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

We propose a nonlinear phase-quantized constantenvelope 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-toanalog converters (DACs) at the base station (BS) and orthogonal frequency-division multiplexing (OFDM) to communicate over frequency-selective channels. We demonstrate that SQUID is able to perform precoding for 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 lowresolution (e.g., 1-bit) DACs. We use error-rate simulations to demonstrate the superiority of SQUID over conventional linearquantized precoders for massive MU-MIMO-OFDM systems.