On the comparisons of system performance and capacity of asynchronous STBC MC-CDMA multirate access schemes

Three multirate access schemes, multicode, variable spreading gain (VSG), and spectral overlaid multiple-symbol-rate (MSR), for asynchronous space-time block coded (STBC) multicarrier code division multiple access (MC-CDMA) systems are proposed. The three possible spectral overlaid configurations for MSR systems are also investigated. The expressions to evaluate the multiple access interferences, bit error rate (BER) performances, and system capacities of a antenna STBC MC-CDMA using the three multirate access schemes are obtained. Transmit power allocation is adjusted according to the service rates and the number of active users in each service class to maintain the link quality and to improve the system capacity. Our numerical results show that systems with multicode access scheme using orthogonal Gold spreading codes and with VSG access scheme have similar system performance and capacity, and both perform in general better than systems with MSR access scheme of any spectrum configurations. In case when non-orthogonal Gold codes are used, multicode access scheme shows degradation in the system capacity as compared to VSG, due to the presence of larger self-interference (SI) among the codes used by each user. The achievable capacities for the three spectral overlaid configurations of MSR multirate systems are also compared. Copyright (C) 2007 John Wiley & Sons, Ltd.