The Performance Evaluation of IEEE 802.16 Physical Layer in the Basis of Bit Error Rate Considering Reference Channel Models

International Journal on Cybernetics & Informatics ( IJCI) Vol.2, No.4, August 2013
DOI: 10.5121/ijci.2013.2403 17
The Performance Evaluation of IEEE 802.16
Physical Layer in the Basis of Bit Error Rate
Considering Reference Channel Models
Arifa Ferdousi1
, Farhana Enam2
and Sadeque Reza Khan3
1
Department of Computer Science and Engineering,
Varendra University, Rajshahi, Bangladesh
arifaferdousi@yahoo.com
2
Dept. of Information and communication Engineering, Rajshahi University, Bangladesh
farhana_ice2008@yahoo.com
3
Dept. of Electronics and Communication Engineering, NITK, Surathkal, India
sadeque_008@yahoo.com
ABSTRACT
Fixed Broadband Wireless Access is a promising technology which can offer high speed data rate from
transmitting end to customer end which can offer high speed text, voice, and video data. IEEE 802.16
WirelessMAN is a standard that specifies medium access control layer and a set of PHY layer to fixed and
mobile BWA in broad range of frequencies and it supports equipment manufacturers due to its robust
performance in multipath environment. Consequently WiMAX forum has adopted this version to develop
the network world wide. In this paper the performance of IEEE 802.16 OFDM PHY Layer has been
investigated by using the simulation model in Matlab. The Stanford University Interim (SUI) channel
models are selected for the performance evaluation of this standard. The Ideal Channel estimation is
considered in this work and the performance evaluation is observed in the basis of BER.
KEYWORDS
IEEE 802.16 standard; OFDM; Pseudo-Random Binary Sequence (PRBS); Bit Error Rate (BER); SUI
Channel.
1. INTRODUCTION
Broadband wireless access (BWA) is about bringing the broadband experience to a wireless
context, which offers users certain unique benefits and convenience. Thus it 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. At present, Digital Subscriber
Line (DSL) technology, which delivers broadband over twisted pair telephone wires, and cable
modem technology, which delivers over coaxial cable TV plant, are the predominant mass-market
broadband access technologies. But the practical difficulties in deployment have prevented them
from reaching many potential broadband internet customers. Many areas throughout the world are
not under the broadband access facilities. Even many urban and sub-urban areas cannot be served
by DSL connectivity. As, it can reach about only three miles from the central switching office.
Again many older cable networks do not have return channel which will prevent to offer internet
access and thus many commercial areas are often not covered by cable network. But the BWA
can overcome these types of situations and difficulties. Because of its wireless nature, it can be
faster to deploy and easier to scale and more flexible, thereby giving it the potential to serve those
customers who are not served or not satisfied by their wired broadband alternatives [1].

18
There are two fundamentally broadband wireless services (BWA). The first type attempts to
provide a set of services similar to that of traditional fixed-line broadband, can be thought of as
alternative to DSL or cable modem. The second type of BWA is called mobile broadband, which
offers the additional functionality of portability, nomad city and mobility.
IEEE 802.16 standard and its associated industry syndicate, Worldwide Interoperability for
Microwave Access WiMAX forum promise to offer high data rate over large areas, to a large
number of users where broadband is unavailable. This is the first industry-wide standard that can
be used for the fixed wireless access with substantially higher bandwidth than more cellular
networks [2]. Wireless broad band systems have been in use for many years, but the development
of this standard enables economy of scale that can bring down the cost of equipment, ensure
interoperability, and reduce investment risk for operators.
The first version of IEEE 802.16 standard operates in the 10-66 GHz frequency band and requires
line of sight (LOS) towers. Later the standard extended its operation through different PHY
specification to 2-11 GHz frequency band enabling the no line of sight (NLOS) communications,
which requires the techniques that efficiently mitigate the impairment of fading and multipath [3].
Taking the advantage of the OFDM technique the PHY is able to provide robust broad band
service in hostile wireless channel.
The OFDM based physical layer of the IEEE 802.16 standard has been standardized in close
cooperation with European Telecommunications Standard Institute (ETSI) High Performance
Metropolitan Area Network (HiperMAN) [4]. Thus the HiperMAN standard and the OFDM
based physical layer of IEEE 802.16 are nearly identical. Both OFDM-based physical layers shall
comply with each other and a global OFDM system should emerge [3]. The WiMAX forum
certified products for BWA comply with the both standards.
The main objective of this research work is to implement and simulate the IEEE 802.16 OFDM
physical layer using the Matlab 7 to have better understanding of the standard and the system
performance. This involves studying, through simulation, the various PHY modulations, coding
schemes in the form of bit error rate (BER).
2. WIRELESS MAN-OFDM PHY LAYER
The WiMAX forum has adopted the version of 256-point OFDM based air interface specification
for the reasons such as lower peak to average ratio, faster fast Fourier transform (FFT) calculation
and less stringent requirements for frequency synchronization compared to 2048-point Wireless
MAN-OFDMA. The size of the FFT point determines the number of subcarriers. Among these
256 subcarriers, 192 are used for user data, 56 are used as null for guard band and 8 are used as
pilot subcarriers for various estimation purposes. The PHY allows to accept variable Cyclic
Prefix (CP) length of 8, 16, 32 or 64 depending on the expected channel delay spread.
3. SIMULATION METHODOLOGY
As the research goal is to evaluate the performance of the IEEE 802.16 OFDM PHY layer, so this
task involves modeling of the physical layer and propagation model. The simulator is developed
by using the Matlab 7. Before discussing the physical layer setup OFDM symbol parameter is
defined.Here there are two types of symbol parameter. One is primitive and one is derived. These
two parameters characterize the OFDM symbol completely. The later one can be derived from the

