Introduction
1.1
Introduction
In
past years, we purely lived on analog system. Both the sources and transmission
system were on analog format but the advancement of technology made it possible
to transmit data digitally. Broadband Wireless Access (BWA) has emerged as a
promising solution for last mile access technology to provide high speed
internet access in the residential as well as small and medium sized enterprise
sectors. Applications like voice, Internet access, instant messaging, SMS,
paging, file transferring, video conferencing, gaming and entertainment etc
became a part of life. We can consider cellular phone systems, WLAN, wide-area
wireless data systems, ad-hoc wireless networks and satellite systems etc as
wireless communication. Wireless technology provide higher throughput, huge
mobility, longer range, robust backbone to thereat. Engineers are trying to
provide smooth transmission of multimedia anywhere on the globe through variety
of applications and devices leading a new concept of wireless communication
which is less expensive and flexible to implement even in odd environment.
Wireless
Broadband Access (WBA) via DSL, T1-line or cable infrastructure is not
available especially in rural or suburban areas. The DSL can covers only up to
near about 18,000 feet (3 miles), this means that many urban, suburban, and
rural areas may not served. The little-bit solution of this problem is to use
Wi-Fi standard broadband connection but for coverage limitation its not
possible in everywhere. But the Urban-area Wireless standard which is called
WiMAX can solve this shortcoming. The wireless broadband connection is much
easier to expose, have long range of coverage, easier to access and more
flexible.
This
connectivity is really important for developing countries and IEEE 802.16
family helps to solve the last mile connectivity problems with BWA
connectivity. IEEE 802.16e can operate in both Line-Of-Sight (LOS) and
Non-Line-Of-Sight (NLOS) environments. In NLOS, the PHY specification is
extended to 211 GHz frequency band which aim is to fight with fading and
multipath propagation. The OFDM physical layer based IEEE 802.16 standard is
almost identical to European Telecommunications Standard Institute’s (ETSI)
High performance Metropolitan Area Network (HiperMAN) as they cooperate with
each other.
This
thesis is all about WiMAX OFDM PHY layer performance where we analyzed the
results using MATLAB simulator with different modulation techniques.
1.2
Why WiMAX
WiMAX
is the next generation broadband wireless technology. It offers high speed, secure,
sophisticate and last mile broadband services along with a cellular pull back
and Wi-Fi hotspots. The evolution of WiMAX began shortly when scientists and
engineers felt the importance of having a wireless Internet access and other
broadband services which works well in rural and urban areas and also in those
areas where it is not possible to establish wired infrastructure. IEEE 802.16,
also known as IEEE Wireless-MAN, explored both licensed and unlicensed band of
2-66 GHz which is standard of fixed wireless broadband and included mobile
broadband application. WiMAX forum, a private organization was formed in June
2001 to coordinate the components and develop the equipment those will be
compatible and inter operable. After several years, in 2007, Mobile WiMAX
equipment developed with the standard IEEE 802.16e got the certification and
they announced to release the product in 2008, providing mobility and nomadic
access. The IEEE 802.16e air interface based on Orthogonal Frequency Division
Multiple Access (OFDMA) which main aim is to give better performance in
non-line-of-sight environments. IEEE 802.16e introduced scalable channel
bandwidth up to 20 MHz, Multiple Input Multiple Output (MIMO) and AMC enabled
802.16e technology to support peak Downlink (DL) data rates up to 63 Mbps in a
20 MHz channel through Scalable OFDMA (S-OFDMA) system. IEEE 802.16e has strong
security architecture as it uses Extensible Authentication Protocol (EAP) for
mutual authentication, a series of strong encryption algorithms, CMAC or HMAC
based message protection and reduced key lifetime.
1.3
Fixed Vs Mobile WiMAX
There
are certain differences between Fixed WiMAX and Mobile WiMAX. 802.16d is known
as Fixed WiMAX and 802.16e standard is fondly referred as Mobile-WiMAX. The
802.16d standard supports fixed and nomadic applications whereas 802.16e
standard supports fixed, nomadic, mobile and portable applications. The 802.16e
carries all the features of 802.16d standard along with new specifications that
enables full mobility at vehicular speed, better QoS performance and power
control but 802.16e devices are not compatible with 802.16d base stations as
802.16e based on TDD whereas 802.16d is on FDD. Due to other compatibility
issues with existing networks, 802.16e adopted S-OFDMA and 2048-FFT size. The
main aim of mobile WiMAX is to support roaming capability and handover between
Mobile Station (MS) and Base Station (BS) [2]. Several countries have already
planned Mobile WiMAX for commercial services. The development included some new
features on the link layer. Such features are, different types of handover
techniques, robust power saving system and multiple broadcast supports etc.
1.4
WiMAX’s Path to Overcome
There
are several challenges for WiMAX. These important issues must be solved to
fulfill its dream of last mile solution. Some of those are mentioned below.
1.4.1
PAL and PAPR
OFDM
has high Peak to Average Power Ratio. A recent analysis of its waveform showed
a large fluctuation in its amplitude which leads to a huge challenge to design
a power amplifier with adequate power back-off. To do so, it has to focus on
different situations like, good sensitivity when the power is low, tolerability
to high power level and tracking ability to track down changes. Clipping and
coding have been used to fight with these effects but still researches needed
in that issue to make it a good wireless communication system.
1.4.2
Attenuation
Each
signal has a specific potency. To reach to a distant receiver, a signal must be
strong enough to be detected by the receiver. When a signal travels in the air,
gradually it becomes weaker over time and this phenomenon is called
Attenuation. WiMAX is considering this issue carefully as it works on both LOS
and NLOS environment.
1.4.3
Multi Path Fading
When
an object comes on the way between a wireless transmitter and a receiver, it
blocks the signal and creates several signal paths known as multi path. Even
though the signal makes till the receiver but with variant time and it is hard
to detect the actual signal. Multi path degrade the quality of the signal.
Several multipath barriers which as follow:
Fast
Fading
Rapid
changes in signal power occur when distance moves about a half wave length. It
is build up by constructive and destructive Interference. This fading occurs
when the coherence time is less than the each symbol period and the Doppler
spread spectrum is high in the channel.
Slow
Fading
Changes
in average received signal power due to the changing distance between
transmitter and the receiver or changes of surroundings when moving. This
fading occurs when the coherence time is greater than the each symbol period
and the Doppler spread spectrum is low in the channel.
Flat
Fading (Non-Selective Fading)
Flat
fading is that type of fading in which all frequency components of the received
signal fluctuates simultaneously in the same proportion[3].This fading occurs
when the channel bandwidth and delay spread spectrum of a signal is less than
the channel bandwidth and symbol period.
Frequency
Selective Fading
Selective
fading affects unequally the different spectral components of a radio signal.
This fading occurs when the channel bandwidth and delay spread spectrum of a
signal is greater than the channel bandwidth and symbol period.
Rayleigh
Fading
NLOS
(indoor, city) Rayleigh fading occurs when there is no multipath LOS between
transmitter and receiver and have only indirect path which is called NLOS to
receive the resultant waves.
Rician
Fading
Rician
fading best characterizes a situation where there is a direct LOS path in
addition to a number of indirect multipath signals between the transmitter and
receiver.
1.4.4
Noise
Different
types of noises create problem in wireless communication which hampers the
transmission quality. Best known noises in wireless media are:
Thermal
Noise
It
occurs due to agitation of electrons and it is present in all electronic
devices and transmission media such as transmitter, channels, repeaters and
receiver. It is more significant in satellite communication.
Principle
equation:
N0
= KT (W/Hz) ———————————————————————————
(1.1)
Where:
N0= noise power density in watts per 1 Hz of bandwidth K = Boltzmann’s constant
= 1.3803 ´10-23J/K T = temperature, in Kelvin (absolute temperature)
If
the noise is assumed as independent, the thermal noise present in a bandwidth
of B Hertz (in watts):
N=
KTB ———————————–
———————————————- (1.2)
Or,
in decibel-watts,
N=10
log k+ 10log T +10log B = -228.6
dbW+10log T +10log B ——————(1.3)
Inter-modulation
noise
It
occurs if the medium has non-linearity. Interference caused by signals produced
at frequencies that are the sum or variety of original frequencies.
Inter
Symbol Interference (ISI)
At
the same time all delayed copies in a pulse may arrive as primary pulse for a
subsequent bit.
Cross
Talk
If
there are unwanted coupling found in a signal path, it is called cross talk. It
creates so many problems in communication media.
Impulse
Noise
When irregular pulses or noise spikes occurs
due to external electromagnetic disturbances, or faults and flaws in the
communications system that is called impulse noise. The behavior of this type
of noise has short duration and relatively high amplitude.
Doppler
Shift Effect
Doppler
shift occurs when a mobile user move towards or away from the transmitter. It
has huge impact on carrier frequency causing the communication poor in
performance and increasing error probability.
Wi-Max
Architecture
2.1
Evolution of IEEE family of standard for BWA:
The
IEEE standard committee introduced standards for networking elements, for an
instance, IEEE 802.16 in 1999. The 802.16 family standards is introduced as
Wireless Metropolitan Area Network (MAN) commercially known as WiMAX (Worldwide
interoperability for Microwave Access) which is an nonprofit, industry-led
organization and responsible for certificating, testing, and promoting the
compatible interoperable wireless products based on IEEE 802.16 working group
and ETSI’s HiperMAN standard. The original IEEE standard addressed 10 to 66 GHz
in licensed bands and 2 to 11 GHz in unlicensed frequency range. They certified
different versions of WiMAX based on different criteria such as carrier based
wireless (single and multi carrier), fixed and portable wireless devices etc.
