MC-CDMA with quadrature spreading over frequency selective fading channels

Multicarrier CDMA (MC-CDMA) schemes resolve the frequency selectivity in multipath fading channels and have good spectral properties. Spreading the information over many subcarriers adds flexibility to the system and a redundancy between subcarriers that can be combined with the derived frequency diversity from the fading channel. The performance of an MC-CDMA scheme over frequency selective, slowly fading channels is studied analytically and by computer simulations. The MC-CDMA system uses a BPSK modulation, quadrature (complex) spreading codes, and a simple correlator receiver. Quadrature spreading codes are used to reduce the effect of the multipath fading channel and restore some of the orthogonality losses between users. The obtained results show considerable performance improvement compared to the conventional orthogonal frequency division multiplexing (OFDM) scheme and MC-CDMA schemes that use non-quadrature spreading. The effect of frequency offsets on the system performance is also addressed