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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 27 / 107

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uplink and downlink. The standard also supports half duplex FDD SS , which may be less expensive as they do not transmit and receive simultaneously [28]. This duplexing technique is common to all the PHY specifications. Access in uplink direction is done by combination of time division multiple access (TDMA) and Demand Assignment Multiple Access (DAMA), exactly the uplink channel is divided into several time slots. Communication on the downlink in PTM Architecture is employed using Time Division Multiplexing (TDM). It also speci ies the randomization, forward error correction (FEC), modulation and coding schemes.

3.2.3 WirelessMAN­SCa™

This is also based on single carrier modulation targeted for 2­11 GHz frequency range. Access is done by TDMA technique both in uplink and downlink, additionally TDM also supported in downlink.

3.2.3 WirelessMAN­OFDM™

This is based on orthogonal frequency division multiplexing (OFDM) with a 256 point transform to support multiple SS in 2­11 GHz frequency band. Access is done by TDMA. The WiMAX forum has adopted this PHY specification for BWA. Because of employing OFDM and other features like multiple forward error correction method, this is the most suitable candidate to provide ixed support in NLOS environment. We have chosen this PHY specification for our simulation model. From next sections our discussion will be on this PHY layer.

3.2.4 WirelessMAN­OFDMA™ :

This PHY specification uses OFDM access (OFDMA) with specified multi­point transform (2048, 1024, 512 or 128) to

Mobile

BWA.

Operation

is

limited

to

below

11

at least a single support of provide combined fixed and GHz licensed band [8].

In

specification multiple access is provided by addressing a subset of the multiple carriers to individual receivers [6].

16

this

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