Physical Layer

The IEEE 802.16 standard uses frequencies from 10 to 66 GHz for LOS transmission, and frequencies from 2 to 11 GHz for NLOS transmission. In the standard, messages can be modulated using different modes, including single carrier, OFDM (orthogonal frequency division multiplexing), and OFDMA (orthogonal frequency division multiple access). Table 2-3 shows the supported physical specifications defined in the IEEE 802.16 standard. In this thesis, we focus on the WirelessMAN-OFDM with TDD (time division duplex) mode for simulating the most commonly used environment of WiMAX ne

Table 2-3 Variants of WiMAX

NLOS Alternative(s) WirelessMAN-SC Point-to-point LOS 10-66 GHz TDD, FDD WirelessMAN-SCa Point-to-point NLOS 2-11 GHz TDD, FDD WirelessMAN-OFDM Point-to-multipoint NLOS 2-11 GHz TDD, FDD WirelessMAN-OFDMA Point-to-multipoint NLOS 2-11 GHz TDD, FDD WirelessMAN-HUMAN Point-to-multipoint NLOS 2-11 GHz TDD

2.2.1. Adaptive Channel Coding and Modulation

uation of signal will be accompan

l coding sed of llow r on,

de (RS al cod ), and g. I ng

a bur rocessed and encoded in order ng

r ed plie ceiv st.

a e h P do-r binary

on code is derived from a systematic RS (N=255, K=239,

N is the number of overall bytes after encoding K is the number of data bytes before encoding

T is the number of data bytes which can be corrected.

This code is shortened and punctured to enable variable block sizes and variable error-correction capability. The RS code is used to correct byte-level error of data The WirelessMAN-OFDM PHY is based on OFDM modulation and designed for NLOS operation in the frequency bands below 11 GHz. It uses OFDM technology with 256-point FFT to resist delay spread, multi-path effect, and inter-symbol interference (ISI). The OFDM uses 192 data sub-carrier of 256 points for carrying data bits. The OFDM technology can help to extend transmission coverage and increase transmission data rate.

Using the OFDM technology, data bits will be modulated into OFDM symbols for transmitting after channel coding. There are four modulation scheme can be used:

BPSK, QPSK, 16-QAM, and 64QAM. Using higher data-rate modulation scheme can convey more information in each OFDM symbol but harsher atten

ied.

Channe is compo the fo ing steps: andomizati

Reed-Solomon co ), convolution e (CC interleavin n transmitti

direction, each dat st should be p . In receivi

direction, reversed orde of decoding proc ures ap s to the re ed data bur Data randomization scr mbles serial bit str ams wit RBS (pseu andom sequence). The Reed-Solom

T=8) code using GF(28) where

block.

The convolutional code is a punctured code and is derived from basic CC with rate

the standard in the order of decreasing robustness. Each block size repre

g per modulation

Modulation block size block size ll

RS code CC code rate

of 1/2, and constraint length equal to 7. The CC is used to correct bit-level errors of bursts. These two coding scheme (RS-CC) can greatly reduce the data error rate of a radio channel.

Table 2-4 shows the defined input/output block sizes used by mandatory channel coding schemes in

sents the data size that can be carried in one OFDM symbol. For example, since an OFDM symbol contains 192 data sub-carrier, it can convey 24 bytes with BPSK modulation per symbol.

All encoded data bits are interleaved by a block interleaver to re-permute the bit order within a block for avoiding burst channel errors. Burst channel errors may make the Convolutional Coding work poorly.

Table 2-4 Mandatory channel codin Uncoded

The IEEE 802.16 standard uses periodical time-based frames to carry MAC

PDUs in physical layer. The frame structure used in TDD mode is illustrated in Figure 2-8. Each frame contains one downlink sub-frame and one uplink sub-frame. The compartment of downlink and uplink sub-frame is adaptive according to the schedule in cu

k sub-frame consists of multiple PHY PDUs transmitted from different subscriber stations. Each PHY PDU is proceeded by a preamble, which is

In the downlink PHY PD ntains control header (FCH) burst which immediately fo he pr The FCH burst transmits the downlink frame prefix (DLFP) using BPSK rate 1/2 with the mandatory coding scheme. The DLFP advertises infor ion about nlink bursts carried in downlink sub-fram

data st are tra itted with e.

Data bursts in the downlink sub-frame m g

r . M AC s are c d in a da cor onding

subscriber station.

ost robust PHY mode and contains broadcast management messages, such as DL-MAP, UL-MAP, DCD, and UCD. All subscriber statio

rrent frame.

A downlink sub-frame carries one PHY PDU for base station transmitting to all other subscriber stations, while an uplin

used for frame synchronization.

U, it co eamble.

a frame llows t

mat dow e.

Each bur nsm different modulation and coding schem ust appear in the order of decreasin

obustness ultiple M PDU arrie ta burst to resp

In the first burst, it uses the m

ns receive the information carried in these messages to process other bursts.

Figure 2-8 OFDM TDD frame structure

In the uplink sub-frame, it contains two contention periods for initial ranging and bandwidth request. Only in these two periods, data transmissions may encounter collisions. All other uplink PHY PDUs are scheduled by the BS and each are transmitted from different SS. Each uplink PHY PDU which is transmitted by a subscriber station contains one leading preamble and one data burst for uplink. The subscriber station transmits the uplink PHY PDU with the PHY mode and opportunity assigned in the UCD message.