Optimal rate and power adaptation for multirate CDMA

We determine the optimal centralized adaptive rate and power control strategy to maximize the total average weighted throughput in a generic multirate CDMA system with slow fading. An average power and instantaneous bit error rate (BER) constraint is assumed at the transmitter with conventional matched filter detection at the receiver. Our results are general enough to apply to several multirate CDMA schemes: multi-code, multi-processing gain, multi-rate modulation or hybrids of these. We obtain compact polyhedral set representations for the optimal rate regions without assuming any symmetries between the users. Moreover we show that as the users move around within the cell, i.e. as the propagation path loss and the distribution of the channel fading of each user changes, the optimal power and rate control strategy can adapt by simply scaling the channel fades of the users. Our numerical results show that increasing the granularity of rate adaptation (i.e. increasing the number of intermediate rate levels) for the same range of rates (i.e. the same maximum and minimum rate) does not significantly increase the average throughput.