Adaptive space-time processing for digital mobile radio communication systems

The performance of digital mobile radio communication systems is primarily limited by cochannel interference and multipath fading. Antenna arrays, with optimum combining (OC), have been shown to combat multipath fading of the desired signal and are capable of reducing the power of interfering signals at the receiver through spatial filtering. With OC, the signals received by several antenna elements are weighted and combined to maximize the output signal-to-interference-plus-noise ratio (SLNR). We derive new closed-form expressions for (1) the probability density function (PDF) of the SINR at the output of the optimum combiner, (2) the average probability of bit error rate (BER) and its upper bound, and (3) the outage probability in a Rayleigh fading environment with multiple cochannel interferers. The study covers both the case when the number of antenna elements exceeds the number of interferers and vice versa. We consider independent fading at each antenna element, as well as the effect of fading correlation. The analysis is also extended to processing using maximal ratio combining (MRC). The performance of the optimum combiner is compared to that of the maximal ratio combiner and results show that OC performs significantly better than MRC. We investigate the performance of OC in a microcellular environment where the desired signal and the cochannel interference can have different statistical characteristics. The desired signal is assumed to have Rician statistics implying that a dominant multipath reflection or a line-of-sight (LOS) propagation exists within-cell transmission. Interfering signals from cochannel cells are assumed to be subject to Rayleigh fading due to the absence of LOS propagation. This is the so called Rician/Rayleigh model. We also study OC for a special case of the Rician/Rayleigh model, the Nonfading/Rayleigh model. We derive expressions for the PDF of the SJNR, the BER and the outage probability for both Rician/Rayleigh and Nonfading/Rayleigh models. Similar expressions are derived with MRC. Another area in which space-time processing may provide significant benefits is when wideband signals (such as code division multiple access (CDMA) signals) are overlaid on existing narrowband user signals. The conventional approach of rejecting narrowband interference in direct-sequence (DS) CDMA systems has been to sample the received signal at the chip interval, and to exploit the high correlation between the interference samples prior to spread spectrum demodulation. A different approach is space-time processing. We study two space-time receiver architectures, referred to as cascade and joint, respectively, and evaluate the performance of a DS-CDMA signal overlaying a narrowband signal for personal communication systems (PCS). We define aild evaluate the asymptotic efficiency of each configuration. We develop new closed-form expressions for the PDF of the SINR at the array output, the BER and its upper bound, for both cascade and joint configurations. We also analyze the performance of this system in the presence of multiple access interference (MAJ)