Diversity Combining for DS CDMA Systems with Synchronization Errors

An eecient series that is used to calculate the probability of error for a BPSK modulated DS CDMA system with chip timing and carrier phase errors in a slowly fading , multipath channel is derived. The receiver is assumed to be a coherent RAKE receiver. Three types of diversity schemes are considered: selection diversity, equal gain diversity combining and maximal ratio diversity combining. The error probability derivation does not resort to the widely used Gaussian approximation for the intersymbol interference and multiple access interference and is very accurate. Systems of 1.25 MHz and 5 MHz are considered for diierent number of diversity branches. Direct-sequence code division multiple access (DS CDMA), which was primarily used in military communications until the late 80's, has recently been the center of attention in cellular radio communications. This is mainly due to the recent developments in the eld which increased the capacity of a CDMA system to a comparable level to the capacities of TDMA and FDMA systems 1]. The performance of a DS CDMA system is usually measured in terms of its bit error rate as a function of the number of active users for a given signal to noise ratio (SNR). We have previously developed an accurate, innnite series expression for the calculation of error probabilities for a coherent DS CDMA system in an AWGN channel without making a Gaussian approximation for the multiple access interference 2, 3]. Furthermore, we did not neglect the presence of synchronization errors in our derivation. In this paper, extending on the procedure of 2, 3], we present a performance analysis for a DS CDMA system in a slowly fading multipath environment. We assume that the system uses coherent reception with a RAKE receiver. We consider three diierent diversity combining schemes, namely, selection diversity , equal gain combining and maximal ratio combining. As in 2, 3], we do not neglect the presence of synchronization errors in the system but rather investigate the degradation in the system capacity due to such errors. An outline of the paper is as follows. The system-fading and multipath model is described in Section II. Section III presents the coherent RAKE receiver and Section IV outlines the use of the Fourier series approach of 4] to develop the innnite series for the bit error rate when selection diversity, equal gain combining and maximal ratio combining are employed. The results are discussed in Section V …