2.2
IEEE 802.16 versions
2.2.1
802.16
The
first 802.16 standard was released in December 2000. It provides a standard
point-to-multipoint broadcast in 10 to 66 GHz frequency range for Line of Sight
(LOS) environment.
2.2.2
802.16a
The
second version of WiMAX standard 802.16a was an amendment of 802.16 standards
and has the capability to broadcast point-to-multipoint in the frequency range
2 to 11 GHz. It was established in January 2003 and assigned both licensed and
unlicensed frequency bands. Unlicensed bands cover maximum distance from 31 to
50 miles. It improves the Quality of Service (QoS) features with supporting
protocols for instance Ethernet, ATM or IP.
2.2.3
802.16c
The
third version of WiMAX standard 802.16c was also an amendment of 802.16
standards which mostly dealt with frequency ranging 10 to 66 GHz. This standard
addressed various issues, for instance, performance evaluation, testing and
detailed system profiling. The system profile is developed to specify the
mandatory features to ensure interoperability and the optional features that
differentiate products by pricing and functionality.
2.2.4
802.16d
In
September 2003, a revision project known as 802.16d began which aimed to align
with a particular view of European Telecommunications Standards Institute
(ETSI) Hiper-MAN. This project was deduced in 2004 with the release of
802.16d-2004 including all previous Performance Evaluation of IEEE 802.16e
(Mobile WiMAX) in OFDM Physical versions’ amendments. This standard supports
mandatory and optional elements along with TDD and FDD technologies. Theoretically,
its effective data rate is 70 Mbps but in reality, the performance is near
about 40 Mbps. This standard improves the Quality of Service (QoS) by
supporting very large Service Data Units (SDU) and multiple polling schemes.
2.2.5
802.16e
802.16e
was an amendment of 802.16d standard which finished in 2005 and known as
802.16e-2005. Its main aim is mobility including large range of coverage.
Sometimes it is called mobile WiMAX. This standard is a technical updates of
fixed WiMAX which has robust support of mobile broadband. Mobile WiMAX was
built on Orthogonal Frequency Division Multiple Access (OFDMA). It mentioned
that, both standards (802.16d-2004 and 802.16e-2005) support the 256-FFT size.
The OFDMA system divides signals into sub-channels to enlarge resistance to
multipath interference. For instance, if a 30 MHz channel is divided into 1000
sub-channels, each user would concede some sub-channels which are based on
distance.
Table
2.1: Comparison of IEEE standard for BWA
|
IEEE |
IEEE |
IEEE802.16 |
IEEE |
Completed |
December |
January |
June |
December |
Spectrum |
10-66 GHz |
2-11 GHz |
2-11 GHz |
2-6 GHz |
Popagation/channel conditions |
LOS |
NLOS |
NLOS |
NLOS |
Bit |
Up (28 channelization) |
Up (20 channelization) |
Up (20 channelization) |
Up MHz channelization) |
Modulation |
QPSK, (optional 64-QAM (optional) |
BPSK, 16-QAM, 64-QAM, 256-QAM (optional) |
256 OFDM, QPSK, 64-QAM, 256-QAM |
Scalable OFDMA, 16-QAM, 64-QAM, 256-QAM (optional) |
Mobility |
Fixed |
Fixed |
Fixed |
Fixed |
Parameter |
Interactive |
Voice |
Streaming |
Data |
Video |
Data |
50Kbps |
4Kbps-64Kbps |
5Kbps-384Kbps |
0.01Mbps-100Mbps |
> |
Applications |
Interactive |
VoIP |
Music, |
Web |
IPTV, |
Packet |
Zero |
<1% |
<1% |
Zero |
<10-8 |
Delay |
Not |
<20ms |
<2sec |
Not |
<2sec |
Delay |
<50ms-150ms |
<100ms |
<250ms |
Flexible |
<100ms |
Specific
|
Operating
|
Duplexing
|
|
||
WirelessMAN-SC™
|
10
|
FDD
|
Single-carrier
|
||
WirelessMAN-SCa™
|
2
|
FDD
|
Single-carrier,
|
||
WirelessMAN-OFDMA™
|
2
|
FDD
|
OFDM
|
||
WirelessHUMAN™
|
2
|
TDD
|
Single-carrier,
|
||
WirelessMAN-OFDMA™
|
2
|
FDD
|
Single
|
|
Modulation |
Uncoded Block (bytes) |
Coded Block (bytes) |
Overall coding |
RS |
CC rate |
BPSK |
12 |
24 |
1/2 |
(12,12,0) |
1/2 |
QPSK |
24 |
48 |
1/2 |
(32,24,4) |
2/3 |
QPSK |
36 |
48 |
3/4 |
(40,36,2) |
5/6 |
16-QAM |
48 |
96 |
1/2 |
(64,48,8) |
2/3 |
16-QAM |
72 |
96 |
3/4 |
(80,72,4) |
5/6 |
64-QAM |
96 |
144 |
2/3 |
(108,96,6) |
3/4 |
64-QAM |
108 |
144 |
3/4 |
(120,108,6) |
5/6 |
Model |
Terrain |
Terrain |
Terrain |
a |
4.6 |
4 |
3.6 |
b |
0.0075 |
0.0065 |
0.005 |
c |
12.6 |
17.1 |
20 |
Channel |
Terrain |
Doppler |
Spread |
LOS |
SUI-1 |
C |
Low |
Low |
High |
SUI-2 |
C |
Low |
Low |
High |
SUI-3 |
B |
Low |
Low |
High |
SUI-4 |
B |
High |
Moderate |
High |
SUI-5 |
A |
Low |
High |
Low |
SUI-6 |
A |
High |
High |
Low |
Cell |
7 |
BTS |
30 |
Receive |
6 |
BTS |
120o |
Receive |
Omni |
Polarization |
Vertical |
Cell |
90% |
SUI |
|||||
|
Tap |
Tap |
Tap |
Units |
|
Delay |
0 |
0.4 |
0.9 |
?s |
|
Power 90% 75%
|
0 4 20 |
-15 0 0 |
-20 0 0 |
dB |
|
Power 90% 75% |
0 16 72 |
-21 0 0 |
-32 0 0 |
dB |
|
Doppler |
0.4 |
0.3 |
0.5 |
Hz |
|
Antenna Gain Normalization F300 = -0.0371dB |
Terrain Omni Overall 300 Overall |
||||
SUI |
|||||
|
Tap |
Tap |
Tap |
Units |
|
Delay |
0 |
0.4 |
1.1 |
?s |
|
Power 90% 75%
|
0 2 11 |
-12 0 0 |
-15 0 0 |
dB |
|
Power 90% 75% |
0 8 36 |
-18 0 0 |
-27 0 0 |
dB |
|
Doppler |
0.2 |
0.15 |
0.25 |
Hz |
|
Antenna Gain Normalization F300 = -0.0768dB |
Terrain Omni Overall 300 Overall |
||||
SUI |
|||||
|
Tap |
Tap |
Tap |
Units |
|
Delay |
0 |
0.4 |
0.9 |
?s |
|
Power 90% 75%
|
0 1 7 |
-5 0 0 |
-10 0 0 |
dB |
|
Power 90% 75% |
0 3 19 |
-11 0 0 |
-22 0 0 |
dB |
|
Doppler |
0.4 |
0.3 |
0.5 |
Hz |
|
Antenna Gain Normalization F300 = -0.3573dB |
Terrain Omni Overall 300 Overall |
||||
SUI |
|||||
|
Tap |
Tap |
Tap |
Units |
|
Delay |
0 |
1.5 |
4 |
?s |
|
Power 90% 75%
|
0 0 1 |
-4 0 0 |
-8 0 0 |
dB |
|
Power 90% 75% |
0 1 5 |
-10 0 0 |
-20 0 0 |
dB |
|
Doppler |
0.2 |
0.15 |
0.25 |
Hz |
|
Antenna Gain Normalization F300 = -0.4532dB |
Terrain Omni Overall 300 Overall |
||||
SUI |
|||||
|
Tap |
Tap |
Tap |
Units |
|
Delay |
0 |
4 |
10 |
?s |
|
Power 90% 75% 50% |
0 0 0 2 |
-5 0 0 0 |
-10 0 0 0 |
dB |
|
Power 90% 75% 50% |
0 0 2 7 |
-11 0 0 0 |
-22 0 0 0 |
dB |
|
Doppler |
2 |
1.5 |
2.5 |
Hz |
|
Antenna Gain Normalization F300 |
Terrain Omni Overall 300 Overall |
||||
SUI |
|||||
|
Tap |
Tap |
Tap |
Units |
|
Delay |
0 |
14 |
20 |
?s |
|
Power 90% 75% 50% |
0 0 0 1 |
-10 0 0 0 |
-14 0 0 0 |
dB |
|
Power 90% 75% 50% |
0 0 2 5 |
-16 0 0 0 |
-26 0 0 0 |
dB |
|
Doppler |
0.4 |
0.3 |
0.5 |
Hz |
|
Antenna Gain Normalization F300 = -0.1184 dB |
Terrain Omni Overall 300 Overall |
||||
Rate |
dFREE |
X |
Y |
XY(punctured |
1/2 |
10 |
1 |
1 |
X1Y1 |
2/3 |
6 |
10 |
11 |
X1Y1Y2 |
3/4 |
5 |
101 |
110 |
X1Y1Y2X3 |
5/6 |
4 |
10101 |
11010 |
X1Y1Y2X3Y4X5 |
|
CP
Mod. |
1/8 |
1/16 |
1/32 |
||||||
Without |
With Coding |
Difference
|
Without |
With Coding |
Difference
|
Without |
With Coding |
Difference
|
|
BPSK |
14 |
7 |
7 |
14 |
7 |
7 |
14.2 |
7.1 |
7.1 |
QPSK |
6 |
4.6 |
1.4 |
9.5 |
8.2 |
1.3 |
6.1 |
5 |
1.1 |
16-QAM |
9.4 |
8.2 |
1.2 |
9.7 |
8.4 |
1.3 |
10.2 |
9.2 |
1 |
64-QAM |
13.5 |
12.2 |
1.3 |
17.5 |
17 |
0.5 |
– |
– |
– |
CP
Mod. |
1/8 |
1/16 |
1/32 |
||||||
Without |
With Coding |
Difference
|
Without |
With Coding |
Difference
|
Without |
With Coding |
Difference
|
|
BPSK |
16.2 |
7.8 |
8.4 |
19 |
9 |
10 |
– |
15 |
– |
QPSK |
13 |
11.5 |
1.5 |
6.8 |
5.5 |
1.3 |
– |
– |
– |
16-QAM |
9.8 |
8.5 |
1.3 |
11.2 |
10.1 |
1.1 |
14 |
13.5 |
0.5 |
64-QAM |
14 |
12.4 |
1.6 |
– |
– |
– |
– |
– |
– |
CP
Mod. |
1/8 |
1/16 |
1/32 |
||||||
Without |
With Coding |
Difference
|
Without |
With Coding |
Difference
|
Without |
With Coding |
Difference
|
|
BPSK |
17.1 |
8.2 |
8.9 |
14.2 |
7.1 |
7.1 |
– |
18 |
– |
QPSK |
8.8 |
7.8 |
1 |
– |
– |
– |
12.5 |
14 |
-1.5 |
16-QAM |
12 |
11.2 |
0.8 |
18 |
– |
– |
– |
– |
– |
64-QAM |
16 |
14.9 |
1.1 |
– |
– |
– |
– |
– |
– |
Features of WiMAX
are certain features of WiMAX those are making it popular day by day. Some
important features of WiMAX are described below:
Interoperability
is the main concern of WiMAX. The IEEE 802.16 standard is internationally
accepted and the standard is maintained and certified by WiMAX forum which
covers fixed, portable and mobile deployments and giving the user the freedom to
choose their product from different certified vendors and use it in different
fixed, portable or mobile networks.
