Joint Beamforming and Broadcasting in Massive MIMO

TL;DR

This paper introduces a joint beamforming and broadcasting technique for massive MIMO systems that improves power efficiency by simultaneously serving terminals with and without channel state information, outperforming traditional orthogonal access methods.

Contribution

The paper proposes and analyzes a novel joint beamforming and broadcasting method for massive MIMO, enabling efficient simultaneous service to different terminal groups with varying CSI availability.

Findings

JBB outperforms orthogonal access in power efficiency.

Broadcasting in nullspace avoids interference with beamforming.

JBB achieves similar or better rates with less power.

Abstract

The downlink of a massive MIMO system is considered for the case in which the base station must concurrently serve two categories of terminals: one group to which imperfect instantaneous channel state information (CSI) is available, and one group to which no CSI is available. Motivating applications include broadcasting of public channels and control information in wireless networks. A new technique is developed and analyzed: joint beamforming and broadcasting (JBB), by which the base station beamforms to the group of terminals to which CSI is available, and broadcasts to the other group of terminals, to which no CSI is available. The broadcast information does not interfere with the beamforming as it is placed in the nullspace of the channel matrix collectively seen by the terminals targeted by the beamforming. JBB is compared to orthogonal access (OA), by which the base station partitions the time-frequency resources into two disjunct parts, one for each group of terminals. It is shown that JBB can substantially outperform OA in terms of required total radiated power for given rate targets.