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Simulation of the W i max (IEEE 802.16 e ) PHYSICAL LAYER (Phase 4)

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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.

WiMAXBlock Diagram (Physical Layer)

FEC Encoding

1.Reed-Solomon

2. Convolutional

3. Optional:

Turbo, LDPC, ..

OFDM

IFFT, + CP..

Randomization

Interleaving

DigitalModulation

(Symbol Mapping)

Data

Channel

AWGN

FECDecoding

1.Reed-Solomon

2. Convolutional

3. Optional:

Turbo, LDPC, ..

OFDM

FFT, - CP..

De-Randomization

De-Interleaving

DigitalDe-Modulation

(Symbol De-Mapping)

Estimated

Data

+

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)

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)

Reed-Solomon encoderThe generator polynomials used to derive its two output code bits, denoted X and Y, are specified in the following expressions:

Convolutional encoderInterleaver

- 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.

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

OFDM DEFINITION- BASIC IDEA : Channel bandwidth is divided into multiple subchannels to reduce ISI and frequency-selective fading.

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 in wimaxOFDM 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.

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.

Inverse Fast Fourier Transform algorithmThe 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.

The cyclic prefixTsym : OFDM symbol time

Tb : useful symbol time

Tg : CP time.

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.

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.

Simulating Sample Spaced Rayleigh Fading Channel Channel

Propagation models for 802.16e

R is the channel symbol rate in MBd

Propagation path parameters are valid for R from 15 to 25 MBd.

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.

OFDM: Fast Fourier Transform, CP removal

Removing the guard bands

Demapping

Deinterleaving

Decoding

Derandomization

Receiver- 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

Measure the channel at the receiver

Feed the measurement back to the transmitter

Adapt the transmission scheme relative to the channel estimate to maximize the data rate, minimize transmit power, or minimize BER

What to adapt?

Constellation size/power

Symbol rate

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)Adaptive modulation and coding (AMC)

Target BER=10-3

- 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.

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

References
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