Long Range
main feature of WiMAX is long range of coverage. Theoretically, it covers up to
30 miles but in practice, it covers only 6 miles. The earlier versions of WiMAX
provide LOS coverage but as technology advanced and the later version of WiMAX,
e.g. mobile WiMAX, can support both LOS and NLOS connections. For that, it must
meet the condition of the range for LOS, 50 kilometers and for NLOS, 10
kilometers. The WiMAX subscriber may connect to WiMAX Base station by Stanford
University Interim (SUI) traffic model from their offices, homes, hotels and so
on.
Mobility
offers immense mobility especially IEEE 802.16e-2005 as it adopted SOFDMA
(Scalable Orthogonal Frequency Division Multiple Access) as a modulation
technique and MIMO (Multiple Input Multiple Output) in its physical layer.
There are two challenges in wireless connectivity, one of them is for session
initiation, which provides a mean to reach to inactive users and continue the
connection service by extending it even the home location of that user has been
changed and the other one provides an ongoing session without interruption
while on moving (specially at vehicular speed). The first is known as roaming
and the second one is handoff. These two are described below.
centralized database keeps current information which sends to the network by
the user base station when it moves from one location to another. To reach
another subscriber station the network pages for it using another base station.
The used subscriber station for paging depends on updating rate and movement of
subscriber station – that means from one station to another. To perform this
operation, there are several networking entities involved such as NSS (Network
Switching Subsystem), HLR (Home Location Register) and VLR (Visitor Location
Register) etc.
(localization and updating of location)
(contains information of current location) and
(sends information to Mobile Station to inform HLR about the changes of
location)
to the absence of handoff technique, the Wi-Fi users may move around a building
or a hotspot and be connected but if the users leave their location, they lose
their connectivity. But with the 802.16e-2005, the mobile users will be
connected through Wi-Fi when they are within a hotspot and then will be
connected to 802.16 if they leave the hotspot but will stay in the WiMAX
coverage area.
Quality of Service
of Service (QoS) refers to the collective effect of service perceived by the
users. Actually it refers to some particular requirements such as throughput,
packet error rate, delay, and jitters etc. The wireless network must support a
variety of applications for instance, voice, data, video, and multimedia. Each
of these has different traffic pattern and requirements which is shown in the
Table 2.2.
2.2: Sample Traffic Parameters for Broadband Wireless Application
Interfacing
installation is another feature of WiMAX. Each base station broadcasts radio
signals to its subscribers to stay with connection. Since each base station
covers limited range so it is necessary to install multiple base stations after
a certain distance to increase the range for network connectivity. Connecting
multiple base stations is not a big deal and it takes only a few hours.
Accessibility
get high speed network connectivity, only necessary thing is to become a
subscriber of WiMAX service providers. Then they will provide hardware that is
very easy to install. Most of time hardware connects through USB ports or
Ethernet and the connection may be made by clicking button.
Scalability
standard supports flexible channel bandwidths for summarize cell planning in
both licensed and unlicensed spectrum. If an operator assigned 15 MHz of
spectrum, it can be divided into three sectors of 5MHz each. By increasing
sector, the operator can increase the number of subscriber to provide better
coverage and throughput. For an instance, 50 of hotspot subscribers are trying
to get the network connectivity in a conference for 3 days. They also require
internet access connectivity to their corporate network via Virtual Private
Network (VPN) with T1 connection. For this connectivity, bandwidth is a big
question as it needs more bandwidth. But in wireless broadband access it’s
feasible to provide service to that location for a small period of time. It
would be very hard to provide through wired connection. Even the operator may
re-use the spectrum in three or more sectors by creating appropriate isolation.
Portability
is another feature as like mobility that is offered by WiMAX. It is not only
offers mobility applications but also offers nomadic access applications.
Last Mile Connectivity
network accesses via DSL, T1-line or cable infrastructure are not available
especially in rural areas. These connections have more limitations which can be
solved by WiMAX standards.
Robust Security
have a robust privacy and key management protocol as it uses Advanced
Encryption Standard (AES) which provides robust encryption policy. It also
supports flexible authentication architecture which is based on Extensible
Authentication Protocol (EAP) which allows variety of subscriber credentials
including subscriber’s username and password, digital certificates and cards.
WiMAX Architecture
architecture comprises of several components but the basic two components are
BS and SS. Other components are MS, ASN, CSN and CSN-GW etc. The WiMAX Forum’s
Network Working Group (NWG) has developed a network reference model according
to the IEEE 802.16e-2005 air interface to make sure the objectives of WiMAX are
achieved. To support fixed, nomadic and mobile WiMAX network, the network
reference model can be logically divided into three parts.
Station (MS)
is for the end user to access the mobile network. It is a portable station able
to move to wide areas and perform data and voice communication. It has all the
necessary user equipments such as an antenna, amplifier, transmitter, receiver
and software needed to perform the wireless communication. GSM, FDMA, TDMA,
CDMA and W-CDMA devices etc are the examples of Mobile station.
Service Network (ASN)
is owned by NAP, formed with one or several base stations and ASN gateways
(ASN-GW) which creates radio access network. It provides all the access
services with full mobility and efficient scalability. Its ASN-GW controls the
access in the network and coordinates between data and networking elements.
Service Network (CSN):
IP connectivity to the Internet or other public or corporate networks. It also
applies per user policy management, address management, location management
between ASN, ensures QoS, roaming and security.
2.1: WiMAX Network Architecture based on IP
Mechanism
is capable of working in different frequency ranges but according to the IEEE
802.16, the frequency band is 10 GHz – 66 GHz. A typical architecture of WiMAX
includes a base station built on top of a high rise building and communicates
on point to multi-point basis with subscriber stations which can be a business
organization or a home. The base station is connected through Customer Premise
Equipment (CPE) with the customer. This connection could be a Line-of-Sight
(LOS) or Non-Line-of-Sight (NLOS).
Line of Sight (LOS)
LOS connection, signal travels in a straight line which is free of obstacles,
means, a direct connection between a transmitter and a receiver. The features
of LOS connections are,
higher frequency between 10 GHz to 66 GHz
coverage areas
throughput
interference
only comes from atmosphere and the characteristic of the frequency
requires most of its first Fresnel zone
should be free of obstacles
2.2: WiMAX in LOS Condition
Non-Line of Sight (NLOS)
NLOS connection, signal experiences obstacles in its path and reaches to the
receiver through several reflections, refractions, diffractions, absorptions
and scattering etc. These signals arrive to the receiver in different times,
attenuation and strength which make it hard to detect the actual signal[6].
WiMAX offers other benefits which works well in NLOS condition,
selective fading can be overcome by applying adaptive equalization
Modulation and Coding (AMC), AAS and MIMO techniques helps WiMAX to works
efficiently in NLOS condition
permits to transmit appropriate power on sub-channels
on the required data rate and channel condition, AMC provides the accurate
modulation and code dynamically
2.3: WiMAX in NLOS Condition
Major shortcomings of WiMAX
are several major shortcomings of WiMAX which are still a headache to the
engineers. Those are as follows:
Error Rate
concept of WiMAX is that, it provides high speed data rate within its maximum
range (30 miles). If WiMAX operates the radio signals to its maximum range then
the Bit Error Rate (BER) increases. So, it is better to use lower bit rates
within short range to get higher data rates.
