  In this paper, we consider a downlink multi-user multiple-input
multiple-output (MU-MIMO) communication assisted by a reconfigurable
intelligent surface (RIS) and study the precoding and RIS configuration design
under practical system constraints. These constraints include the
limited-capacity fronthaul at the transmitter side and the finite resolution of
RIS elements. We investigate the sum mean squared error (MSE) minimization
problem and propose an algorithm based on the block coordinate descent method
to optimize the precoding, RIS configuration, and receiver gains. We compute
the precoding vectors and RIS configuration using the Schnorr-Euchner sphere
decoding (SESD) method which delivers the optimal MSE-minimizing solution. We
numerically evaluate the performance of the proposed SESD-based methods and
corroborate their effectiveness in improving the system performance.
