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Introduction

This article is a discussion of the OFDMA support of the wimax air interface as currently defined in the wimax specifications. The wimax specs define four different forms of air interface, including both single channel and multiple channel mechanisms - the latter using the OFDM technology. In this article, we shall focus on the AI as defined using the OFDM/OFDMA technologies. Here, we shall explain some of the unique aspects of the wimax/wireless lan specifications. The user is invited to read this for an introduction to the ofdm technology.

Overview of OFDMA in the wimax air interface

The wimax air interface is fairly typical of a system which is being designed to deal with an evolving technology. Since it is currently not known what the optimal combination of features is, the wimax specification incorporates practically every feature which may be required. In this article we will focus on some of the key features

Interleaved subchannel allocation.

Support for smart antenna features, such as spatial multiplexing, space-time diversity coding and adaptive antennae.

Ongoing support for mobility.

Wimax OFDMA sub-channel allocation

In Wimax OFDMA, the entire spectrum is divided into sub-channels, which can be considered the basic unit of allocation. A single sub-channel can be shared by multiple users, and a single user can potentially be allocated multiple sub-channels. Each sub-channel defines a particular combination of sub-carriers (these are the basic narrow band carriers into which the overall spectrum is broken), which may change over time (even from symbol to symbol). However, the ensemble properties of the link remains statistically constant over all.

OFDMA offers three main mechanisms for sub-channel allocation. The three differ in the interleaving density, pilot density and consequently, the radio-environment that they offer. We provide a brief summary below

Fully Usable Shared Channels (FUSC)

The FUSC allocation spreads the sub-carriers for a given sub-channel over the entire available spectrum. There is approximately one pilot per 12 sub-carriers. There are some constant pilots (the sub-carrier for the pilot does not change) and volatile pilots (sub-carriers move alternately left and right symbol to symbol, shifting by about six sub-carriers in each symbol). FUSC is designed for situations where the noise is spread evenly over the entire spectrum and there is no single dominant source of interference. It achieves very high frequency re-use.

Partially Usable Shared Channels (PUSC)

The PUSC allocation reduces the interleaving to some extent. It breaks the sub-carriers up into clusters, which are interleaved. Each cluster contains 14 adjacent sub-carriers. Thus, the sub-carriers are still distributed, but in bunches of 14. The pilot ratio is 1:7; this typically leads to better performance in fast fading or high delay spreads [Samsung]. PUSC allows multi-sector allocation, by limiting groups of clusters to individual sectors in a cell; this allows for combating of self-interference between different subscriber stations.

Adaptive Modulation and Coding (AMC)

The AMC allocation has no interleaving at all; sub-channels comprise of bands of 54 sub-carriers, including one pilot in the middle of every 9 sub-carriers and 48 data-carriers per sub-channel. The pilot ratio is the least, but because there is no interleaving, the resistance to delay spread is the highest. AMC is specifically designed for adaptive antennae usage and can be used to bypass parts of the spectrum with heavy interference.