Rates
WiMAX uses Customer Premises Equipment (CPE) which is attached to computers
(either desktop or laptop or PDA) and a lower gain Omni-directional antenna is
installed which is difficult to use compared to fixed WiMAX.
and NLOS coverage
WiMAX covers 10 kilometers with 10 Mbps speeds in line -of-sight (LOS)
environment but in urban areas, it is only 2 kilometers coverage due to
non-line-of-sight problem. In this situation, mobile WiMAX may use higher gain
directional antenna for excellent coverage and throughput but problem is that
it loose its mobility.
all above shortcomings, there is a major impact of weather conditions like
rain, fog and droughts etc on WiMAX networks.
IEEE 802.16 Protocol Layers
802.16 standard WiMAX gives freedom in several things compared to other
technologies. The focus is not only on transmitting tens of megabits of data to
many miles distances but also maintaining effective QoS (Quality of Services)
and security. This chapter gives an overview of IEEE 802.16 protocol layers and
OFDM features. WiMAX 802.16 is mainly based on the physical and data link layer
in OSI reference model. Here, Physical layer can be single-carrier or
multi-carrier (PHY) based and its data link layer is subdivided into two layers
Link Control (LLC) and
Access Control (MAC)
is further divided into three sub-layers:
Sub-layer (CS)
Part Sub-layer (CPS) and
Sub-layer (SS).
Physical Layer (PHY)
layer set up the connection between the communicating devices and is
responsible for transmitting the bit sequence. It also defines the type of
modulation and demodulation as well as transmission power. WiMAX 802.16 physical
layer considers two types of transmission techniques OFDM and OFDMA. Both of
these techniques have frequency band below 11 GHz and use TDD and FDD as its
duplexing technology. After implementing OFDM in IEEE 802.16d, OFDMA has been
included in IEEE 802.16e to provide support in NLOS conditions and mobility.
The earlier version uses 10 to 66 GHz but the later version is expanded to use
up the lower bandwidth from 2 to 11 GHz which also supports the 10 to 66 GHz
frequency bands. There are some mandatory and some optional features included
with the physical layer specification.
2.4: WiMAX Physical and MAC layer architecture
OSI 7 layer reference model, WiMAX only uses the physical layer and MAC of
datalink layer.
are specific names for each physical layer interface. The Table summarizes IEEE
802.16 physical layer’s features.
2.3: IEEE 802.16 standard air interface’s description
MAC layer
basic task of WiMAX MAC is to provide an interface between the physical layer
and the upper transport layer. It takes a special packet called MAC Service
Data Units (MSDUs) from the upper layer and makes those suitable to transmit
over the air. For receiving purpose, the mechanism of MAC is just the reverse.
In both fixed and mobile WiMAX, it included a convergence sub-layer which is
able to interface with upper layer protocols such as ATM, TDM, Voice and other
advanced protocols. WiMAX MAC has unique features to identify and address the
SS and BS. Each SS carries 48-bit IEEE MAC address whereas BS carries 48-bit
Base Station ID in which 24-bit uses for operator indicator. Other features
are, 16-bit CID, 16-bit SAID and 32-bit SFID. MAC supports a variety of
applications and mobility features such as,
for MAC security and PKMV2 for Extensible Authentication Protocol (EAP)
handover and strong mobility management
normal, sleep and idle mode power levels
Sub-layers
MAC layer is divided into three sub-layers such as Service Specific Convergence
Sub-layer (SSCS), Common Part Sub-layer (CPS) and Security Sub-layer (SS).
2.5: Purposes of MAC Layer in WiMAX
Service Specific Convergence Sub-layer (SSCS)
stays on the top of MAC layer architecture which takes data from the upper
layer entities such as router and bridges. It is a sub-layer that is service
dependent and assures data transmission. It enables QoS and bandwidth
allocation. Payload header suppression and increase the link efficiency are
other important task of this layer. IEEE 802.16 specifies two types of SSCS for
mapping function.
Convergence sub-layer: is a logical interface which is responsible for
Asynchronous Transfer Mode (ATM) services. In the operation, it accepts ATM
cells from ATM layer classify and then sends CS PDUs to MAC SAP. It
differentiates Virtual path switched ATM connection and assigns Channel ID
(CID)
Convergence sub-layer: It’s a packet based protocol which performs packet
mapping such as IP, IPv4, IPv6, IEEE 802.3 Ethernet LAN, VLAN and PPP.
Common Part Sub-layer (CPS)
stays underneath of SSCS and above the Security Sub-layer and defines the
multiple access mechanism. CPS is responsible for the major MAC functionalities
like system access, establishing the connection and maintain and bandwidth
management etc. As WiMAX MAC is connection oriented so it provides service
flows after each Subscriber Station’s registration. Other responsibilities are,
providing QoS for service flows and managing connection by adding or deleting
or modifying the connection statically or dynamically. On downlink channel,
only the BS transmits and it does not need any coordination function. SS
receives only those messages which are addressed to them. On uplink channel,
three major principles defines the transmission right,
bandwidth permission
and
procedures
Security Sub-layer (SS)
one of the most important part of MAC as it provides authentication, secure key
exchange, encryption and integrity of the system. IEEE 802.16 standard defines
both ways data encryption connection between subscriber and base station. A set
of cryptographic suites such as data encryption and authentication algorithm
has been defined which made security sub-layer of WiMAX MAC very robust. A
secure distribution of keying data from base station to subscriber station is
assured by providing an authentication and a PKM protocol. On top of that, in
SS, the addition of a digital certificate strengthen the privacy of data and in
BS, the PKM assured the conditional access to the network services and
applications. Further improvement of PKM protocol is also defined with some additional
features and with a new name named PKMv2 which strongly controls integrity,
mutual authentication and handover mechanisms.
WiMAX forum and adaptation of IEEE 802.16
Worldwide Interoperability for Microwave Access (WiMAX) forum is an alliance of
telecommunication equipments and components manufacturers and service
providers, formed to promote and certify the compatibility and interoperability
of BWA products employing the IEEE 802.16 and ETSI HiperMAN wireless
specifications. WiMAX Forum Certified™ equipment is proven interoperable with
other vendors’ equipment that is also WiMAX Forum Certified™. So far WiMAX
forum has setup certification laboratories in Spain, Korea and China.
Additionally, the WiMAX forum creates what it calls system profiles, which are
specific implementations, selections of options within the standard, to suit
particular ensembles of service offerings and subscriber populations. WiMAX
forum has adopted two version of the IEEE 802.16 standard to provide different
types of access:
access: The WiMAX forum has adopted IEEE802.162004 And ETSI Hyper MAN standard
for fixed and nomadic access. This uses Orthogonal Frequency Division
Multiplexing and able to provide supports in Line of Sight (LOS) and Non Line of
Sight (NLOS) propagation environment. Both outdoor and indoor CPEs are
available for fixed access. The main focus of the WiMAX forum profiles are on
3.5 GHz and 5.8 GHz frequency band.
Access: The forum has adopted the IEEE 802.16e version of the standard, which
has been optimized for mobile radio channels. This uses Scalable OFDM Access
and provides support for handoffs and roaming. IEEE 802.16e based network is
also capable to provide fixed access. The WiMAX Mobile WiMAX profiles will cover
5, 7, 8.75, and 10 MHz channel bandwidths for licensed worldwide spectrum
allocations in the 2.3 GHz, 2.5 GHz, 3.3 GHz and 3.5 GHz frequency bands. The
first certified product is expected to be available by the end of 2007.
Application of IEEE 802.16 based network:
802.16 supports ATM, IPv4, IPv6, Ethernet and Virtual Local Area Network (VLAN)
services. SO, it can provide a rich choice of service possibilities to voice
and data network service providers. It can be used for a wide selection of
wireless broadband connection and solutions.
Backhaul: IEEE 802.16 wireless technology can be an excellent choice for back
haul for commercial enterprises such as hotspots as well as point to point back
haul applications due to its robust bandwidth and long range.
Broadband: Practical limitations like long distance and lack off return channel
prohibit many potential broadband customers reaching DSL and cable technologies.
IEEE 802.16 can fill the gaps in cable and DSL coverage.
areas: In many rural areas, especially in developing countries, there is no
existence of wired infrastructure. IEEE 802.16 can be a better solution to
provide communication services to those areas using fixed CPE and high gained
antenna.
Best Connected: As IEEE 802.16e supports mobility, so the mobile user in the
business areas can access high speed services through their IEEE 802.16/WiMAX
enabled handheld devices like PDA, Pocket PC and smart phone.
2.6: Application scenarios
Modulation Techniques
variation of the property of a signal, such as its amplitude, frequency or
phase is called modulation. This process carries a digital signal or message.
Different types of modulation techniques are available such as, Amplitude Shift
Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK). This
section discusses on different modulation techniques along with WiMAX’s special
modulation technique which is called Adaptive Modulation technique.
ASK, FSK and PSK
modulation techniques consist on three parts. Which as follows,
Shift Keying (ASK)
Shift Keying (FSK)
Shift Keying (PSK)
Amplitude-Shift Keying (ASK)
difference of carrier frequency is called ASK. In this, the phase and the
frequency are always constant. The principle is based on the mathematical
equation,
(3.1)
of ASK
to be affected by sudden changes of gain.
modulation technique compared to other techniques.
the voice transmission lines such as telephone, used up to 1200 bps.
in optical fibres to transmit digital data.
