Introduction

One way to mitigate the frequency-selective fading seen in a wideband channel is to use a multi-carrier technique, which subdivides the channel into smaller sub-bands, or subcarriers. In conventional single-carrier systems, a single fade or interferer can cause the entire link to fail, but in multi-carrier systems, only a small percentage of the subcarriers will be affected. Error correction coding can then be used to correct the few erroneous subcarriers. OFDM is a multi-carrier technique which uses orthogonal subcarriers to convey information. In the frequency domain, since the bandwidth of a subcarrier is designed to be smaller than the coherence bandwidth, each subchannel

is seen as a flat fading channel, which simplifies the channel equalization process. In the time domain, by splitting a high-rate data stream into a number of lower-rate data streams that are transmitted in parallel, OFDM resolves the problem of intersymbol interference in wideband communications [6].