Analysis and optimization of DS-CDMA systems with time-limited partial response chip waveforms

Conventional direct sequence code division multiple access systems (DS-CDMA) using offset quadrature phase shift key (OQPSK) usually employ strictly band-limited partial response square-root raised cosine pulse as the chip waveform. They have the disadvantage of large envelope fluctuation that will incur performance degradation due to the inter-modulation and bandwidth enlargement caused by post nonlinear processing. To improve the performance of DS-CDMA system, the chip waveform and receiver should be properly selected. This paper is to present a systematic performance analysis of matched filter receiver and zero-forcing filter (ZF) receiver for DS-CDMA using time-limited partial response chip waveform. Nevertheless the systematic performance analysis is applicable to band-limited chip pulse as well. For the zero-forcing filters, we propose to select their frequency responses that satisfy the first Nyquist criterion. With this class of filters, we can choose the roll-off factor to minimize the total power of multiple access interference and noise power. The zero-forcing filter with proper choice of roll-off factor, referred to as optimum ZF, will yield a performance better than the matched filter counterpart. The bit error rate (BER) performance of the optimum ZF with superposed quadrature amplitude modulation signal as the time pulse waveform is evaluated. It is shown that the optimum ZF provides better BER performance than conventional OQPSK and minimum shift keying, and its envelope uniformity is much better than that of OQPSK.