Nonlinear Precoding for Phase-Quantized Constant-Envelope Massive MU-MIMO-OFDM

TL;DR

This paper introduces a nonlinear precoding algorithm for massive MU-MIMO-OFDM systems that enables constant-envelope transmission with phase quantization, improving performance with low-resolution DACs.

Contribution

It adapts the SQUID precoder to OFDM systems with oversampling DACs, facilitating constant-envelope signals suitable for power-efficient hardware.

Findings

SQUID-OFDM outperforms linear-quantized precoders in error-rate simulations.

The precoder generates signals compatible with low-resolution DACs.

Constant-envelope signals enable power-efficient RF circuitry.

Abstract

We propose a nonlinear phase-quantized constant-envelope precoding algorithm for the massive multi-user (MU) multiple-input multiple-output (MIMO) downlink. Specifically, we adapt the squared-infinity norm Douglas-Rachford splitting (SQUID) precoder to systems that use oversampling digital-to-analog converters (DACs) at the base station (BS) and orthogonal frequency-division multiplexing (OFDM) to communicate over frequency-selective channels. We demonstrate that the proposed SQUID-OFDM precoder is able to generate transmit signals that are constrained to constant envelope, which enables the use of power-efficient analog radio-frequency circuitry at the BS. By quantizing the phase of the resulting constant-envelope signal, we obtain a finite-cardinality transmit signal that can be synthesized by low-resolution (e.g., 1-bit) DACs. We use error-rate simulations to demonstrate the superiority of SQUID-OFDM over linear-quantized precoders for massive MU-MIMO-OFDM systems.