Performance analysis of forward link DS-CDMA systems using random and orthogonal spreading sequences

The characteristics of spreading sequences significantly affect the performance of the forward link DS-CDMA system in a fading channel. The bit error rate (BER) is a function of the signal-to-interference ratio (SIR) which has different characteristics depending upon the spreading sequence. In this paper, the SIR is analyzed when orthogonal codes and pseudo random codes are used as the spreading sequence. The BER performance is analyzed with and without the use of convolutional codes. The use of orthogonal codes can cancel out the interference signals having the same path delay, but the use of random codes cannot, resulting in significant performance degradation. As a result, the BER performance of the orthogonal spreading system is much better than that of the random spreading system. Even in the case of many multipaths, the number of users achieving a fixed BER value in the orthogonal spreading system is twice larger than that in the random spreading system. The analytical results are verified by computer simulation.