  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.
