FDD Massive MIMO: How to Optimally Combine UL Pilot and Limited DL CSI Feedback?

TL;DR

This paper proposes a novel method combining uplink CSI and limited downlink feedback to accurately reconstruct downlink channels in FDD Massive MIMO systems, significantly reducing feedback overhead while maintaining high spectral efficiency.

Contribution

It introduces a MSE-optimal downlink channel reconstruction method that jointly exploits uplink CSI and quantized phase feedback, with an optimal feedback bit allocation strategy.

Findings

Achieves spectral efficiency comparable to perfect CSI using limited feedback.

Derives the MSE as a function of feedback bits for phase quantization.

Provides an optimal feedback bit allocation method to minimize MSE.

Abstract

In frequency-division duplexing (FDD) multiple-input multiple-output (MIMO) systems, obtaining accurate downlink channel state information (CSI) for precoding is vastly challenging due to the tremendous feedback overhead with the growing number of antennas. Utilizing uplink pilots for downlink CSI estimation is a promising approach that can eliminate CSI feedback. However, the downlink CSI estimation accuracy diminishes significantly as the number of channel paths increases, resulting in reduced spectral efficiency. In this paper, we demonstrate that achieving downlink spectral efficiency comparable to perfect CSI is feasible by combining uplink CSI with limited downlink CSI feedback information. Our proposed downlink CSI feedback strategy transmits quantized phase information of downlink channel paths, deviating from conventional limited methods. We put forth a mean square error (MSE)-optimal downlink channel reconstruction method by jointly exploiting the uplink CSI and the limited downlink CSI. Armed with the MSE-optimal estimator, we derive the MSE as a function of the number of feedback bits for phase quantization. Subsequently, we present an optimal feedback bit allocation method for minimizing the MSE in the reconstructed channel through phase quantization. Utilizing a robust downlink precoding technique, we establish that the proposed downlink channel reconstruction method is sufficient for attaining a sum-spectral efficiency comparable to perfect CSI.