  This paper considers a multi-user multiple-input multiple-output (MU-MIMO)
system where the precoding matrix is selected in a baseband unit (BBU) and then
sent over a digital fronthaul to the transmitting antenna array. The fronthaul
has a limited bit resolution with a known quantization behavior. We formulate a
new sum rate maximization problem where the precoding matrix elements must
comply with the quantizer. We solve this non-convex mixed-integer problem to
local optimality by a novel iterative algorithm inspired by the classical
weighted minimum mean square error (WMMSE) approach. The precoding optimization
subproblem becomes an integer least-squares problem, which we solve with a new
algorithm using a sphere decoding (SD) approach. We show numerically that the
proposed precoding technique vastly outperforms the baseline of optimizing an
infinite-resolution precoder and then quantizing it. We also develop a
heuristic quantization-aware precoding that outperforms the baseline while
having comparable complexity.
