Large System Analysis of Cooperative Multi-Cell Downlink Transmission via Regularized Channel Inversion with Imperfect CSIT

In this paper, we analyze the ergodic sum-rate of a multi-cell downlink system with base station (BS) cooperation using regularized zero-forcing (RZF) precoding. Our model assumes that the channels between BSs and users have independent spatial correlations and imperfect channel state information at the transmitter (CSIT) is available. Our derivations are based on large dimensional random matrix theory (RMT) under the assumption that the numbers of antennas at the BS and users approach to infinity with some fixed ratios. In particular, a deterministic equivalent expression of the ergodic sum-rate is obtained and is instrumental in getting insight about the joint operations of BSs, which leads to an efficient method to find the asymptotic-optimal regularization parameter for the RZF. In another application, we use the deterministic channel rate to study the optimal feedback bit allocation among the BSs for maximizing the ergodic sum-rate, subject to a total number of feedback bits constraint. By inspecting the properties of the allocation, we further propose a scheme to greatly reduce the search space for optimization. Simulation results demonstrate that the ergodic sum-rates achievable by a subspace search provides comparable results to those by an exhaustive search under various typical settings.

I. Introduction

Multi-user multiple-input multiple-output (MU-MIMO) has been well recognized as an effective means to increase capacity in the downlink [1]–[3]. However, challenges arise in practical cellular systems where inter-cell interference remains the bottleneck limiting the achievable performance. Therefore, base station (BS) cooperation was recently proposed as a way to alleviate the issue, e.g., [4]–[10], which is greatly motivated by the fact that BSs may be connected via high-speed backhaul links and the channel state information (CSI) and/or data and/or precoding matrices can be shared among the BSs for coordinated transmission. Such BS cooperation in the downlink can improve sum-rates and reduce outage as compared to the conventional or single-cell signal processing where the interference (often from other cells) is treated as noise.