Recital on multicarrier communications: space-time coded versus adaptive OFDM/MC-CDMA

This overview portrays the evolution of multi-carrier systems from their conception and demonstrates that they are capable of supporting the requirements of future wireless systems of the next generation. The a novel multiple access scheme based on slow frequency hopping multicarrier direct-sequence code-division multipleaccess (SFHIMC DS-CDMA) is highlighted, which can be rendered compatible with the existing 2nd-generation narrow-band CDMA, and 3rd-generation wideband CDMA mobile communication systems. A RAKE receiver structure with maximum ratio combining is considered and its performance is evaluated over the range of Nakagami multipath fading channels, under the assumption that the receiver has an explicit knowledge of the associated frequency-hopping (FH) patterns invoked. It is argued furthermore that symbol-by-symbol adaptive Orthogonal Frequency Division Multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput in comparison to their fixed-mode counterparts. By contrast, various diversity techniques, such as Rake receivers and space-time coding, mitigate the channel quality variations in their effort to obtain a reduced BER. This paper investigates a combined system constituted by a constant-power adaptive modem employing spacetime coded diversity techniques in the context of both OFDM and MC-CDMA. The combined system is configured to produce a constant uncoded BER and exhibits virtually error free performance, when a turbo convolutional code is concatenated with a space-time block code. It was found that the advantage of the adaptive modem erodes, as the overall diversity-order increases.