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Presented by: Ahmad Salim. Simulation of the W i max (IEEE 802.16 e ) PHYSICAL LAYER (Phase 4). 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).

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Presentation Transcript
  • 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
WiMAXBlock Diagram (Physical Layer)

FEC Encoding


2. Convolutional

3. Optional:

Turbo, LDPC, ..


IFFT, + CP..




(Symbol Mapping)






2. Convolutional

3. Optional:

Turbo, LDPC, ..


FFT, - CP..




(Symbol De-Mapping)




  • 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
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)
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)

Reed-Solomon encoder
convolutional encoder

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

Convolutional encoder
  • 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 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
FDM versus OFDM
  • Frequency Division Multiplexing
  • OFDM frequencydividing

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 in wimax

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.

Inverse Fast Fourier Transform algorithm
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.

The cyclic prefix

Tsym : 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.

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.

Simulating Sample Spaced Rayleigh Fading 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






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
adaptive modulation and coding amc

Basic Idea:

„ 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)
adaptive modulation and coding amc1

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)
conclusions and future work


  • 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.
Conclusions and future work

IEEE Standard for Local and metropolitan area networks Part16: Air Interface for Broadband Wireless Access Systems (