Content area
Abstract
In this thesis, diversity techniques for direct sequence code division multiple access (DS/CDMA) systems are investigated. Specifically, the focus is on packet combining employed with multiuser detection, and multiple transmit and receive antennas with iterative multiuser detection/decoding.
Optimum detection of combined packets and suboptimum multiuser detection schemes in conjunction with packet combining are proposed. The results indicate that for high multiple access interference and background noise, the suggested schemes with joint multiuser detection and packet combining achieve considerable performance gains compared to its counterparts without packet combining, in both additive white Gaussian noise (AWGN) and Rayleigh fading channels. Multi-user detection retains its performance advantage over conventional detection when packet combining is employed. The throughput analysis also suggests that for the considered schemes there exists the offered load for which the maximum throughput is achieved. Since packet combining can be seen as an adaptive repetition coding technique, we conclude that there is an optimum repetition code rate. Sensitivity analysis shows that in presence of various imperfections multiuser detection with packet combining preserves its gain over conventional detection.
The proposed packet combining with multiuser detection is an adaptive scheme with considerably improved robustness and capacity. Such a joint multiuser detection-packet combining technique is very attractive for the systems with variable channel conditions where a certain delay between the transmission and the reception can be tolerated.
Multiple transmit and receive antennas is another approach to achieve diversity in multiuser communications. A novel scheme for a turbo coded system with transmit and receive antenna diversity that employs iterative multiuser detection and decoding is proposed. Multiuser detection is embedded into single-user turbo decoding. In addition to optimum, MAP multiuser detector, suboptimum, greedy based multiuser detector has been considered. The scheme achieves excellent performance close to multiuser capacity for multiple antennas. It is shown that depending on the channel characteristics, for a fixed spectral efficiency, that is for a fixed number of virtual users Kv = KTx, there exists a trade-off between the number of active users K and the number of transmit antennas Tx. Obtained results also demonstrate that substantial gain can be achieved with multiple receive diversity only, where performance approaches the multiuser capacity limit for systems with receive diversity. The proposed greedy multiuser detector for the iterative scheme with antenna diversity offers the reduction in complexity, obtained at the expense of insignificant performance loss, that depends on the extent of complexity reduction. Sensitivity analysis of the scheme shows its robustness towards imperfect amplitude estimation and correlated fading among antennas.
