simulation of the w i max (ieee 802.16 e ) physical layer (phase 5 ) multiuser environment
DESCRIPTION
Presented by: Ahmad Salim. Simulation of the W i max (IEEE 802.16 e ) PHYSICAL LAYER (Phase 5 ) Multiuser environment. Introduction . - PowerPoint PPT PresentationTRANSCRIPT
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.
3
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
32
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