Multi-Cell Random Beamforming: Achievable Rate and Degrees of Freedom Region

TL;DR

This paper analyzes multi-cell random beamforming (RBF) in multi-antenna downlink systems, deriving achievable rates and degrees of freedom (DoF) regions, and demonstrates RBF's optimality in high-SNR regimes with large user scaling.

Contribution

It extends RBF analysis to multi-cell systems with inter-cell interference, deriving sum-rate and DoF region expressions, and shows RBF's optimality without full CSI at high SNR.

Findings

Multi-cell RBF achieves asymptotic sum-rate scaling with many users.

The DoF region of multi-cell RBF matches interference-free upper bounds.

RBF is optimal in high-SNR regimes with sufficient user scaling.

Abstract

Random beamforming (RBF) is a practically favourable transmission scheme for multiuser multi-antenna downlink systems since it requires only partial channel state information (CSI) at the transmitter. Under the conventional single-cell setup, RBF is known to achieve the optimal sum-capacity scaling law as the number of users goes to infinity, thanks to the multiuser diversity enabled transmission scheduling that virtually eliminates the intra-cell interference. In this paper, we extend the study of RBF to a more practical multi-cell downlink system with single-antenna receivers subject to the additional inter-cell interference (ICI). First, we consider the case of finite signal-to-noise ratio (SNR) at each receiver. We derive a closed-form expression of the achievable sum-rate with the multi-cell RBF, based upon which we show an asymptotic sum-rate scaling law as the number of users goes to infinity. Next, we consider the high-SNR regime and for tractable analysis assume that the number of users in each cell scales in a certain order with the per-cell SNR. Under this setup, we characterize the achievable degrees of freedom (DoF) for the single-cell case with RBF. Then we extend the analysis to the multi-cell RBF case by characterizing the DoF region. It is shown that the DoF region characterization provides useful guideline on how to design a cooperative multi-cell RBF system to achieve optimal throughput tradeoffs among different cells. Furthermore, our results reveal that the multi-cell RBF scheme achieves the "interference-free DoF" region upper bound for the multi-cell system, provided that the per-cell number of users has a sufficiently large scaling order with the SNR. Our result thus confirms the optimality of multi-cell RBF in this regime even without the complete CSI at the transmitter, as compared to other full-CSI requiring transmission schemes such as interference alignment.