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Future multimedia wireless communications systems will require higher spectral efficiencies, with decreased receiver complexity. MC-CDMA systems can increase spectral efficiency when employing the proper spreading code. Short codes can be used in MC-CDMA, since a high processing gain is not required. Short codes allow the use of maximum likelihood detection, thereby providing a simple form of multi-user detection. In contrast, correlation receivers are also feasible, since orthogonal codes are possible in an MC-CDMA structure. Correlation receivers offer low complexity for long code lengths, allowing an increase in frequency diversity. We emphasise the benefits of orthogonal codes for correlation detectors because of their effectiveness in reducing the crest factor, as well as providing low complexity receiver solutions.