On the Capacity and Design of Limited Feedback Multiuser MIMO Uplinks

TL;DR

This paper analyzes the capacity of multiuser MIMO uplinks with limited feedback, proposing codebook design methods to optimize sum-rate and demonstrating near-optimal performance with few feedback bits.

Contribution

It derives the capacity region for MIMO MAC with partial feedback and introduces a finite codebook design approach for maximizing sum-rate under feedback constraints.

Findings

Capacity increases with more feedback bits.

Proposed codebook design achieves near-optimal performance with limited feedback.

Beamforming is optimal under certain conditions.

Abstract

The theory of multiple-input multiple-output (MIMO) technology has been well-developed to increase fading channel capacity over single-input single-output (SISO) systems. This capacity gain can often be leveraged by utilizing channel state information at the transmitter and the receiver. Users make use of this channel state information for transmit signal adaptation. In this correspondence, we derive the capacity region for the MIMO multiple access channel (MIMO MAC) when partial channel state information is available at the transmitters, where we assume a synchronous MIMO multiuser uplink. The partial channel state information feedback has a cardinality constraint and is fed back from the basestation to the users using a limited rate feedback channel. Using this feedback information, we propose a finite codebook design method to maximize sum-rate. In this correspondence, the codebook is a set of transmit signal covariance matrices. We also derive the capacity region and codebook design methods in the case that the covariance matrix is rank-one (i.e., beamforming). This is motivated by the fact that beamforming is optimal in certain conditions. The simulation results show that when the number of feedback bits increases, the capacity also increases. Even with a small number of feedback bits, the performance of the proposed system is close to an optimal solution with the full feedback.