Performance analysis of a spread spectrum M-ary orthogonal modulation under multipath fading

The performance of orthogonal Walsh functions spread spectrum modulation under multipath, selective Rayleigh fading channel is analyzed. This modulation scheme is used for a DS CDMA mobile communication system which is being investigated. Orthogonal modulation and noncoherent detection seems to be suitable for the up-link of a DS CDMA system in which the multi-user interference are more severe than on the down-link. For simplicity, the analysis has been carried out only for a fixed path reception. The results of analysis show that although orthogonal modulation is an effective method to combat the noise and interference in an AWGN channel, under the multipath fading channel and reception based on one path, the performance is still poor. However, a computer simulation under a more practical condition shows that this modulation scheme combined with the RAKE receiver and convolutional encoding will achieve an acceptable performance. A comparison of the simulation results for orthogonal modulation and DPSK shows that the performance of a 64-ary Walsh modulation is nearly 7 dB better than DPSK, while the bandwidth of the latter is 1.5 times more than the former