Iterative Equalizer for OFDM-CDMA Multiple Access Communication Systems

In this paper, the receivers with adaptive equalizer and turbo decoder are investigated for orthogonal frequency division multiplexing-code division multiple access (OFDM-CDMA) systems in multipath channels. The de-spreading equalizer which combines de-spreading and equalization is used to mitigate the MAI and ISI. The stochastic gradient based LMS algorithm is used as the adaptive algorithm for the equalizer. Three receiver architectures are investigated: The first one is the traditional receiver called de-spreading equalizer disjointed with turbo decoder. In this scheme, the equalizer and turbo decoder operate independently and the equalizer relies on slicer output to adapt its filter coefficients. However, the extrinsic information from turbo decoder can provide more reliable information of the transmitted data. Hence the second receiver architecture is called as de-spreading equalizer jointed with turbo decoder, where turbo decoder gives the extrinsic information as feedback to the adaptive equalizer as the desired information. Finally, the equalizer jointed with turbo decoder is extended to differential coded signaling in OFDM-CDMA system and its performance is presented and discussed.