Per-antenna Constant Envelope Precoding for Large Multi-User MIMO Systems

TL;DR

This paper investigates constant envelope precoding in large multi-user MIMO systems, demonstrating that array gain is achievable with increasing antennas under CE constraints, and proposes a near-optimal precoding scheme with low complexity.

Contribution

It introduces a novel CE precoding scheme for large MIMO systems that achieves near-optimal performance with linear power reduction and low computational complexity.

Findings

Array gain is achievable under CE constraints with large N.

Total transmit power can be reduced linearly with N.

Proposed precoding performs close to sum-capacity schemes.

Abstract

We consider the multi-user MIMO broadcast channel with $M$ single-antenna users and $N$ transmit antennas under the constraint that each antenna emits signals having constant envelope (CE). The motivation for this is that CE signals facilitate the use of power-efficient RF power amplifiers. Analytical and numerical results show that, under certain mild conditions on the channel gains, for a fixed $M$, array gain is achievable even under the stringent per-antenna CE constraint (essentially, for a fixed $M$, at sufficiently large $N$ the total transmitted power can be reduced with increasing $N$ while maintaining a fixed information rate to each user). Simulations for the i.i.d. Rayleigh fading channel show that the total transmit power can be reduced linearly with increasing $N$ (i.e., an O(N) array gain). We also propose a precoding scheme which finds near-optimal CE signals to be transmitted, and has O(MN) complexity. Also, in terms of the total transmit power required to achieve a fixed desired information sum-rate, despite the stringent per-antenna CE constraint, the proposed CE precoding scheme performs close to the sum-capacity achieving scheme for an average-only total transmit power constrained channel.