Frequency Shift Keying (FSK)
difference near carrier frequency is called FSK. In this, the phase and the
amplitude are always constant. There are several types of FSK. Most common are,
Binary Frequency Shift Keying (BFSK) and Multiple Frequency Shift Keying
(MFSK).
Binary Frequency Shift Keying (BFSK)
frequencies represent two binary values in this technique. The principle lies
on the equation,
(3.2)
of BFSK
affected by errors than ASK.
voice transmission lines such as telephone, range till 1200bps.
is used for high-frequency (3 to 30 MHz) radio frequency.
for LANs that use coaxial cables.
Multiple Frequency Shift Keying (MFSK)
than two frequencies are used to represent signaling elements. The principle
lies on the equations,
(3.3)
of MFSK
frequencies are used
bandwidth efficient but very much affected by errors
requirement is 2Mfd in total.
signal element encodes L bits (M=2L).
Phase-Shift Keying (PSK)
of carrier signal is digital modulation scheme which conveys data by modulating
or changing of carrier wave. The most common and widely used are Binary Phase
shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK). Other PSKs are
Differential Phase Shift Keying (DPSK) and Multilevel Phase Shift Keying (MPSK)
etc. As WiMAX uses Adaptive Modulation Techniques, so, here we will broadly
discuss only BPSK, QPSK and QAM.
Binary Phase Shift Keying (BPSK)
is also known as two-level PSK as it uses two phases separated by 180º to
represent binary digits. The principle equation is,
——————————–(3.6)
kind of phase modulation is very effective and robust against noises especially
in low data rate applications as it can modulate only 1bit per symbol.
3.1: BPSK, (a) Block Diagram (b) Constellation
Quadrature Phase Shift Keying (QPSK)
is also known as four-level PSK where each element represents more than one
bit. Each symbol contains two bits and it uses the phase shift of ?/2, means
90º instead of shifting the phase 180º. The principle equation of the technique
is:
this mechanism, the constellation consists of four points but the decision is
always made in two bits. This mechanism can ensure the efficient use of bandwidth
and higher spectral efficiency.
3.2: QPSK, (a) Block Diagram (b) Constellation
Quadrature Amplitude Modulation (QAM)
is the most popular modulation technique used in various wireless standards. It
combined with ASK and PSK which has two different signals sent concurrently on
the same carrier frequency but one should be shifted by 90º with respect to the
other signal. At the receiver end, the signals are demodulated and the results
are combined to get the transmitted binary input [16]. The principle equation
is:
3.3: QAM Modulator Diagram
16-QAM
is called 16-states Quadrature Amplitude Modulation which means four different
amplitude levels would be used and the combined stream would be one of 16 = 4 *
4 states. In this mechanism, each symbol represents 4 bits[16].
3.4: 16-QAM Constellation
64-QAM
is same as 16-QAM except it has 64-states where each symbol represents six bits
(26= 64). It is a complex modulation techniques but with greater efficiency
[16]. The total bandwidth increases according to the increasing number of
states for each symbol. Mobile WiMAX uses this higher modulation technique when
the link condition is high.
3.5: 64-QAM Constellation
Adaptive Modulation and Coding
specified modulation scheme in the DL (DownLink) and UL( UpLink) are BPSK
(Binary Phase Shift Keying) ,QPSK(Quadrature PSK), 16-QAM (16- Quadrature
Amplitude Modulation) and 64-QAM to modulate bits to the complex constellation
points. The FEC options are paired with the modulation schemes to form burst
profiles. The PHY specifies seven combinations of modulation and coding rate,
which can be allocated selectively to each subscriber, in both UL and DL [17].
There are tradeoffs between data rate and robustness, depending on the
propagation conditions. Table 3.1 shows the combination of those modulation and
coding rate.
3.1: Mandatory channel coding per modulation
Frequency Division Multiplexing
OFDM BASIC:
idea of OFDM comes from Multi Carrier Modulation (MCM) transmission technique.
The principle of MCM describes the division of input bit stream into several
parallel bit streams and then they are used to modulate several sub carriers as
shown in Figure 4.1. Each subcarrier is separated by a guard band to ensure
that they do not overlap with each other. In the receiver side, bandpass
filters are used to separate the spectrum of individual subcarriers. OFDM is a
special form of spectrally efficient MCM technique, which employs densely
spaced orthogonal subcarriers and overlapping spectrums. The use of bandpass
filters is not required in OFDM because of the orthogonality nature of the
subcarriers. Hence, the available bandwidth is used very efficiently without
causing the InterCarrier Interference (ICI). In figure 4.2, the effect of this
is seen as the required bandwidth is greatly reduced by removing guard band and
allowing subcarrier to overlap. It is still possible to recover the individual
subcarrier despite their overlapping spectrum provided that the orthogonality
is maintained. The Orthogonality is achieved by performing Fast Fourier
Transform (FFT) on the input stream. Because of the combination of multiple low
data rate subcarriers, OFDM provides a composite high data rate with long
symbol duration. Depending on the channel coherence time, this reduces or
completely eliminates the risk of InterSymbol Interference (ISI), which is a
common phenomenon in multipath channel environment with short symbol duration.
The use of Cyclic Prefix (CP) in OFDM symbol can reduce the effect of ISI even
more, but it also introduces a loss in SNR and data rate.
4.1: Block diagram of a generic MCM transmitter.
4.2: Comparison between conventional FDM and OFDM
OFDM SYSTEM IMPLEMENTATION
principle of OFDM was already around in the 50’s and 60’s as an efficient MCM
technique. But, the system implementation was delayed due to technological
difficulties like digital implementation of FFT/IFFT, which were not possible
to solve on that time. In 1965, Cooley and Tukey presented the algorithm for
FFT calculation and later its efficient implementation on chip makes the OFDM
into application.
digital implementation of OFDM system is achieved through the mathematical
operations called Discrete Fourier Transform (DFT) and its counterpart Inverse
Discrete Fourier Transform (IDFT). These two operations are extensively used
for transforming data between the time domain and frequency domain. In case of
OFDM, these transforms can be seen as mapping data onto orthogonal subcarriers.
order to perform frequency domain data into time domain data, IDFT correlates
the frequency domain input data with its orthogonal basis functions, which are
sinusoids at certain frequencies. In other ways, this correlation is equivalent
to mapping the input data onto the sinusoidal basis functions. In practice,
OFDM systems employ combination of fast fourier transform (FFT) and Inverse
fast Fourier transform (IFFT) blocks which are mathematical equivalent version
of the DFT and IDFT.
the transmitter side, an OFDM system treats the source symbols as though they
are in the frequency domain. These symbols are feed to an IFFT block which
brings the signal into the time domain. If the N numbers of subcarriers are
chosen for the system, the basis functions for the IFFT are N orthogonal
sinusoids of distinct frequency and IFFT receive N symbols at a time. Each of N
complex valued input symbols determines both the amplitude and phase of the
sinusoid for that subcarrier. The output of the IFFT is the summation of all N
sinusoids and makes up a single OFDM symbol. The length of the OFDM symbol is
NT where T is the IFFT input symbol period. In this way, IFFT block provides a
simple way to modulate data onto N orthogonal subcarriers.
4.3: Basic OFDM transmitter and receiver
the receiver side, The FFT block performs the reverse process on the received
signal and bring it back to frequency domain. The block diagram in Figure 4.3
depicts the switch between frequency domain and time domain in an OFDM system.
Data transmission
transmission is high enough compared to FDM as OFDM follows multicarrier
modulation. For this, OFDM splits high data bits into low data bits and sends
each sub-stream in several parallel sub-channels, known as OFDM subcarriers.
These subcarriers are orthogonal to each other and the each subcarrier
bandwidth is much lesser than the total bandwidth. Inter Symbol Interference is
reduced in OFDM technique as the symbol time Ts of each sub-channel is higher
than the channel delay spread .
of Single and Multi-carrier signals
the figure 4.4, it is clear that OFDM resists the multipath effect by adopting
smaller frequency bandwidth and longer period of time which leads to get better
spectral efficiency.
Parameters
implementation of OFDM physical layer is different for two types of WiMAX. For
fixed WiMAX, FFT size is fixed for OFDM-PHY and it is 256 but for mobile WiMAX,
the FFT size for OFDMA-PHY can be 128, 512, 1024 and 2048 bits. This helps to
combat ISI and Doppler spread. Other difference between OFDM-PHY and OFDMA-PHY
is, OFDM splits a single high bit rate data into several low bit rate of data
sub-stream in parallel which are modulated by using IFFT whereas OFDMA accepts
several users’ data and multiplex those onto downlink sub-channel. Uplink
multiple access is provided through uplink sub-channel. OFDM-PHY and OFDMA-PHY
parameters are discussed briefly in the following subsection.
OFDM-PHY
this, FFT size is fixed and it is 256 bits in which number of used data
subcarrier is 192, 8 pilot subcarriers to perform synchronization and channel
estimation and 56 Null subcarriers [20]. The channel bandwidth for fixed WiMAX
is 3.5 MHz but it varies due to spacing of subcarrier. Subcarrier spacing rises
in higher bandwidth which decreases the symbol time eventually increases the
delay spread. To avoid delay spreading, OFDM-PHY allocates a large fraction of
guard space. For OFDM-PHY, the suitable symbol time is 64 ?s, symbol duration
is 72 ?s and guard time spacing is 15.625 kHz.
