SIMULATION OF THE
W
i
MAX
(IEEE 802.16
e
) PHYSICAL LAYER
(PHASE 4)
Presented by:
Ahmad Salim
2
INTRODUCTION
The acronym
WiMAX
stands for “Worldwide
Interoperability for Microwave Access”. It
is based
on IEEE 802.16
standard for
Wireless Metropolitan
Area Network (Wireless MAN).
It
specifies the air interface for fixed, portable, and
mobile broadband wireless access (BWA) systems
supporting multimedia services
.
W
i
MAX
Block Diagram (Physical Layer)
3
FEC Encoding
1.Reed

Solomon
2.
Convolutional
3. Optional:
Turbo, LDPC, ..
OFDM
IFFT, + CP..
Channel
+
Randomization
Interleaving
Data
Digital
Modulation
(Symbol Mapping)
AWGN
FEC
Decoding
1.Reed

Solomon
2.
Convolutional
3. Optional:
Turbo, LDPC, ..
OFDM
FFT,

CP..
De

Randomization
De

Interleaving
Estimated
Data
Digital
De

Modulation
(Symbol De

Mapping)
RANDOMIZER
Uncorrelates
long sequence of 1s or 0s by
XORing
with the synchronization frame data.
The purpose of randomization is to maintain better data integrity. Also the output of the
randomizer has equal number of 0’s and 1’s for given binary FEC block input.
The random sequence generator is a 2
15
− 1 Pseudo

Noise (PN) sequence generator with the
initial sequence set as

1 0 0 1 0 1 0 1 0 0 0 0 0 0 0
The initial sequence is reloaded for each FEC frame.
The random sequence generation is synchronized with the receiver which descrambles the
data.
From IEEE Std 802.16

2004 [
1]
FEC ENCODER
The 802.16* standards propose the following can be
used
–
Reed Solomon concatenated Convolution Coder (Mandatory)
Convolutional Turbo Codes (mandatory for Mobile Wimax)
Block Turbo Codes (Optional)
Low Density Parity Check Codes (Optional)
ENCODER
WiMAX modulation and coding schemes
AMC
Modulation
RS code
CC code rate
Overall
code
rate
1
BPSK
(12,12,0)
1/2
1/2
2
QPSK
(32,24,4)
2/3
1/2
3
QPSK
(40,36,2)
5/6
3/4
4
16

QAM
(64,48,4)
2/3
1/2
5
16

QAM
(80,72,4)
5/6
3/4
6
64

AQM
(108,96,6)
3/4
2/3
7
64

QAM
(120,108,6)
5/6
3/4
REED

SOLOMON ENCODER
A Reed

Solomon code is specified by RS(n, k, t).
The encoder takes k data symbols of l bits each and adds 2t
parity symbols to construct an n

symbol codeword.
n: number of bytes after encoding,
k: number of data bytes before encoding,
t: number of data bytes that can be corrected.
As
speciﬁed
in the standard, the Reed

Solomon encoding shall
be derived from a systematic RS( 255, 239, 8)
CONVOLUTIONAL
ENCODER
The generator polynomials used to derive its two output code bits,
denoted X and Y, are specified in the following expressions:
1
2
171 for X,
133 for Y
OCT
OCT
G
G
INTERLEAVER
Distribute the coded bits over subcarriers. A first
permutation ensures that adjacent coded bits are
mapped on to nonadjacent subcarriers.
The second permutation insures that adjacent coded
bits are mapped alternately on to less or more
significant bits of the constellation, thus avoiding long
runs of bits of low reliability.
MODULATION
MAPPER
BPSK, 4

QAM and 16

QAM constellation maps. (using Gray
mapping)
OFDM DEFINITION
OFDM = Orthogonal FDM
Carrier centers are put on orthogonal frequencies
ORTHOGONALITY

The peak of each signal coincides
with
trough
of other signals
Subcarriers are spaced by 1/
Ts
BASIC IDEA :
Channel bandwidth is divided into multiple
subchannels
to reduce ISI and frequency

selective fading.
FDM
VERSUS OFDM
Frequency
Division
Multiplexing
OFDM
frequency
dividing
Increase
In spectral
efficiency
OFDM
IN
WIMAX
WiMAX
speciﬁcations
for the 256

point FFT OFDM PHY layer
deﬁne
three types of subcarriers; data, pilot and null.
200 of the total 256 subcarriers are used for data and pilot
subcarriers, eight of which are pilots permanently spaced
throughout the OFDM spectrum.
The rest of the potential carriers are
nulled
and set aside for
guard bands.
OFDM frequency description.
The remaining 55 carriers, that are zero subcarriers appended at the
end of the cited structure, act as guard bands with the purpose to
enable the naturally decay of the signal.
These guard bands are used to decrease emissions in adjacent
frequency channels.
the structure of the subcarriers before and after appending the guard bands.
INVERSE FAST FOURIER TRANSFORM ALGORITHM
The IFFT is used to produce a time domain
signal.
each of the discrete samples before applying
the IFFT algorithm corresponds to an individual
subcarrier.
Besides ensuring the
orthogonality
of the
OFDM subcarriers, the IFFT represents also a
rapid way for modulating these subcarriers in
parallel.
THE CYCLIC PREFIX
The robustness of any OFDM transmission
against multipath delay spread is achieved by
having a long symbol period with the purpose
of minimizing the inter

