Joint Power and Bandwidth Allocation in Downlink Transmission

We formulate and analyze the problem of optimal downlink scheduling with instantaneous channel and queue information in a CDMA cell when both power and bandwidth may be adaptively split among multiple users. We derive throughput optimal solutions of low computational complexity, as well as faster and simpler approximations, to various versions of this problem when the power, rate and bandwidth allocations to the users can all take continuous values. For this case, we show that the optimal schedule requires transmission to no more than two users during each time slot when users can receive at arbitrary rates, and even when the user rate per code is upper bounded by the best available modulation scheme. Our methods also extend easily to incorporate other intuitive constraints, such as upper limits on user rates to improve frame fill efficiency. Simulation results suggest that the simple approximations work nearly as well as the throughput optimal schedules when continuous bandwidth and power partitions are allowed. In practice, the rates and bandwidth assignments to users take discrete values, and we apply heuristic methods motivated by the continuous optimum to this discrete case. Simulations suggest reasonably good performance from simple heuristics, although the optimality gaps here could be greater.