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).
Simulation of the Wimax (IEEE 802.16e) PHYSICAL LAYER (Phase 4)
Turbo, LDPC, ..
IFFT, + CP..
Turbo, LDPC, ..
FFT, - CP..
From IEEE Std 802.16-2004 
WiMAX modulation and coding schemes
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)
The generator polynomials used to derive its two output code bits, denoted X and Y, are specified in the following expressions:
BPSK, 4-QAM and 16-QAM constellation maps. (using Gray mapping)
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
Increase In spectral efficiency
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.
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 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 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.
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
OFDM: Fast Fourier Transform, CP removal
Removing the guard bands
Multipath (Type 1) with QPSK, R=1/2
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?
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
(Configuration 1, Channel 1)
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)