Capacity-enhancing diversity techniques for CDMA systems
Various capacity-enhancing diversity techniques are considered in DS-CDMA systems. Problems are defined from three perspectives. The first thrust aims to exploit the multiple-input, multiple-output techniques in the asynchronous DS-CDMA reverse link. The Alamouti code, spatial multiplexing, and full-rate, full-diversity space-time code are considered at the transmitter. The receiver exploits both the chip-level oversampling and sphere decoder to. Analysis shows that the performance is determined by the product measure of space-time codes and the effective SINR. Furthermore, three expurgated union bounds are proposed to approximate the BER for different space-time schemes and SNRs. Based on the link-level performance, single-cell spectral efficiency is also analyzed for both the ideal and noisy channel state information at the receiver. The second thrust seeks to investigate the performance variation of MC-DS-CDMA systems over fast fading channels. A receiver structure, which features both Doppler diversity and frequency diversity, is proposed to counteract the effect of large Doppler spreads. The second-order statistics of received signals are derived that lead to a closed-form expression of BER. Under a constant power constraint, it is shown that the Doppler diversity outperforms the frequency diversity as long as the Doppler spread is larger than certain threshold. Numerical results are also provided to show that the multiple-access interference tends to degenerate into a wide-sense stationary process as Doppler spreads become large. Finally, the throughput of a multiple-input, single-output downlink system is examined with noisy channel information feedback. A cross-layer approach is employed to optimize the physical layer precoding matrix with a throughput metric. The optimal precoding matrix reduces to beamforming when the received SNR and SNR of channel feedback both become large. The rate allocation, even with a fairly good channel feedback, needs to be reduced to a utilization factor around 0.8. For low SNR and large transmission delay, we propose a two-mode transmission scheme, switching between beamforming and spatial diversity, such that the throughput is very close to the optimal precoding scheme.
Lehnert, Purdue University.
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