OFDMA-PHY
mobile WiMAX FFT size can varies between 128 and 2048 and to keep the
subcarrier spacing at 10.94 KHz, the FFT size should be adjusted which is
helpful to minimize Doppler spreads. Since there are different channel
bandwidth like, 1.25, 5, 10 and 20 MHz etc, so FFT sizes are 128, 512, 1024 and
2048 respectively. For OFDMA-PHY, the suitable symbol time is 91.4 ?s and the
symbol duration is 102.9 ?s and number of symbols in 5 ms frames is 48.0.
Sub-channelization
divides the available subcarriers into several groups of subcarriers and
allocates to different users based on channel conditions and requirement of
users. This process is called sub-channelization. Sub-channeling concentrates
the transmit power to different smaller groups of subcarrier to increase the
system gain and widen up the coverage area with less penetration losses that
cause by buildings and other obstacles. Without sub-channelization, the link
budget would be asymmetric and bandwidth management would be poor[6]. Fixed
WiMAX based OFDM-PHY permits a little amount of sub-channelization only on the
uplink. Among 16 standard sub-channel, transmission can takes place in 1, 2, 4,
8 or all sets of sub-channels in the uplink of the SS. SS controls the
transmitted power level up and down depending on allotted sub-channels. When
the allotted sub-channels increase for uplink users, the transmitted power
level increases and when the power level decreases, it means the allotted
sub-channels decreased. The transmitted power level is always kept below the
maximum level. In fixed WiMAX, to improve link budget and the performance of
the battery of the SS, the uplink sub-channelization permits SS to transmit
only a fraction of the bandwidth usually below 1/16 allocated by the BS.
WiMAX’s OFDMA-PHY permits sub-channelization in both uplink and downlink
channels. The BS allocates the minimum frequency and sub-channels for different
users based on multiple access technique. That is why this kind of OFDM is
called OFDMA (Orthogonal Frequency Division Multiple Access). For mobile
application, frequency diversity is provided by formation of distributed
subcarriers. Mobile WiMAX has several distributed carrier based
sub-channelization schemes. The mandatory one is called Partial Usage of
Sub-Carrier (PUSC). Another sub-channelization scheme based on unbroken
subcarrier is called Adaptive Modulation and Coding (AMC) in which multiuser
diversity got the highest priority. In this, allocation of sub-channels to
users is done based on their frequency response. It is a fact that, contiguous
sub-channels are best suited for fixed and low mobility application, but it can
give certain level of gain in overall system capacity.
4.5: Downstream transmission of OFDM spectrum
4.6: Upstream transmission of OFDM spectrum
4.6 shows the upstream transmission of OFDM spectrum from a CPE where the
carriers are quarter in size compared to fig 4.5 downstream transmission from
BS.
4.7: Upstream transmission of OFDM spectrum from the CPE[7]
4.7 illustrates the transmitted upstream OFDM spectrum from a CPE where the
carriers are as same as BS in size and range but with small capacity.
BENEFITS AND DRAWBACKS of OFDM:
the earlier section, we have stated that how an OFDM system combats the ISI and
reduces the ICI. Besides those benefits, there are some other benefits as
follows:
spectral efficiency because of overlapping spectra
implementation by fast fourier transform
receiver complexity as the transmitter combat the channel effect to some
extends.
for high datar ate transmission
flexibility in terms of link adaptation
complexity multiple access schemes such as orthogonal frequency division
multiple access (OFDMA)
is possible to use maximum likelihood detection with reasonable complexity.
the other side, few drawbacks of OFDM are listed as follows
OFDM system is highly sensitive to timing and frequency offsets . Demodulation
of an OFDM signal with an offset in the frequency can lead to a high bit error
rate.
OFDM system with large number of subcarriers will have a higher peak to average
power ratio (PAPR) compared to single carrier system. High PAPR of a system
makes the implementation of Digital to analog (DAC) and Analog to Digital
Conversion (ADC) extremely difficult.
APPLICATION
has gained a big interest since the beginning of the 1990s as many of the
implementation difficulties have been overcome. OFDM has been in used or
proposed for a number of wired and wireless applications. Digital Audio
Broadcasting (DAB) was the first commercial use of OFDM technology. OFDM has
also been used for the Digital Video Broadcasting. OFDM under the acronym of
Discrete Multi Tone (DMT) has been selected for asymmetric digital subscriber
line (ADSL). The specification for Wireless LAN standard such as IEEE 802.11a/g
and ETSI HIPERLAN2 has employed OFDM as their PHY technologies. IEEE 806.16
standard for Fixed/Mobile BWA has also accepted OFDM for PHY technologies.
University Interim Channel
SUI Channel Basic concepts:
term channel refers to the medium between the transmitting antenna and the
receiving antenna.
characteristics of wireless signal changes as it travels from the transmitter
antenna to the receiver antenna. These characteristics depend upon the distance
between the two antennas, the path(s) taken by the signal, and the environment
(buildings and other objects) around the path. The profile of received signal
can be obtained from that of the transmitted signal if we have a model of the
medium between the two. This model of the medium is called channel model.
general, the power profile of the received signal can be obtained by convolving
the power profile of the transmitted signal with the impulse response of the
channel. Convolution in time domain is equivalent to multiplication in the
frequency domain. Therefore, the transmitted signal x, after propagation
through the channel H becomes y:
———————————————————————————–(5.1)
H(f) is channel response, and n(f) is the noise. Note that x, y, H, and n are
all functions of the signal frequency f.
five key components of the channel response are
loss
delay line
speed
Empirical Path Loss Models:
environments are too complex to model accurately. In practice, most simulation
studies use empirical models that have been developed based on measurements
taken in various real environments.
commonly used empirical models are-
Model
231 Extension to Hata Model
231-Walfish-Ikegami Model
Model
University Interim (SUI) Channel Models
Erceg Model
model is based on extensive experimental data collected by AT&T Wireless
Services across the United States in 95 existing macro cells at 1.9GHz. The
terrains are classified in three categories. Category A is hilly terrain with
moderate-to-heavy tree density and has a high path loss. Category C is mostly
flat terrain with light tree density and has a low path loss. Category B is
hilly terrain with light tree density or flat terrain with moderate-to-heavy
tree density. Category B has an intermediate path loss. For all three
categories, the median path loss at distance
d > d0 is given by:
? is the wavelength in meters, ? is the path-loss exponent with:
(between 10 m and 80 m), d0 = 100 m, and a, b, c are constants dependent on the
terrain category. These parameters are listed in the table below.
5.1: Parameters of the ERCEG model
represents the shadowing effect and follows a lognormal distribution with a
typical standard
of 8.2 to 10.6 dB.
above model is valid for frequencies close to 2 GHz and for receive antenna heights
close to 2 m. For other frequencies and antenna heights (between 2 m and 10 m),
the following correction terms are recommended :
PL, is the path loss given earlier, ?PLf is the frequency term, and ?PLh is the receive antenna height correction
terms given as follows:
is a set of 6 channel models representing three terrain types and a variety of
Doppler spreads, delay spread and line-of-sight/non-line-of-site conditions
that are typical of the continental US as follows[31]:
5.2: Terrain type and Doppler spread for SUI channel model
terrain type A, B,C are same as those defined earlier for Erceg model. The
multipath fading is modeled as a tapped delay line with 3 taps with non-uniform
delays. The gain associated with each tap is characterized by a Rician
Distribution and the maximum Doppler frequency. In a multipath environment, the
received power r has a Rician distribution, whose pdf is given by:
I0 (x) is the modified Bessel function of the first kind, zero order. A is zero
if there is no LOS component and the pdf of the received power becomes:
is the Raleigh distribution. The ratio K = A2/(2?2 ) in the Rician case
represents the ratio of
component to NLOS component and is called the “K-Factor” or
“Rician Factor.” For NLOS case, K-factor is zero and the Rician
distribution reduces to Raleigh Distribution.
general structure for the SUI channel model is as shown below in Figure 5.1.
This structure is for Multiple Input Multiple Output (MIMO) channels and
includes other configurations like Single Input Single Output (SISO) and Single
Input Multiple Output (SIMO) as subsets. The SUI channel structure is the same
for the primary and interfering signals.
Channel Models
Mixing Matrix
part models correlation between input signals if multiple transmitting antennas
are used.
Delay Line Matrix
part models the multipath fading of the channel. The multipath fading is
modeled as a tapped delay line with 3 taps with non-uniform delays. The gain
associated with each tap is characterized by a distribution (Rician with a
K-factor > 0, or Raleigh with K-factor = 0) and the maximum Doppler
frequency.
Mixing Matrix
part models the correlation between output signals if multiple receiving
antennas are used. Using the above general structure of the SUI Channel and
assuming the following scenario, six SUI channels are constructed which are
representative of the real channels.
Scenario for modified SUI channels
5.3: Scenario for SUI Channel Models
the following models, the total channel gain is not normalized. Before using a
SUI model, the specified normalization factors have to be added to each tap to
arrive at 0dB total mean power. The specified Doppler is the maximum frequency
parameter. The Gain Reduction Factor (GRF) is the total mean power reduction
for a 30° antenna compared to an Omni antenna. If 30o antennas are used the
specified GRF should be added to the path loss. Note that this implies that all
3 taps are affected equally due to effects of local scattering. K-factors have
linear values, not dB values. K-factors for the 90% and 75% cell coverage are
shown in the tables, i.e., 90% and 75% of the cell locations have K factors
greater or equal to the K-factor value specified, respectively. For the SUI
channels 5 and 6, 50% K-factor values are also shown.
