Optimizing large-scale MIMO cellular downlink: Multiplexing, diversity, or interference nulling?

A base-station (BS) equipped with multiple antennas can use its spatial dimensions in three ways: (1) serve multiple users to achieve a multiplexing gain, (2) provide diversity to its users, and/or (3) null interference at a chosen subset of out-of-cell users. The main contribution of this paper is to answer the following question: what is the optimal balance between the three competing benefits of multiplexing, diversity and interference nulling? We answer this question in the context of the downlink of a cellular network in which each user chooses its best serving BS, and requests nearby interfering BSs for interference nulling. BSs are equipped with a large number of antennas, serve multiple single-antenna users using zero-forcing beamforming and equal power assignment, and null interference at a subset of out-of-cell users. The remaining spatial dimensions provide transmit diversity. We assume perfect channel state information at the BSs and users. Utilizing tools from stochastic geometry, we show that, surprisingly, to maximize the per-BS ergodic sum rate, at the optimal allocation of spatial resources, interference nulling does not bring tangible benefit. A close-to-optimal strategy is to use none of the spatial resources for interference nulling, while reserving 60% of spatial resources for achieving multiplexing and the rest for providing diversity.