M-ary MC-CDMA using quadrature spreading codes for wireless communications system

We present a new modulation method, i.e., M-ary MC-CDMA system with quadrature spreading codes. In the transmitter, M-ary orthogonal modulation is applied to serial-to-parallel converted data bits. The modulated symbol is then spread in the frequency domain using each chip of complex orthogonal codes. Each spread symbol is then modulated on corresponding subcarrier, finally the multiple branches are combined and transmitted. Considering the frequency selective slowly fading, we investigate the system's downlink performance. Using equal gain combination (EGC) and maximum ratio combination (MRC), we study the effect of complex quadrature spreading codes on the system performance and compare it with that of the single spreading code case. Also we analyse the effect of the guard time on the system and compare it with that of no guard time.