Optimizing the MIMO Cellular Downlink: Multiplexing, Diversity, or Interference Nulling?

TL;DR

This paper investigates how to optimally allocate multiple antennas at base stations among multiplexing, diversity, and interference nulling in cellular networks, finding that interference nulling offers limited sum-rate benefits but improves cell-edge user performance.

Contribution

It provides a stochastic geometry-based analysis showing that interference nulling is not essential for maximizing sum-rate, but enhances cell-edge user experience with adaptive nulling strategies.

Findings

Interference nulling does not significantly increase sum-rate.

Allocating resources to multiplexing and diversity is nearly optimal for sum-rate.

Nulling improves performance for cell-edge users with adaptive strategies.

Abstract

A base-station (BS) equipped with multiple antennas can use its spatial dimensions in three different ways: (1) to serve multiple users, thereby achieving a multiplexing gain, (2) to provide spatial diversity in order to improve user rates and (3) to null interference in neighboring cells. This paper answers the following question: What is the optimal balance between these three competing benefits? We answer this question in the context of the downlink of a cellular network, where multi-antenna BSs serve multiple single-antenna users using zero-forcing beamforming with equal power assignment, while nulling interference at a subset of out-of-cell users. Any remaining spatial dimensions provide transmit diversity for the scheduled users. Utilizing tools from stochastic geometry, we show that, surprisingly, to maximize the per-BS ergodic sum rate, with an optimal allocation of spatial resources, interference nulling does not provide a tangible benefit. The strategy of avoiding inter-cell interference nulling, reserving some fraction of spatial resources for multiplexing and using the rest to provide diversity, is already close-to-optimal in terms of the sum-rate. However, interference nulling does bring significant benefit to cell-edge users, particularly when adopting a range-adaptive nulling strategy where the size of the cooperating BS cluster is increased for cell-edge users.