19
former one because of fixed relation between them. Here, in MATLAB implementation the
primitive parameters are calculated as ‘IEEE80216params’ which can be accessed globally.
The used OFDM parameters are given in the following table 1:
Table 1: OFDM Symbol Parameters
4. PHYSICAL LAYER SETUP [4]
Figure 1 shows the base band part of the implemented transmitter and receiver. It corresponds to
the PHY layer of the IEEE 802.16-2004 WirelessMAN-OFDM air interface. In this setup only the
mandatory features are implemented. Channel coding part consists of three steps; Randomization,
Forward Error Correction (FEC) and interleaving. Again FEC is done through two phases; outer
phase is Reed-Solomon (RS) and Convolutional Code (CC). At the receiving end the
complementary operations are implemented in the reverse order at the channel coding. The
complete channel encoding setup is shown in figure 2 and the corresponding decoding set is
shown in figure 3.
Fig: 1: Simulation Setup
Fig: 2: Channel Encoding Setup

20
Fig: 3: Channel Decoding Setup
Fig: 3: Channel Decoding Setup
The Pseudo-Random Binary Sequence (PRBS) generator used for randomization is shown in
figure 4. Each data byte to be transmitted enters sequentially into the randomizer, with the Most
Significant Byte (MSB) first. Preambles are not randomized. The randomizer sequence is applied
only to information bits.
Fig: 4: PRBS generator used for data randomization in OFDM and OFDMA PHY. (From IEEE
Std 802.16-2004.)
4.2.RS Encoder
The RS encoder of OFDM PHY is denoted as an (N, K) = (255, 239) code, which is capable of
correcting up to eight symbol errors per block. This Reed–Solomon encoding uses GF(28
), where
GF is the Galois Field operator. The Reed–Solomon encoder and decoder require Galois field
arithmetic. The following polynomials are used for the OFDM RS systematic code, an RS code
that leaves the data unchanged before adding the parity bits:
       HEX
T
xxxxG 02,............. 1210
 
 ………………………………………(1)
1)( 2348
 xxxxxp ………………………………………………………………....(2)
The coding rate of the OFDM PHY RS encoder is then 239/255 (very close to one). The standard
indicates that this code can be shortened and punctured to enable variable block sizes and variable
error-correction capabilities.
4.3. CC Encoder
The outer RS encoded block is fed into inner binary convolutional encoder. The convolution code
has an original coding rate of 1/2, has a constraint length of 7. In order to achieve variable code
rate a puncturing operation is performed on the output of the convolutional encoder in accordance
to table 2. In this table “1” denotes that the corresponding convolutional encoder output is used,

21
while “0” denotes that the corresponding output is not used. At the receiver Viterbi decoder is
used to decode the convolutional codes.
Table 2: Puncturing output configuration of the convolutional code.
Rate dFREE X output Y output XY( punctured
output)
1/2 10 1 1 X1Y1
2/3 6 10 11 X1Y1Y2
3/4 5 101 110 X1Y1Y2X3
5/6 4 10101 11010 X1Y1Y2X3Y4X5
4.4. Interleaver
Interleaving is used to protect the transmission against long sequences of consecutive errors,
which are very difficult to correct. The encoded data bits are interleaved by a block inter-leaver
with a block size corresponding to the number of coded bits per allocated sub-channels per
OFDM symbol. The Matlab implementation of the interleaver was performed calculating the
index value of the bits after first and second permutation using the following equation 3 and 4
respectively
    1,...2,1,0,.....12.12 12mod  cbpscbpsk NkwherekfloorkNf ……………………....(3)
  1,...2,1,0,,))/.12((/. )mod(  cbpsscbpskcbpskk NkwhereNmfloorNmsffloorss
…(4)
where s=ceil )2/( cbpsN , while cbpsN stands for the number of coded bits per subscriber, i.e. ,1, 2,
3, 4 or 6 for BPSK, QPSK, 16-QAM OR 64-QAM, respectively.
The default number of subchannels i.e. 16 is used for this implementation.
The receiver also performs the reverse operation by following two permutation using equation 5
and 6
)mod())/.12(()/(. scbpsj Njfloorjsjfloorsf  , j=0,1,2,……… cbpsN -1 ……...........(5)
))/.12().1(.12 cbpsjcbpsjj NffloorNfs  , j=0,1,2,……… cbpsN -1 ………….........(6)
4.5. Constellation Mapper
The interleaved data are then entered serially to the constellation mapper. The Matlab
implemented constellation mapper support BPSK, grey-mapped QPSK, 16-QAM and 64-QAM.
The complex constellation points are normalized with the specified multiplying factor for
different modulation scheme so that equal power is achieved for the symbols.
4.6. IFFT
The grey-mapped data are then sent to IFFT for time domain mapping. Mapping to time domain
needs the application of the Inverse Fast Fourier Transform (IFFT). In this research the MATLAB