5.4: Characteristic of SUI-1
5.6: Characteristic of SUI-3
5.7: Characteristic of SUI-4
Model
chapter describes the simulation part of the thesis. A brief description of
time and frequency division duplex is described first and then the simulation
procedure is explained step by step with appropriate diagrams. We have employed
Matlab 9.0 to develop the simulator. Before going for the physical layer setup,
let us first define the OFDM symbol parameter used in our study.
Physical Layer Setup
physical layer handles error correction and signal connectivity, as well as
registration, initial ranging, connectivity channels and bandwidth request for
data and management. Physical layer consists of some sequence of equal length
frames which transmit through modulation and coding of RF signals. OFDM
technology has been using by WiMAX technology. Different user assigning
different sub carries which are allowed in orthogonal frequency division
multiplexing (OFDM) techniques. It is durable to multi-path which helps to
overcome multipath signals hitting the receiver. OFDM signals divide into 256
carries in IEEE-802.16 standard and IEEE 802.16e use scalable OFDMA. Wide range
of frequencies supported by IEEE 802.16 standard and physical layer contains
several multiplexing and modulation forms. Modulation methods in the uplink
(UL) and downlink (DL) are Binary Phase Shift Keying (BPSK), Quadrature Phase
Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM).
6.1: IEEE 802.16 Protocol Layer (IEEE-2004)
supports both full and half duplex. Two types of transmission supported by IEEE
802.16,
Division Duplex (TDD)
Division Duplex (FDD)
Time Division Duplex (TDD)
division duplex framing is adaptive (when input changes it output behavior is
automatically change). It consists fixed duration which consists one downlink
frame and uplink frame. Base station (BS) sends complete downlink (DL-MAP,
UL-MAP). Up and Down link share same frequency but they are separated in time.
Frequency Division Duplex (FDD)
transmission in frequency division, multi-path is scheduled by DL-MAP and
UL-MAP. Downlink and uplink can be done in same time, but on different
frequency. UL and DL channels grouped into some continuous blocks of some
paired channel. FDD system provide full duplex where we can make some
application like voice, where DL and UP traffic requirement need more or less
symmetric. In Base station (BS) to base station interface kept in minimum, in
this technique, network for radio communication planning is easier.
Simulation Procedure:
6.2: Mobile WiMAX Performance Simulation Block Diagram
6.3: Data
6.4: Data Decoding
diagram 6.2 shows the whole process of
the thesis work. Every part of the diagram is described below:
Transmitter Module
subsection describes the transmitter module used for the simulation.
Mersenne Twister-Random Number Generator Algorithm
Twister is a pseudo random number generator that produces a sequence of zeros
and one bits. It might be combined into sub-sequences of zeros and ones or
blocks of random numbers. There are two types of random number which is called
deterministic and nondeterministic. We are dealing deterministic random
numbers. A deterministic Random Number Generator (RNG) produces a sequence of
bits from an initial value which is called seed. The seed value is 19,937 bits
long and stored in 624 element array. The RNG algorithm has a period of
2**19937-1. A Pseudo Random Number Generator (PRNG) produces values based on a
seed and current values. In our simulation we used this algorithm as function
rand () to generate the random input value for evaluate the performance of
WiMAX.
Modulation
passed the random values through the adaptive modulation schemes according to
the constellation mapped. The data was modulated depending their size and on
the basis of different modulation schemes like BPSK, QPSK, 16-QAM and 64-QAM.
The modulation has done on the basis of incoming bits by dividing among the
groups of i. That is why there are 2i points. The total number of bits
represented according to constellation mapped of different modulation
techniques. The size of i for BPSK, QPSK, 16-QAM and 64-QAM is 1, 2, 4 and 16
respectively.
ReedSolomon Encoder
randomized data are arranged in block format before passing through the encoder
and a single 0X00 tail byte is appended to the end of each burst. The
implemented RS encoder is derived from a systematic RS (N=255, K=239, T=8) code
using GF (28). The following polynomials are used for code generator and field
generator:
= (x+?0)( x+?0)… (x+?2T-1), ? = 02HEX —————————————-(6.1)
= x8 + x4 + x3 + x2 + 1 ———————————————————–(6.2)
encoder support shortened and punctured code to facilitate variable block sizes
and variable error correction capability. A shortened block of k´ bytes is
obtained through adding 239k´ zero bytes before
data block and after encoding, these 239k´ zero bytes are discarded. To obtain
the punctured pattern to permit T´ bytes to be corrected, the first 2T´ of the
16 parity bytes has been retained.
Convolutional Encoder
outer RS encoded block is fed to inner binary convolutional encoder. The
implemented encoder has native rate of 1/2, a constraint length of 7 and the
generator polynomial in Equation (6.3) and (6.4) to produce its two code bits.
The generator is shown in Figure 6.5.
= 171OCT For X ————————————————————————(6.3)
= 133OCT For Y ————————————————————————(6.4)
6.5: Convolutional encoder of rate ½
6.1: Puncturing configuration of the convolution code
order to achieve variable code rate a puncturing operation is performed on the
output of the convolutional encoder in accordance to Table 6.1. In this Table
“1” denotes that the corresponding convolutional encoder output is used, while
“0” denotes that the corresponding output is not used. At the receiver Viterbi
decoder is used to decode the convolutional codes.
Interleaver
encoded data are interleaved by a block interleaver. The size of the block is
depended on the numbers of bit encoded per subchannel in one OFDM symbol,
Ncbps. In IEEE 802.16, the interleaver is defined by two step permutation. The
first ensures that adjacent coded bits are mapped onto nonadjacent subcarriers.
The second permutation ensures that adjacent coded bits are mapped alternately
onto less or more significant bits of the constellation, thus avoiding long
runs of unreliable bits [1].
Matlab implementation of the interleaver was performed calculating the index
value of the bits after first and second permutation using Equation (6.5) and
(6.6) respectively.
0,1,2,… … ..Ncbps1 —
————— –(6.5)
–floor(12.mk/Ncbps))mod(s) k=0,1,2, Ncbps1 (6.6)
s= ceil(Ncpc/2) , while Ncpc stands for the number of coded bits per
subcarrier, i.e.,
or 6 for BPSK,QPSK 16-QAM, or 64-QAM, respectively.
default number of subchannels i.e 16 is used for this implementation.
receiver also performs the reverse operation following the two step permutation
equations (6.7) and (6.8) respectively.
= s. floor(j/s)+(j+floor(12.j/Ncbps))mod(s) j=0,1,… … ..Ncbps1 ———–(6.7)
= 12.fj – (Ncbps 1). floor(12.fj/Ncbps) j=0,1,2… … .Ncbps1 ————(6.8)
Constellation Mapper
bit interleaved data are then entered serially to the constellation mapper. The
Matlab implemented constellation mapper support BPSK, QPSK, 16-QAM, and 64-QAM
. The complex constellation points are normalized with the specified
multiplying factor for different modulation scheme so that equal average power is achieved for the
symbols. The constellation mapped data are assigned to all allocated data
subcarriers of the OFDM symbol in order of increasing frequency offset index.
IFFT
OFDM symbol threats the source symbols to perform frequency-domain into
time-domain. If we chose the N number of subcarriers for the system to evaluate
the performance of WiMAX the basic function of IFFT receives the N number of
sinusoidal and N symbols at a time. The output of IFFT is the total N
sinusoidal signals and makes a single OFDM symbol. The mathematical model of
OFDM symbol defined by IFFT which would be transmitted during our simulation as
given bellow:
Subcarriers
OFDM system, the carriers are sinusoidal. Two periodic sinusoidal signals are
called orthogonal when their integral product is equal to zero over a single
period. Each orthogonal subcarrier has an integer number of cycles in a single
period of OFDM system. To avoid inter channel interference these zero carriers
are used as a guard band in this system.
OFDM Symbol Description
WiMAX Transmitter, IFFT (Inverse Fast Fourier Transform) used to create OFDM
waveform with the help of modulated data streams. On the other hand in WiMAX
receiver end the FFT used to demodulate the data streams. This time duration is
defined to as symbol time, Tb. A copy of symbol period, Tg which is termed of
Cyclic Prefix (CP) used to collect multipath where maintaining the
orthogonality of the codes. The following fig 6.6 shows the OFDM symbol in the
time domain.
6.6: OFDM Symbol Structure in Time Domain
any OFDM symbol, the transmitted signal voltage to the antenna in time domain
can be written as
= Time elapsed with 0 ? t ? TS
= Complex number where k is the frequency offset index of transmitted data
Carrier spacing
= FS/Nfft where FS = Sampling frequency and Nfft = Number of points of FFT/IFFT
OFDM system, the number of sub-carriers is 256 which is equal to the FFT size.
Each OFDM symbol consists of the following four types of carriers .
sub-carriers (OFDM) or sub-channels (OFDMA): used for data transmission
sub-carriers: used for various estimation purposes
sub-carriers: used as center frequency
sub-carriers/Guard bands: used for keeping space between OFDM and OFDMA signals
following fig 6.7 shows the OFDM symbol in frequency domain,
avoid Intersymbol Interference (ISI) the Cyclic Prefix (CP) is inserted in OFDM
system before each transmitted symbol. In wireless transmission the transmitted
signals might be distort by the effect of echo signals due to presence of
multipath delay. The ISI is totally eliminated by the design when the CP length
is greater than multipath delay. After performing Inverse Fast Fourier
Transform (IFFT) the CP will be add with each OFDM.
