Maximizing the Mutual Information of Multi-Antenna Links Under an Interfered Receiver Power Constraint

TL;DR

This paper addresses the problem of maximizing the mutual information of a MIMO link while limiting interference to a primary receiver, providing a novel optimal power allocation solution under interference constraints.

Contribution

It introduces the first evaluation of the mutual information of a secondary MIMO link under interference constraints in spectrum sharing scenarios, deriving the optimal power allocation and precoding policies.

Findings

Optimal power allocation under interference constraints is derived.

The proposed solution outperforms traditional water-filling in interference-limited scenarios.

Interference penalties significantly impact the capacity of secondary links.

Abstract

Single-user multiple-input / multiple-output (SU-MIMO) communication systems have been successfully used over the years and have provided a significant increase on a wireless link's capacity by enabling the transmission of multiple data streams. Assuming channel knowledge at the transmitter, the maximization of the mutual information of a MIMO link is achieved by finding the optimal power allocation under a given sum-power constraint, which is in turn obtained by the water-filling (WF) algorithm. However, in spectrum sharing setups, such as Licensed Shared Access (LSA), where a primary link (PL) and a secondary link (SL) coexist, the power transmitted by the SL transmitter may induce harmful interference to the PL receiver. While such co-existing links have been considered extensively in various spectrum sharing setups, the mutual information of the SL under a constraint on the interference it may cause to the PL receiver has, quite astonishingly, not been evaluated so far. In this paper, we solve this problem, find its unique optimal solution and provide the power allocation policy and corresponding precoding solution that achieves the optimal capacity under the imposed constraint. The performance of the optimal solution and the penalty due to the interference constraint are evaluated over some indicative Rayleigh fading channel conditions and interference thresholds. We believe that the obtained results are of general nature and that they may apply, beyond spectrum sharing, to a variety of applications that admit a similar setup.