22
‘ifft’ function is called to do so. This block delivers a vector of 256 elements, where each
complex number element represents one sample of the OFDM symbol.
4.7. Cyclic Prefix (CP) insertion
A cyclic prefix is added to time domain data to combat the effect of the multipath. Four different
duration of cyclic prefix are available in the standard. The ratio of CP time to OFDM symbol time
can be equal to 1/32, 1/6, 1/8, 1/4.
5. CHANNEL MODEL
To get a perfect evaluation of the performance of the developed communication system, an
accurate description of the wireless channel is required to address its propagation, and the
wireless channel is characterized by the factors like environment Path loss, Multipath delay
spread, Fading characteristics, Doppler spread, Co-channel and adjacent channel interference [5].
While simulating the system these factors are considered. To take care of these requirement an
empirical model is chosen, the Stanford University Interim (SUI) channel models. SUI channel
models are an extension of the earlier work by AT&T Wireless and Erceg et al [6]. In this model
a set of six channels was selected to address three different terrain types [7]. This model is used
for simulation, design, and testing the technologies suitable for fixed broadband wireless
applications [8]. The parameters for the model were selected based upon some statistical models.
The parametric view of the six SUI channels is depicted in the following table 3 and 4.
Table 3: Terrain type for SUI channels
Table 4: General Characteristics of SUI Channels
A scenario has been assumed with the following parameters:
 Cell size: 7 Km
 BTS antenna height: 30m
 Receive antenna height: 6m
 BTS antenna beam width: 1200
 Receive antenna beam width: omni directional
 Polarization: Vertical only.
 90% cell coverage with 99.9% reliability at each location covered

23
6. SUI CHANNEL MODEL IMPLEMENTATION
The goal of the model implementation is to simulate the channel coefficients. Channel
coefficients with the specified distribution and spectral power density are generated using the
method of filtered noise. A set of complex zero-mean Gaussian distributed number is generated
with a variance of 0.5 for the real and imaginary part for each tap to achieve the total average
power of this distribution is 1. In this way, a Rayleigh distribution (equivalent to Ricean with
K=0) is got for the magnitude of the complex coefficients. In case of a Ricean distribution (K>0),
a constant path component m has to be added to the Rayleigh set of coefficients. The K-factor
specifies the ratio of powers between this constant part and the variable part. The distribution of
the power is given below:
The total power P of each tap:
22
 mp ................................................................................................................(7)
Where m is the complex constant and 2
 is the variance of the complex Gaussian set
The ratio of the power is:
2
2

m
k  ........................................................................................................................(8)
From the above two equations the power of the complex Gaussian:
1
1
.2


K
p ..............................................................................................................(9)
and the power of the constant part as :
1
.
2


K
k
pm ...........................................................................................................(10)
this SUI channel model address a specific power spectral density (PSD) function for the scatter
component channel coefficients is given by:
 







1,0
1,785.072.11
0
0
4
0
2
0
f
fff
fS ........................................................................(11)
where the function is parameterized by a maximum Doppler frequency mf and
mf
ff 0 ......................................................................................................................(12)
To generate a set of channel coefficients with this PSD function, original coefficients are
correlated with a filter which amplitude frequency response is:
   fSfH  ...........................................................................................................(13)
For efficient implementation, a non recursive filter and frequency-domain overlap-add method
has been used.
There are no frequency components higher than mf (for the construction formula of S(f)): So the
channel can be represented with a minimum sampling frequency of 2 mf according to the Nyquist
theorem. For this reason the sampling frequency is chosen 2 mf . The power of the filter has to be
normalized to 1, so that the total power of the output is equal to input one.

