Downlink Throughput Maximization for Interference Limited Multiuser Systems: TDMA versus CDMA

We consider the downlink throughput maximization problem for interference limited multiuser systems. Our goal is to characterize the optimum base station transmission strategy, i.e., whether the base station transmits to one-user (TDMA) or multiple users (CDMA). Specifically, we aim at determining the optimum number of users to be scheduled and finding the corresponding power allocation. We model the interference by the aid of the orthogonality factor, and determine the throughput maximizing transmission strategy for a range of the values of the orthogonality factor, and the channel gains, subject to a total power constraint. Although the resulting optimization problem may turn out to be non-convex, we show that valuable observations regarding the structure of the optimum solution can be obtained by examining the performance metric from an individual user´s point of view. We propose an exact and a near-exact algorithm to determine whether one-user-transmission is the optimum strategy, or more than one user should be transmitted to. Numerical results to support our analysis, as well as the modifications to the proposed algorithms in the presence of individual power constraints are presented.