A Distributed Downlink Scheduling Method for Multi-user Communication with Zero-Forcing Beamforming

Abstract

A distributed scheduling approach to multi-user beamforming at a base-station (BS) is proposed. The scheme assumes the BS is equipped with multiple antennas and uses zero-forcing beamforming to schedule multiple users on the same time-frequency uplink/downlink channels. The optimal user set that the BS can schedule is the one that maximizes the sum-rate for the communication channel. The computational complexity of finding the optimal user set becomes intractable as the number of users exceeds 20-30. Sub-optimal schemes for maximizing the sum-rate generally require large control overhead. We propose an effective sum-rate performance metric that accounts for the control overhead required by the scheduling method. We also propose a user scheduling method that has both low complexity and low overhead. The complexity and overhead are reduced by adding one user at a time and distributing the computation of the scheduling problem across the users. Each user calculates the sum-rate that would result if they were added to the existing user set using their own channel and knowledge of the channels of existing users that is broadcast by the BS. The user with greatest sum-rate increase is scheduled. We use simulations to show that this approach can achieve significantly higher effective sum rates than previously proposed sub-optimal schemes. We also derive an approximate expression for the expected sum-rate with our distributed scheduling method and demonstrate its accuracy with simulations.