  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 on the rate achievable under such coarse quantization. Our
results reveal that the performance attainable with infinite-resolution DACs
can be approached using DACs having 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 for linear precoders is
about 8 dB.
