Massive multiuser (MU) multiple-input multiple-output (MIMO) is foreseen to be one of the key technologies in fifth-generation wireless communication systems. In this paper, we investigate the problem of downlink precoding for a narrowband massive MU-MIMO system with low-resolution digital-to-analog converters (DACs) at the base station (BS). We analyze the performance of linear precoders, such as maximal-ratio transmission and zero-forcing, subject to coarse quantization. Using Bussgang's theorem, we derive a closed-form approximation of the achievable rate of the coarsely quantized system. Our results reveal that the infinite-resolution performance can be approached with DACs using only 3 to 4 bits of resolution, depending on the number of BS antennas and the number of user equipments (UEs). For the case of 1-bit DACs, we also propose novel nonlinear precoding algorithms that significantly outperform linear precoders at the cost of an increased computational complexity. Specifically, we show that nonlinear precoding incurs only a 3 dB penalty compared to the infinite-resolution case for an uncoded bit error rate of 10^-3 in a system with 128 BS antennas that uses 1-bit DACs and serves 16 single-antenna UEs; in contrast, the penalty is about 8 dB for linear precoders.
['Sven Jacobsson', 'Giuseppe Durisi', 'Mikael Coldrey', 'Tom Goldstein', 'Christoph Studer']
Quantized Precoding for Massive MU-MIMO