24
7. RESULT AND DISCUSSION
The simulator is developed in Matlab using modular approach. Each block of transmitter, receiver
and channel is developed in separate ‘m’ file. The main procedure calls each block in a manner,
so that a communication system works. The main procedure also contains the initialization
parameters, input data, and it delivers results. The parameters like, number of OFDM symbols,
CP length, modulation type, coding rate, range of SNR values and SUI channel models, can be set
at the time of initialization. The input data stream is generated randomly. Output variables are
available at the Matlab workspace. And the bit error rate (BER) for different SNR are stored in
the text files to draw the plots. Each single block of transmitter is tested with its counter part of
the receiver side to confirm that it works perfectly. Now the various BER vs. SNR plots are
shown for all mandatory modulation and coding profiles as specified in the standard on same
channel models. Figure 5, 6 and 7 show the performance on SUI-1, SUI-2, and SUI-3 channel
models respectively. It is observed from the figures that the lower modulations and coding
schemes provide better performance with less SNR. Larger distance between the adjacent points
can tolerate larger noise at the cost of coding rate. Adaptive modulation schemes can be attain the
highest transmission speed with a target BER. SNR required attaining at BER level 10-3
are listed
in the following table 5.
Figure 5: BER vs SNR plot for different coding profiles on SUI-1 channel.

25
8. CONCLUSION
The summery of this research is the implementation of the IEEE 802.16 OFDM PHY layer using
Matlab in order to evaluate the PHY layer performance under reference channel model. The
implemented PHY layer supports all the modulation schemes. To keep the research work doing
over-sampling of the data is avoided before using the channel models. This can be implemented
by minor modification. On the receiver side, perfect channel estimation was assumed to avoid the
effect of any particular estimation method on the simulation results, though insertion of pilot
subcarriers in the OFDM symbols make use of any comb-type estimator possible. In this
performance evaluation, the effects of forward error correction (FEC) on different modulation
under reference channel are not observed because of shortage of time. If it was done the
performance evaluation will be completed perfectly.
Simulation is the methodology used to investigate the PHY layer performance. The performance
evaluation method was mainly concentrated on the effect of channel coding on the PHY layer.
The overall system performance was also evaluated under different channel conditions. Scatter
plots were generated to validate the model in terms of general trends in reception quality as the
different parameters were varied. A key performance measure of a wireless communication
system is BER and BLER. Here the BER curves are used to compare the performance of different
modulation and coding scheme used.

26
REFERENCES
1. Book: Jeffrey G. Andrews, Arunabha Ghosh, Riaz Muhamed, “Fundamentals of WiMAX:
Understanding Broadband Wireless Networking”, Published by Prentice & Hall,2007.
2. Ghosh, A.,: Wolter, D.R.; Andrews, J.G.; Chen, R., “Broadband wireless access with WiMax/802.16:
current performance benchmark and future potential”, Communications Magazine, IEEE, Vol.43,
Iss.2, Feb.2005.
3. Koffman, I.; Roman, V., “Broadband wireless access solutions based on OFDM access in IEEE
802.16”Communications magazine, IEEE, Vol.40, Iss.4, April 2002.
4. ETSI Broadband Radio Access Networks (BRAN); HIPERMAN; Physical (PHY) Layer. Standard
TS102 177, 2003.
5. Book: Kaveh Pahlavan, Prasant Krishnamurty, “Principals of Wireless Networks”, Published by
Prentice & Hall, 2007.
6. Book: Bernard Sklar, “Digital communications: Fundamentals and Applications, 2nd edition”,
January 11, 2001.
7. V. Erceg, et al, “An empirically based path loss models for wireless channels in suburban
environments”, IEEE JSAC, vol. 17, no.7, July 1999.
8. Book: V. Erceg, K. V. S. Hari, M. S. Smith, D. S. Baum et al, “Channel Models for Fixed wireless
applications”, February 1, 2001.
Authors
Arifa Ferdousi received B.Sc. and M.Sc. degree in ICE from University of Rajshahi,
Bangladesh, in the year of 2007 and 2009 respectively. Currently she is working as a
lecturer in the department of CSE in Varendra University, Rajshahi, Bangladesh. Her
research interest includes electronics system designing, OFDM, Advanced LTE Wi-
Max and Bangla speech recognition system using Neural Network. She is the member
of Bangladesh Electronic Society (BES).
Sadeque Reza Khan received B.Sc. degree in Electronics and Telecommunication
Engineering from University of Liberal Arts Bangladesh and continuing his M.Tech in
VLSI from National Institute of Technology Kernataka (NITK), India. Currently he is
in study leave from his Institution where he was working as a lecturer in the
department of Electrical and Electronic Engineering in Prime University, Bangladesh.
His research interest includes VLSI, Microelectronics, Control System Designing and
Embedded System Designing.

The Performance Evaluation of IEEE 802.16 Physical Layer in the Basis of Bit Error Rate Considering Reference Channel Models

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