Coordinate ascent scheduler for the multi-user MIMO uplink channel

Orthogonal frequency division multiple access (OFDMA) transmission has been chosen for the next generation of wireless networks. Multiple input multiple output (MIMO) techniques are well suited for OFDMA and will be used for increased transmission efficiency for both downlink and uplink. In this paper we focus on space division multiple access (SDMA) techniques for the uplink of such a network. In particular, we consider the problem of scheduling uplink users so as to maximize the uplink resources while satisfying the QoS requirements of each userpsilas flows. 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, the scheduling problem becomes multi-dimensional and, in general, its computational complexity grows non-linearly with the number of users. In addition to choosing the users to be scheduled one must also determine the optimal transmission power for each scheduled user. For the case of two users per frame we show that the optimal power allocation scheme is such that either both users transmit with full power or only one transmits with full power. We then propose a coordinate ascent algorithm for solving the utility-based scheduling problem. Simulations are then used to show that the solution produced by this approach is close to optimal and is quite robust.