The MAI performance of orthogonal codes for channel cover in Asynchronous CDMA system

It is widely believed that the walsh code is the best to provide covering for a CDMA channel due to its orthogonality. However, it exhibits a severe aperiodic ACF and variation of its aperiodic ACF pattern for each code. Hence, the comparison of MAI performances of walsh sequences with other three orthogonal sequences as a channel cover code in DS-CDMA was established. The aperiodic ACF and CCF that are directly correspondent with average system performance are examined. It is shown that walsh codes have the worst among the others in term of aperiodic ACF and CCF. The average system performance is assessed by AIP-based computation and simulation based on the gaussian approximation method. However, the result shows that the other orthogonal codes provide slightly lower BER performance in the simulation. The computation shows both Luke type I and Cubic polyphase codes produce strongest resistance to MAI with significant margin compared with walsh codes, while OCPS codes generate the weakest resistance to MAI. It is due to that the aperiodic auto correlation of both polyphase codes which directly influence the AIP is better than walsh and OCPS code. In contrast, the simulation shows walsh code outperform the polyphase codes and OCPS for Eb/No > 4 dB, due to the effect of filtering and complex modulation that are not taken into account in AIP-based computation.