Low-Complexity Space-Time-Frequency Scheduling for MIMO Systems With SDMA

In this contribution we propose a low-complexity fair scheduling algorithm for wireless multi-user MIMO communication systems in which users are multiplexed via time-, frequency-, and space-division multiple access schemes, TDMA, FDMA, and SDMA, respectively. In such systems, the transmission quality degrades considerably if users with spatially correlated channels are scheduled to be served at the same time and frequency. The approach presented here works with both zero-forcing and non zero-forcing SDMA precoding schemes by deciding, for each time and frequency slot, which users are to be served in order to maximize the precoding performance. The number of users is not a fixed parameter of the algorithm (as often assumed for other schedulers present in the literature), but is also adjusted according to the channel conditions. While smaller SDMA groups allow us to transmit with higher average power per user, larger groups lead to higher multiplexing gains. Our algorithm ProSched is based on a novel interpretation of the precoding process using orthogonal projections which permit us to estimate the precoding results of all user combinations of interest with significantly reduced complexity. Additionally, the possible user combinations are efficiently treated with the help of a tree-based sorting algorithm. The ProSched takes advantage of perfect channel state information, when available, or alternatively of second order channel statistics. The individual user quality of service (QoS) requirements can be considered in the decision making process. The effectiveness of the algorithm is illustrated with simulations based on the IlmProp channel model, which features realistic correlation in space, time, and frequency.