Optimal Space Time Scheduling for Block Fading Channels with Partial Power Feedback

The multi-user multiple-input-multiple-output (MIMO) space- time scheduling problem is the prime focus of the paper. Using multiple transmit (nT) and receive antennas (nR), it is well- known that the link-level throughput will be increased linearly with respect to min[nT, nR] without increasing the bandwidth and power budget. However, optimizing the link level performance of the MIMO system does not always imply achieving scheduling- level optimization. Therefore, the design optimization across the link layer and the scheduling layer is very important to fully exploit the temporal and spatial dimensions of the communication channel. In this paper, we address the design of the optimal space-time scheduler for the multi-user MIMO system based on an information theoretical approach. It is well-known that for full feedback, the optimal transmission strategy is a cascade of power control matrix and an eigen-beamforming matrix. Since full knowledge of channel matrix at the transmitter requires a feedback channel capacity not scalable with respect to nT (number of transmit antennas at mobiles) and nR (number of receive antennas at base station), we shall assume a partial power feedback channel where only the power control matrix is adaptive. This paper is an extension to the single-input- single-output (SISO) investigation in [1] where the optimal scheduler design was to allocate all the power to at most a single user at any particular instant. We found that in the multi-user MIMO system with K ?nR (K is the total number of users), the optimal scheduler should allocate all the power to at most nR users at any particular instant. The scheduling gain is attributed to the distributed MIMO configuration formed between mobile users and base station as well as the selection diversity among independent users. We considered two cases of partial power feedback, namely the scalar feedback and the per-antenna vector feedback. With scalar feedback, the optimal scheduling for system performance is to focus the power on a single transmit antenna only. In other words, from a system performance perspective, a 2 x 2 system (with uniform power allocation on the 2 transmit antenna) is worse than a 1 x 2 system. With the per-antenna vector feedback, both the scheduling performance and the link-level performance are enhanced with nT. Finally, scheduling performance improves as K increases because it is more probable to find a set of good users to form the distributed MIMO configuration.