Frame Structures for Massive MIMO Communications Under Channel Aging

We study the problem of optimizing the frame structure of a massive MIMO system under channel aging. We argue that the conventional TDD frame structure with lumped training is suboptimal under rapidly aging channels. We, therefore discuss a generalized frame structure allowing for the training of a fraction of the total number of users, with the conventional lumped and interspersed training frames as its special cases. We then derive the achievable uplink and downlink rates for this system, incorporating the cost for switching from uplink to downlink and vice versa. The analysis, and the subsequent numerical results clearly bring out the dependence of the rates achievable in a massive MIMO system on the choice of the frame structure.