Capacity enhancing techniques for code division multiple access systems.

摘要

The use of diversity and interference suppression techniques to enhance the capacity of a code division multiple access (CDMA) system is considered, and analytical techniques are developed to quantify this performance improvement.; The effect of spatially constrained spatial (receive) diversity on initial acquisition is presented, where the spatial constraints result in spatial correlations, thus causing a loss of diversity. An optimum, noncoherent receiver is derived for the spatially correlated case, and its performance is evaluated using a technique developed here in terms of the probability of incorrect synchronization, thus quantifying the tradeoff between the diversity gain and the noncoherent combining loss. The optimum number of antenna elements for a linear antenna array with a fixed length, which minimizes the probability of incorrect synchronization, is obtained for a given correlation model based on the angular spread of the signal arrival. The derivation of an optimum receiver and the analysis of that receiver are then extended to a wideband, multicarrier, CDMA system both with and without spatial diversity.; Next, the effect of the spatial correlation on both the diversity gain and the accuracy of channel estimation of a BPSK-based receive diversity system using pilot symbol assisted modulation is evaluated in terms of the probability of bit error. The optimum number of receive antennas in an arbitrarily correlated Rayleigh fading channel is obtained using an analytical technique developed here. In addition, an exact, simple, and closed-form expression for the probability of bit error in a 16-QAM system using pilot symbol assisted modulation is obtained. Transmit diversity is analyzed in the presence of impairments, such as fade-correlation and imperfect channel estimates, and the effects of diversity gain, noncoherent combining loss, and inter-antenna interference on the probability of bit error are analyzed.; Finally, a minimum mean square error receiver is used in conjunction with a 16-QAM constellation to suppress interference and to reduce the probability of error. This receiver is compared with an equal data-rate QPSK-based receiver under various channel conditions, and the performance advantage of the 16-QAM system in certain interference limited CDMA systems is presented in terms of the probability of symbol error.