Joint Space–Frequency User Scheduling for MIMO Random Beamforming With Limited Feedback

This paper presents a joint space-frequency user scheduling approach to enhance the random beamforming (RBF) with limited feedback in multiple-input -multiple-output (MIMO) broadcast channels. Such scheme was shown to obtain the optimal scaling law of sum rate in the large number of user regime. However, for a small number of user cases, the system degrees-of-freedom cannot be well exploited, and the accuracy of predicting users' signal-to-interference-plus-noise ratios (SINRs) is also degraded. Motivated by these, two strategies are proposed to solve the problems. Specifically, the conventional spatial domain RBF is first extended to the space-frequency domain. The first strategy aims to maximize the number of active users based on users' initially predicted SINRs; the second strategy is to schedule the maximum number of users, whose preferred beams can coexist with each others, with more accurate users' SINRs prediction method. Computer simulations are carried out to examine the proposed strategies in terms of the sum rate and the number of active users. It is shown that the first strategy achieves close performance to the corresponding brute-force search with lower complexity. Moreover, the second strategy improves the performance by accurately predicting users' SINRs at the price of relatively increased complexity and feedback overhead.