symbol interference.
T
sym
: OFDM symbol time
T
b
: useful symbol time
T
g
: CP time.
g
b
T
G
T
Each OFDM symbol is preceded by a periodic
extension of the signal itself.
CP is a copy of the last portion of the data
symbol.
When eliminating ISI, it has to be taken into
account that the CP must be longer than the
dispersion of the channel.
SIMULATING SAMPLE SPACED RAYLEIGH
FADING CHANNEL
By sample spaced channel taps, we mean that the
difference in delays between different waves is either
some sampling interval T
s
or a multiple of it.
This channel can easily be implemented using a 3

tap
FIR filter as the sampling frequency is fixed.
CHANNEL
Propagation model
Tap number
i
Tap amplitude
C
i
Tap delay T
i
(ns)
Clear LOS (Type 0)
1
1.0 0
0
Multipath (Type 1)
1
0.995
0
2
0.0995 exp(

j0.75)
400/R
Multipath (Type 2)
1
0.286 exp(

j0.75)
0
2
0.953
400/R
3

0.095
800/R
R is the channel symbol rate in
MBd
Propagation path parameters are valid for R from 15 to 25
MBd
.
Propagation models for 802.16e
Multipath (Type 1) Channel
Specifications
No. of Taps = 2
Ex: R= 20MBd
Tap Weights and Delays
First Tap = 0 dB with
delay of 0 nanoseconds
Second Tap =

10 dB with
delay of 20 nanoseconds
we will make 2 correlated
Rayleigh faded channel
taps, each will be fed
samples taken from Jakes
filter.
0
2
4
6
8
10
12
14
16
18
20
0.2
0
0.2
0.4
0.6
0.8
1
1.2
Power delay profile
Arrival time for each multipath (ns)
Mean power for each multipath normalized by direct wave
RECEIVER
OFDM: Fast Fourier Transform, CP removal
Removing the guard bands
Demapping
Deinterleaving
Decoding
Derandomization
Simulator Description
Each block of the transmitter, receiver and channel is written in
separate ’m’ file
The main procedure call each of the block in the manner a
communication system works
initialization parameters: number of simulated OFDM symbols, CP
length, modulation and coding rate, range of SNR for simulation.
The input data stream is randomly generated
NUMERICAL RESULTS
AWGN
1
1.5
2
2.5
3
3.5
4
4.5
5
10
2
10
1
10
0
E
b
/N
0
(dB)
BER
AWGN
Multipath (Type 1) with QPSK, R=1/2
0
5
10
15
10
0.9
10
0.8
10
0.7
10
0.6
10
0.5
10
0.4
E
b
/N
0
(dB)
BER
0
2
4
6
8
10
12
10
2
10
1
10
0
Error rate
E
b
/N
0
(dB)
QPSK (R=1/2)  Multipath Type 1
BER
ADAPTIVE MODULATION AND CODING (AMC)
Basic Idea:
1.
„ Measure the channel at the receiver
2.
„ Feed the measurement back to the transmitter
3.
„ Adapt the transmission scheme relative to the
channel estimate to
maximize the data rate,
minimize transmit power, or minimize BER
„ What to adapt?
1.
„ Constellation size/power
2.
„ Symbol rate
3.
„ Coding rate/scheme
ADAPTIVE MODULATION AND CODING (AMC)
Bit rate shifting is achieved using adaptive
modulation
„ When the MS is close to the BS, it is offered high
bit rate (higher speed)
When the MS is far from the BS, the reliability
decreases and it is offered a lower bit rate
ADAPTIVE MODULATION AND CODING (AMC)
5
10
15
20
25
30
10
6
10
5
10
4
10
3
10
2
10
1
10
0
E
b
/N
0
(dB)
BER
BPSK 1/2
QPSK 1/2
QPSK 3/4
16QAM 1/2
16QAM 1/2
64QAM 2/3
64QAM 3/4
Target BER=10

3
DETAILED
RESULTS
(Configuration 1, Channel 1)
CONCLUSIONS AND FUTURE WORK
Conclusion
Lower modulation and coding scheme provides better performance at lower
SNR
Results obtained from the simulation can be used to set threshold SNR to
implement adaptive modulation scheme to attatin highest transmission
speed with a target BER
Future Work
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. Also, the use of the optional LDPC codes can provide
an improvement in the performance provided that the word length is long
enough.
REFERENCES
IEEE Standard for Local and metropolitan area networks Part16: Air Interface for Broadband Wireless Access Systems
(
http://standards.ieee.org/about/get/802/802.16.html
)
http://www.wimaxforum.org/
http://grouper.ieee.org/groups/802/16/
http://en.wikipedia.org/wiki/IEEE_802.16m#802.16e

2005_Technology
http://ecee.colorado.edu/~ecen4242/WiMax/WiMAX_802_16e.htm#_edn1
http://www.scribd.com/doc/2945438/PHY

Layer

of

WiMAX
http://www.google.com.sa/search?q=channel+wimax&ie=utf

8&oe=utf

8&aq=t&rls=org.mozilla:en

US:official&client=firefox

a&safe=on
http://www.wimax360.com/forum/topics/610217:Topic:61844?groupUrl=wimaxradioengineering&id=610217%3ATo
pic%3A61844&groupId=610217%3AGroup%3A18095&page=2#comments
http://dspdotcomm.blogspot.com/2008/11/simulating

sample

spaced

rayleigh.html
http://www.mathworks.com/matlabcentral/fx_files/18869/1/ChannelModelingWhitePaper.pdf
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