.Andrews, Arunabha Gosh and Rias Muhamed; Fundamentals of
WiMAX, understanding broadband Wireless Networking; Prentice Hall, 2007
2. Loutfi Nuaymi; WiMAX technology for broadband access, John Viley & Sons,
2007
3. IEEE; The 802.16-2005 standard
4. IEEE; The 802.16-2004 standard
5. Syed Ahson and Mohammad Ilyas; WiMAX Applications,CRC Press, 2008
6. Syed Ahson and Mohammad Ilyas; WiMAX Technologies, Performance
Analysis and QoS, CRC Press, 2008
7. Syed Ahson and Mohammad Ilyas; WiMAX Standards and Security, CRC Press,
2008
8. Yang Xiao; WiMAX MobileFi: Advanced Research and Technology, Auerbach
Publications, 2008
9. Clint Smith and John Meyer; 3G Wireless with WiMAX and Wi-Fi,McGraw-Hill
Professional Engineering, 2004
10. Daniel Sweeney; WiMAX Operator’s Manual: Building 802.16 Wireless
Networks, Apress, 2006
11. Louis Litwin and Michael Pugel; The principles of WiMAX, www.rfdesign.com,
2001
12. WiMAX Forum: Senza Fili Consulting; Fixed, nomadic, portable and mobile
applications for 802.16-2004 and 802.16e WiMAX networks, WiMAX Forum ,
November 2005
13. WiMAX Forum; Mobile WiMAX Part I: A technical overview and Performance
Evaluation, WiMAX Forum, August 2006
14. WiMAX Forum; Documents of www.wimaxforum.org
15. Parviz Yegani; WiMAX Overview, Cisco Systems, 2005
16. Rohde & Schwarz; WiMAX: General Information about the standard 802.16
17. Michael Richardson and Patrick Ryan; WiMAX: opportunity or hype?,
University of Colorado, 2006
18. AirSpan Networks Inc, Mobile WiMAX Security, 2007
84
13.- REFERENCES
19. Documents of IEEE: www.ieee802.org
20. Rafael Herradón Diez, Comunicaciones móviles digitales, DIAC- Polytehnic
Universtity of Madrid, 2007
21. WiMAX 360; WiMAX Know-How from Engineers, Technicians and WiMAX
Professionals, www.wimax360.com
22. Dictionary Spanish- English: www.wordreference.com
85
WiMAX: IEEE 802.16
Table of figures:
Figure 1: Protocol Stack ........................................................................................... 19
Figure 2: Cellular System ......................................................................................... 25
Figure 3: BPSK Constellation .................................................................................. 28
Figure 4: QPSK Constellation .................................................................................. 29
Figure 5: 16QAM Constellation ............................................................................... 29
Figure 6: OFDM Spectrum ....................................................................................... 31
Figure 7: OFDMA subcarriers .................................................................................. 34
Figure 8: OFDM transmission chain ....................................................................... 38
Figure 9: OFDMA transmission chain ..................................................................... 38
Figure 10: Convolutional Encoder ........................................................................... 39
Figure 11: OFDMA PHY Convolutional Turbo Code (CTC) Encoder ................... 40
Figure 12: OFDM PHY DL subframe ...................................................................... 45
Figure 13: OFDM PHY UL subframe ...................................................................... 46
Figure 14: TDD frame structure for Mobile WiMAX .............................................. 47
Figure 15: MAC Frame Structure ............................................................................. 50
Figure 16: Network Entry procedure ........................................................................ 54
Figure 17: PMP Topology ....................................................................................... 58
Figure 18: Mesh Topology ...................................................................................... 58
Figure 19 : Network Reference Level ...................................................................... 64
Figure 20 : Protocol stack of the security sublayer .................................................. 70
Figure 21: WiMAX Access-Control Structure ......................................................... 73
Figure 22: Encrypted payload format in AES-CCM ................................................ 74
Figure 23 : Main usages of WiMAX ........................................................................ 75
Figure 24: WMAN Network..................................................................................... 76
Figure 25 : Cellular Backhaul ................................................................................... 77
86
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