Downlink SDMA with Limited Feedback in Interference-Limited Wireless Networks

TL;DR

This paper analyzes the performance of SDMA with limited feedback in interference-limited wireless networks, revealing that limited feedback significantly degrades capacity and that single-stream transmission is often optimal.

Contribution

It provides closed-form expressions for key performance metrics and demonstrates the necessity of increasing feedback with antennas and SINR to maintain multiplexing gains.

Findings

Per-user feedback rate must increase linearly with antennas and SINR for full multiplexing.

Single-stream transmission is optimal in most practical interference-limited scenarios.

SDMA is generally not advisable for interference-limited wireless networks.

Abstract

The tremendous capacity gains promised by space division multiple access (SDMA) depend critically on the accuracy of the transmit channel state information. In the broadcast channel, even without any network interference, it is known that such gains collapse due to interstream interference if the feedback is delayed or low rate. In this paper, we investigate SDMA in the presence of interference from many other simultaneously active transmitters distributed randomly over the network. In particular we consider zero-forcing beamforming in a decentralized (ad hoc) network where each receiver provides feedback to its respective transmitter. We derive closed-form expressions for the outage probability, network throughput, transmission capacity, and average achievable rate and go on to quantify the degradation in network performance due to residual self-interference as a function of key system parameters. One particular finding is that as in the classical broadcast channel, the per-user feedback rate must increase linearly with the number of transmit antennas and SINR (in dB) for the full multiplexing gains to be preserved with limited feedback. We derive the throughput-maximizing number of streams, establishing that single-stream transmission is optimal in most practically relevant settings. In short, SDMA does not appear to be a prudent design choice for interference-limited wireless networks.