Downlink Throughput Maximization: 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 policy (TDMA or CDMA), and find the corresponding power allocation. We model the interference by the aid of the orthogonality factor and determine the transmission strategy for a range of the values of the orthogonality factor, and users’ channel gains, so as to maximize the system throughput, subject to a total power constraint. Although the resulting optimization problem may turn out to be non-convex depending on the value of the orthogonality factor, we show that valuable observations regarding the optimum solution can be obtained based on the properties of the problem. In particular, we propose an exact and a near-exact algorithm to determine whether TDMA is the optimum strategy, or simultaneous users (CDMA) should be transmitted to.