To catch up with the fast changes of the information challenges, providing multimedia services has become a very important requirement for future wireless communications. A proper system, capable of supporting multi-rate transmissions as well as handling high quality of service (QoS) requirements in hostile wireless communication environments, should be sought. Multi-carrier CDMA (MC-CDMA), a combination of multi-carrier modulation (MCM) and direct-sequence CDMA (DS-CDMA), appears to be one of the most elegant solutions. In this dissertation, four multi-rate access schemes, termed uncoded fixed spreading length (UFSL), coded fixed spreading length (CFSL), multi-code fixed spreading length (MFSL) and variable spreading length (VSL), are constructed for MC-CDMA. Due to different sub-carrier assignment strategies, they present different properties in spectral utilization efficiency (SUE), rate matching capability, receiver structure and bit-error-rate (BER) performance in correlated Rayleigh fading channels. With these schemes, different information traffic such as voice, video and higher rate data can be transmitted seanilessly through one MC-CDMA infrastructure.
The performance of the multi-rate MC-CDMA is mainly limited by multiple access interference (MAI). For example, in the MFSL MC-CDMA systems, the interference is not only presented among different users, but also among different symbols of the same user transmitted in parallel on different spreading codes. To mitigate this problem, a nonlinear zero-forcing successive interference cancellation (ZF-SIC) receiver and a minimum mean square error SIC (MMSE-SIC) receiver are applied in the MFSL MC-CDMA systems. It is well known that SIC is sensitive to the receive power distribution. By providing channel state information (CSI) at the receiver and reliable feedback of power distribution from the receiver to the transmitter, SIC can be integrated with power distribution control (PDC), which improves the system capacity significantly.
In this dissertation, the PDC algorithms, under both a short-term power constraint (STPC) and a long-term power constraint (LTPC) are investigated for two different SIC receivers. For the ZF-SIC receiver, the PDC under the equal BER criterion, which ensures the same performance after SIC for all parallel transmit symbols, is first considered. It is found that for a multi-code system, such equal BER PDC is only suboptimal from the viewpoint of minimizing each user's BER, hence, an optimal PDC algorithm is proposed, which significantly outperforms the equal BER PDC, particularly under the STPC and highly-loaded systems. For the MMSE-SIC, the PDC under the equal BER criterion is derived, which cancels interference very effectively, resulting in a performance of a fully-loaded system close to the single user bound (SUB). In comparison to the nonlinear matched-filter SIC (MF-SIC) with the equal BER PDC, studied extensively in the literature, the ZF-SIC and MMSE-SIC with the proposed PDC algorithms present remarkable performance advantage.
Finally, the effect of channel estimation errors (CEE) on the performance of the MMSE-SIC with the equal BER PDC is analyzed. A method of second-order approximation is used to estimate the mean excess MSE (MEMSE) of the parallel transmit symbols, under a given decision order. The approximation accuracy is confirmed by simulation results. Furthermore, it is also interesting to find out that the MMSE-SIC with the equal BER PDC presents significant robustness to CEE.
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