Linear Precoding with Low-Resolution DACs for Massive MU-MIMO-OFDM Downlink

TL;DR

This paper analyzes the performance of massive MU-MIMO-OFDM systems with low-resolution DACs, providing theoretical bounds and demonstrating that 3-4 bits suffice for reliable communication with minimal performance loss.

Contribution

It offers the first analysis of low-resolution DAC effects in wideband OFDM MU-MIMO systems, including BER and sum-rate bounds, considering oversampling and practical channel conditions.

Findings

3-4 DAC bits achieve BER of 10^-4 with negligible performance loss

Derived closed-form bounds for sum-rate with low-resolution DACs

Highlight the impact of spatial and temporal correlations due to low-precision DACs

Abstract

We consider the downlink of a massive multiuser (MU) multiple-input multiple-output (MIMO) system in which the base station (BS) is equipped with low-resolution digital-to-analog converters (DACs). In contrast to most existing results, we assume that the system operates over a frequency-selective wideband channel and uses orthogonal frequency division multiplexing (OFDM) to simplify equalization at the user equipments (UEs). Furthermore, we consider the practically relevant case of oversampling DACs. We theoretically analyze the uncoded bit error rate (BER) performance with linear precoders (e.g., zero forcing) and quadrature phase-shift keying using Bussgang's theorem. We also develop a lower bound on the information-theoretic sum-rate throughput achievable with Gaussian inputs, which can be evaluated in closed form for the case of 1-bit DACs. For the case of multi-bit DACs, we derive approximate, yet accurate, expressions for the distortion caused by low-precision DACs, which can be used to establish lower bounds on the corresponding sum-rate throughput. Our results demonstrate that, for a massive MU-MIMO-OFDM system with a 128-antenna BS serving 16 UEs, only 3--4 DAC bits are required to achieve an uncoded BER of 10^-4 with a negligible performance loss compared to the infinite-resolution case at the cost of additional out-of-band emissions. Furthermore, our results highlight the importance of taking into account the inherent spatial and temporal correlations caused by low-precision DACs.