Coordinate ascent scheduler for the multi-user MIMO broadcast channel

Multiple Input Multiple Output (MIMO) ^{antenna systems} are being used to increase the spectral efficiency for the next generation of wireless networks (LTE, WiMAX). In this paper we focus on beamforming techniques for the downlink channel. In particular, we consider the problem of downlink user selection to maximize the downlink resources while satisfying the QoS requirements of all users. We do this by assigning suitable utility functions to user flows that are designed to maintain the specified QoS constraints. Since multiple users can be simultaneously scheduled in a time-bandwidth resource block, the scheduling problem becomes multi-dimensional and, in general, its computational complexity grows non-linearly with the number of users. We propose a simple but sub-optimal coordinate ascent algorithm for scheduling users. Prior work on this problem included simple greedy algorithms but with inferior performance or near-optimal algorithms that are computationally intensive. We will demonstrate that our proposed solution, although computationally simple, provides near-optimal performance.