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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 29 / 107





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user data, 56 are nulled for guard band and 8 are used as pilot subcarriers for various estimation purposes. The PHY allows to accept variable CP length of 8, 16, 32 or 64 depending on the expected channel delay spread. In the following sub sections, we will discuss about the other mechanism of the PHY layer.

3.4.1 Flexible Channel Bandwidth:

The channel bandwidth can be an integer multiple of 1.25 MHz, 1.5 MHz, 1.75 MHz, 2MHz and 2.75 MHz with a maximum of 20 MHz [1]. But the WiMAX forum has initially narrowed down the large choice of possible bandwidth to a few possibilities to ensure interoperability between different vendor’s products [2].

3.4.2 Robust Error Control Mechanism

Forward Error Correction (FEC) is done on two phases through the outer Reed­Solomon (RS) code and inner Convolutional code (CC). The RS coder corrects burst error at the byte level. It is particularly useful for OFDM links in the presence of multipath propagation. The CC corrects independent bit errors. The puncturing functionality in CC made the concatenated codes rate compatible as per speci ication. The support of Turbo coding is left as an optional feature to increase the coverage and/or capacity [2] with the expense of increased decoding latency and complexity.

3.4.3 Adaptive Modulation and Coding

The speci ied modulation scheme in the downlink (DL) and uplink (UL) are binary phase shift keying (BPSK), quaternary PSK (QPSK), 16 quadrature amplitude modulation (QAM) and 64­QAM to modulate bits to the complex constellation points. The FEC options are paired with the modulation schemes to form burst profiles. The PHY specifies seven combinations of modulation and coding rate, which can be allocated selectively to each subscriber, in both UL and DL [4]. There are trade­offs between data rate and robustness, depending on the propagation conditions. Table 3.2 Shows the combination of those modulation and coding rate.


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