Analysis of System Capacity of Grant-free Access with Massive MIMO

Massive sporadic access is a crucial scenario in the future wireless communications, where grant-free access is preferred for reducing signaling overhead and latency. To deal with much more active user devices, massive multiple-input-multiple-output (MIMO) is introduced to increase the system capacity. However, the length of pilot sequences has to be increased with more users and antennas to perform joint activity detection and channel estimation (JADCE), occupying more radio resource. Thus there is a trade-off in the pilot sequence length to maximize the system capacity, and other parameters such as the base station (BS) antenna number and the active rate of users also influence the optimization results. Therefore, it is necessary to explore how these parameters influence the capacity. Toward this end, this paper conducts simulations of a single-cell grant-free system with massive MIMO. It firstly shows that the capacity loss compared to granted access mainly comes from activity detection rather than channel estimation with non-orthogonal pilot sequences. Then the paper investigates the optimization of the pilot sequence length and the number of active users with different channel assumptions. A noteworthy result is that the capacity grows more slowly than the number of BS antennas does in the massive MIMO regime. Additionally, this paper proposes a modified approximate message passing (AMP) algorithm for the multi-antenna user situations, to maximize the system capacity by adjusting the number of user antennas.