SIMULATION OF THE WiMAX (IEEE 802.16e) PHYSICAL LAYER (PHASE 5)

MULTIUSER ENVIRONMENT

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.

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WiMAX Block Diagram (Physical Layer)

FEC Encoding1.Reed-Solomon2. Convolutional3. Optional: Turbo, LDPC, ..

OFDMIFFT, + CP..

Channel

+

Randomization InterleavingData Digital Modulation(Symbol Mapping)

AWGNFEC Decoding1.Reed-Solomon2. Convolutional3. Optional: Turbo, LDPC, ..

OFDMFFT, - CP..De-Randomization De-Interleaving

EstimatedData

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 215 − 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 1/22 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 3/46 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

GG

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.

Tsym : OFDM symbol timeTb : useful symbol timeTg : CP time.

g

b

TG

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 Ts 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 Ci Tap delay Ti (ns)

Clear LOS (Type 0)

1 1.0 0 0

Multipath (Type 1)

1 0.995 02 0.0995 exp(-

j0.75)400/R

Multipath (Type 2)

1 0.286 exp(-j0.75) 02 0.953 400/R3 -0.095 800/R

R is the channel symbol rate in MBdPropagation 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)

Mea

n po

wer

for e

ach

mul

tipat

h no

rmal

ized

by

dire

ct w

ave

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 4.5 510

-2

10-1

100

Eb/N0 (dB)

BE

R

AWGN

Multipath (Type 1) with QPSK, R=1/2 (Without compensation)

0 2 4 6 8 10 1210

-2

10-1

100

Err

or r

ate

Eb/N0 (dB)

QPSK (R=1/2) --- Multipath Type 1

BER

Multipath (Type 1) with QPSK, R=1/2 (Without compensation)

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 3010

-6

10-5

10-4

10-3

10-2

10-1

100

Eb/N0 (dB)

BE

R

BPSK 1/2QPSK 1/2QPSK 3/416-QAM 1/216-QAM 1/264-QAM 2/364-QAM 3/4

Target BER=10-3

DETAILED RESULTS (Configuration 1, Channel 1)

figs (Configuration 1, Channel 1).rar

OFDMA The IEEE 802.16e/ WiMax use OFDMA

as Multiple access technique Bandwidth options 1.25, 5, 10, or 20 MHz Entire bandwidth divided into 128, 512,

1024 or 2048 sub carriers 20 MHz bandwidth with 2048 sub carriers

has 9.8 KHz spacing between sub carriers

OFDMA Each terminal occupies a subset of sub-carriers Subset is called an OFDMA traffic channel Each traffic channel is assigned exclusively to one

user at any time

user1

user2

user3

user4

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ADVANTAGES OF OFDMA

Multi-user Diversity broadband signals experience frequency

selective fading OFDMA allows different users to transmit

over different portions of the broadband spectrum (traffic channel)

Different users perceive different channel qualities, a deep faded channel for one user may still be favorable to others

SCHEDULING

Scheduling is a method of allowing multiple users to share a common resource (such as band width) to optimize a measure of goodness like SER, delay, etc.

Common scheduling algorithms: 1. Round robin Scheduling (RR): routes the transmission

of packets equally across users.(used in TDMA)2. Greedy Scheduling: routes each transmission to the user

with the best CSI.3. Proportional Fair (PF): assigns a user for transmission

when its instantaneous channel capacity is high relative to its average channel condition.

4. Opportunistic Round Robin (ORR): routes the transmission to the best user and after that he will be excluded from competition of the coming time slots of the frame. 33

SINGLE USER

0 2 4 6 8 1010

-4

10-3

10-2

10-1

Eb/N0 (dB)

Err

or r

ate

QPSK transmission

BER

GREEDY SCHEDULING

0 2 4 6 8 1010

-7

10-6

10-5

10-4

10-3

10-2

10-1

Eb/N0 (dB)

Err

or r

ate

QPSK transmission

Nu= 1

Nu= 2

Nu= 5

Nu=10

ERROR RATE AT EB/N0=4 DB (QPSK -- GREEDY SCHEDULING)

1 2 3 4 5 6 7 8 9 1010

-7

10-6

10-5

10-4

10-3

10-2

Number of Users

BE

R

Error rate at Eb/N0=4 dB (QPSK -- Greedy scheduling)

RR VERSUS GREEDY

1 2 3 4 5 6 7 8 9 1010

-7

10-6

10-5

10-4

10-3

10-2

Number of Users

BE

R

Error rate at Eb/N0=4 dB (QPSK )

Round Robin SchedulingGreedy scheduling

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%3ATopic%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

Thank You