Optimal and low complex suboptimal transmission schemes for MIMO-OFDM broadcast channels

In this paper, we address the problem of optimal power allocation for multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) broadcast channels with a sum power constraint when transmitter and receiver have perfect channel state information (CSI). We derive optimality conditions for power allocation and present an efficient algorithm achieving the optimal power allocation. Furthermore, we obtain necessary and sufficient conditions for the optimality of frequency division multiple access (FDMA) transmission and single user transmission. It turns out that the assignment of multiple users to one subcarrier is optimal in general. We propose a low complexity algorithm using a pure FDMA-scheme without multiple assignment of users and characterize the achievable rate region in comparison to the complete capacity region.