Stability region of arrival rates and optimal scheduling for MIMO-MAC - a cross-layer approach

With the imminent proliferation of new services in wireless standards beyond 3G, which require low-delay and/or high-rate uplinks, is associated a need for flexible and fair uplink scheduling. In this work we give an overview of our recent results regarding optimal scheduling schemes and stability properties of a system of bit-queues in the multiple input multiple output multiple access channel (MIMO-MAC). Our results originate from joint optimization of the physical layer and data link layer, which is a field of so called cross layer design of network communication stack. We make use of the notions of so called weak and strong stability of the queue system and interpret the behaviour of queue system complying with each notion. We show, that the ergodic capacity region of the MIMO-MAC corresponds to the stability region of the MIMO-MAC. We specify the optimal scheduling policy, achieving the largest possible stability region and using spatial channelization of user signals, with event-dependent changes of successive interference cancellation order. Presented optimal scheduling policy determines optimal signal processing on the transmitter and on the receiver side, based on easy to handle, convex optimization problem. Our results hold for both individual and sum-power constraints in the MIMO-MAC. We interpret them in terms of geometry, characterizing in this way specific points and subregions of the capacity region together with transmit strategies achieving them.