CP Insertion
maintain the frequency orthogonality and reduce the delay due to multipath
propagation, cyclic prefix is added in OFDM signals. To do so, before
transmitting the signal, it is added at the beginning of the signal. In
wireless transmission the transmitted signals might be distort by the effect of
echo signals due to presence of multipath delay. The ISI is totally eliminated
by the design when the CP length L is greater than multipath delay. After
performing Inverse Fast Fourier Transform (IFFT) the CP will be add with each
OFDM symbol.
Channel Module / Wireless Channel
wireless communication, the data are transmitting through the wireless channel
with respective bandwidth to achieve higher data rate and maintain quality of
service. The transmitting data has to take environmental challenges when it is
on air with against unexpected noise. That’s why data has to encounter various
effects like multipath delay spread, fading, path loss, Doppler spread and
co-channel interference. These environmental effects play the significant role
in WiMAX Technology. To implement an efficient wireless channel have to keep in
mind the above fact. In this section we are presenting the wireless channels.
White Gaussian Noise (AWGN)
Fading Channel
University Interim (SUI)
Additive White Gaussian Noise (AWGN)
is a noise channel. This channel effects on the transmitted signals when
signals passes through the channel. This noise channel model is good for
satellite and deep space communication but not in terrestrial communication
because of multipath, terrain blocking and interference. AWGN is used to
simulate background noise of channel. The mathematical expression as in
received signal r(t) = s(t) + n(t) is shown in figure 6.8 which passed through
the AWGN channel where s(t) is transmitted signal and n(t) is background noise.
6.8: AWGN Channel
Rayleigh Fading Channel
Fading is one kind of statistical model which propagates the environment of
radio signal. According to Rayleigh distribution magnitude of a signal which
has passed though the communication channel and varies randomly. Rayleigh
Fading works as a reasonable model when many objects in environment which
scatter radio signal before arriving of receiver. When there is no propagation
dominant during line of sight between transmitter and receiver on that time
Rayleigh Fading is most applicable. On the other hand Rician Fading is more
applicable than Rayleigh Fading when there is dominant line of sight. During
our simulation we used Rayleigh Fading when we simulate the performance of Bit
Error Rate Vs Signal to Noise Ratio.
Receiver Module
directional Antenna is the most popular antenna in WiMAX, which can be used for
point-to-multipoint configuration. The main feature of Omni Directional antenna
is that, it can be deployed broad-casting in 3600 angle. This is the limitation
of its range and ultimately it shows its signed strength. Omni directional
antennas are mostly user friendly when lots of subscribers stay very close to
the base station.
CP Removal
transmitting module, to deal the frequency orthogonality and reduce the delay,
cyclic prefix added in each OFDM signals. That’s why, before transmitting the
signal, the CP added at the beginning of the signal. After performing Inverse
Fast Fourier Transform (IFFT) the CP will be add with each OFDM symbol. In
receiver module, after synchronization the received data contains the Cyclic
Prefix of each OFDM signal which is ignored.
FFT
using number of samples FFT converts time domain signal into frequency domain signal.
The FFT frequency domain signal defined as 1/ Ts_tot (where Ts_tot is total
number of samples). In transmitter module, IFFT converts the OFDM signals from
frequency domain to time domain which is exactly reverse work of FFT. To
perform of OFDM 256 points, the zeros are padded beginning and ending of the
OFDM signal. These zero pads will be removed from the corresponding places at
the receiving module.
Channel Equalizer
our simulation we used Zero-Force Bock Equalizer (ZFE) and Minimum Mean Square
Equalizer (MMSE) which are described below.
Block Equalizer (ZFE)
force channel equalizer removes the output of equalizer Inter symbol
interference (ISI) from the channel. This equalizer works as a noise remover
but if the channel has no noise then it remain ideal condition.
6.9: Block Diagram of a Simple Transmission in Zero-Force Equalize
estimates data of the output of channel is,
implies that,
—————————————-(6.12)
equation leads to,
——————————–(6.13)
Z-transform of this above equation is,
—————————–(6.14)
G (Z), F (Z) and B (Z) are the Z-transform of gn , fn and bn. This means that
the zero force equalizer is constructed by the inverse filter.
Mean square Error Equalizer (MMSE)
order to minimize the minimum square error at the output of the equalizer
defined as:
E[x] indicates the expected value of x. The MMSE equations are as follows:
6.10: Block Diagram of a Simple Transmission in MMSE Equalizer
B (Z) = channel transfer function, F (Z) = transfer function of equalizer, So,
the output of the MMSE equalizer is,
Demodulation
works to extract the original data from a modulated waveform. At the receiver,
an electronic circuit works to recover the different base-band signals which
have already transmitted from the transmitter end which is called demodulator
[30].
Results
this chapter the simulation results are shown and discussed. In the following
sections, first we will present the structure of the implemented simulator and
then we will present the simulation results.
Bit Error Rate (BER)
number of bits error occurs within one second in transmitted signal then we
called Bit Error Rate (BER). In another sentence Bit Error rate is one type of
parameter which used to access the system that can transmit digital signal from
one end to other end. We can define BER as follows,
transmitter and receiver’s medium are good in a particular time and
Signal-to-Noise Ratio is high, then Bit Error rate is very low. In our thesis
simulation we generated random signal when noise occurs after that we got the
value of Bit error rate.
SNR
per bit to noise power spectral density ratio is important role especially in
simulation. Whenever we are simulating and comparing the Bit Error rate (BER)
performance of adaptive modulation technique is very necessary Eb/N0. The
normalized form of Eb/N0 is Signal-to-
Noise Ratio (SNR). In telecommunication, Signal-to-Noise ratio is the form of
power ratio between a signal and background noise,
P is mean power. In this case the signal and the background noise are measured
at the same point of view if the measurement will take across the same
impedance then SNR would be obtained by measuring the square of the amplitude
ratio.
BER Vs SNR
Bit Error Rate (BER) defined as the probability of error (Pe). On the other
hand Signal-to- Noise is the term of power ratio between a signal and
background noise. There are three variables like,
error function (erf)
energy per bit (Eb)
noise power spectral density (N0)
modulation scheme has its own value for the error function. That is why each
modulation scheme performs in different manner due to the presence of
background noise. For instance, the higher modulation scheme (64-QAM) is not
robust but it carries higher data rate. On the contrary, the lower modulation
scheme (BPSK) is more robust but carries lower data rate. The energy per bit, Eb
defined by dividing the carrier power and measured of energy with the unit of
Joules. Noise power spectral density (N0) is power per hertz with the unit of
Joules per second. So, it is clear that the dimension of SNR is cancelled out.
So we can agree on that point that, the probability of error is proportional to
Eb/N0.
Physical layer performance results
basic goal of this thesis is to analyze the performance of WiMAX OFDM physical
layer based on the simulation results. In order to analyze, the BER Vs SNR plot
was investigated.
BPSK
7.3: SUI-1 BER over SNR for 16-QAM
7.4: SUI-1 BER over SNR for 64-QAM
for different modulation and coding profile for SUI-1
7.2: SNR required at BER level 10-2 for different modulation and coding profile
for SUI-2
7.11: SUI-3 BER over SNR for 16-QAM
7.3: SNR required at BER level 10-2 for different modulation and coding profile
for SUI-3
7.18: SUI-5 BER over SNR for QPSK
7.19: SUI-5 BER over SNR for 16-QAM
7.20: SUI-5 BER over SNR for 64-QAM
7.21: SUI-6 BER over SNR for BPSK
7.22: SUI-6 BER over SNR for QPSK
7.23: SUI-6 BER over SNR for 16-QAM
7.24: SUI-6 BER over SNR for 64-QAM
Conclusion:
all conditions we applied and the results we got we can conclude our work as
follows,
studied WiMAX OFDM physical layer, mobile systems, modulation techniques and
features of WiMAX networks properly, with the help of necessary figures and
tables.
studied SUI-1 to SUI-6 channel model and also implemented it through Matlab
simulation to evaluate the performance of Mobile WiMAX.
also used and understood the adaptive modulation techniques like, BPSK, QPSK,
16-QAM and 64-QAM according to IEEE 802.16d standard.
all aspects of adaptive modulation technique, we can conclude the performance
of Mobile WiMAX as,
Phase Shift Keying (BPSK) is more power efficient and needs less bandwidth.
the other hand 64-Qadrature Amplitude Modulation (64-QAM) has higher bandwidth
with very good output.
another case, Quadrature Phase Shift Keying (QPSK) and 16-QAM modulation
techniques are in middle of those two (BPSK and 64-QAM) and they requires
higher bandwidth.
and 16-QAM are less power efficient than BPSK.
all simulations we got, BPSK has the lowest BER and 64-QAM has the highest BER
than other modulation techniques.
also add some more things in here,
included Cyclic Prefix (CP) and random signals which reduced noise resulting
lower Bit error Rate (BER) for OFDM system but increased the complexity in the
system.
Prefix requires higher power but non Cyclic Prefix requires lower power.
Future Work:
lot of works can be done for future optimization of Wireless communication
especially in WiMAX system. Adaptive modulation techniques and WiMAX physical
layer can be adopted with High Amplitude Platform (HAP) and Long Term
Evaluation (LTE).
implemented PHY layer model still needs some improvement. The channel estimator
can be implemented to obtain a depiction of the channel state to combat the
effects of the channel using an equalizer. The IEEE 802.16 standard comes with
many optional PHY layer features, which can be implemented to further improve
the performance. The optional Block Turbo Coding (BTC) can be implemented to
enhance the performance of FEC. Space Time Block Code (STBC) can be employed in
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