Transmission Systems IB-DFE receivers with space diversity for CP-assisted DS-CDMA and MC-CDMA systems

MC-CDMA (MultiCarrier Code Division Multiple Access), currently regarded as a promising multiple access scheme for broadband communications, is known to combine the advantages of an OFDM-based (Orthogonal Frequency Division Multiplexing), CP-assisted (Cyclic Prefix) block transmission with those of CDMA systems. Recently, it was recognized 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 FFT-based (Fast Fourier Transform), chip level FDE (Frequency-Domain Equalization) techniques. When employing a linear FDE with both MC-CDMA and DS-CDMA, the FDE coefficients can be optimized under the MMSE criterion (Minimum Mean Squared Error), 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 frequencydomain spreading. We consider the use of IB-DFE (Iterative Block Decision Feedback Equalization) 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 BER (Bit Error Rate) that can be close to the single-code matched filter bound (especially for the CP-assisted DS-CDMA alternative), even with full code usage.