IB‐DFE receivers with space diversity for CP‐assisted DS‐CDMA and MC‐CDMA systems

Multi‐Carrier Code Division Multiple Access (MC‐CDMA), currently regarded as a promising multiple access scheme for broadband communications, is known to combine the advantages of an Orthogonal Frequency Division Multiplexing (OFDM)‐based, Cyclic Prefix (CP)‐assisted block transmission with those of CDMA systems. Recently, it was recognised that DS‐CDMA (Direct Sequence) implementations can also take advantage of the benefits of the CP‐assisted block transmission approach, therefore enabling an efficient use of Fast Fourier Transform (FFT)‐based, chip level Frequency‐Domain Equalisation (FDE) techniques. When employing a linear FDE with both MC‐CDMA and DS‐CDMA, the FDE coefficients can be optimised under the Minimum Mean Squared Error (MMSE) criterion, so as to avoid significant noise enhancement. The residual interference levels can be very high, especially for fully loaded scenarios, since the FDE/MMSE does not perform a perfect channel inversion. This paper deals with CP‐assisted DS‐CDMA systems and MC‐CDMA systems with frequency‐domain spreading. We consider the use of Iterative Block Decision Feedback Equalisation (IB‐DFE) FDE techniques as an alternative to conventional, linear FDE techniques, and derive the appropriate IB‐DFE parameters in a receiver diversity context. Our performance results show that IB‐DFE techniques with moderate complexity allow significant performance gains in both systems, with bit error rate (BER) that can be close to the single‐code matched filter bound (MFB) (especially for the CP‐assisted DS‐CDMA alternative), even with full code usage. Copyright © 2007 John Wiley & Sons. Ltd.