  Rate-Splitting Multiple Access (RSMA) has recently appeared as a powerful and
robust multiple access and interference management strategy for downlink
Multi-user (MU) multi-antenna communications. In this work, we study the
precoder design problem for RSMA scheme in downlink MU systems with both
perfect and imperfect Channel State Information at the Transmitter (CSIT) and
assess the role and benefits of transmitting multiple common streams. Unlike
existing works which have considered single-antenna receivers (Multiple-Input
Single-Output--MISO), we propose and extend the RSMA framework for
multi-antenna receivers (Multiple-Input Multiple-Output--MIMO) and formulate
the precoder optimization problem with the aim of maximizing the Weighted
Ergodic Sum-Rate (WESR). Precoder optimization is solved using Sample Average
Approximation (SAA) together with the proposed vectorization and Weighted
Minimum Mean Square Error (WMMSE) based approach. Achievable sum-Degree of
Freedom (DoF) of RSMA is derived for the proposed framework as an increasing
function of the number of transmitted common and private streams, which is
further validated by the Ergodic Sum Rate (ESR) performance using Monte Carlo
simulations. Conventional MU-MIMO based on linear precoders and Non-Orthogonal
Multiple Access (NOMA) schemes are considered as baselines. Numerical results
show that with imperfect CSIT, the sum-DoF and ESR performance of RSMA is
superior than that of the two baselines, and is increasing with the number of
transmitted common streams. Moreover, by better managing the interference, RSMA
not only has significant ESR gains over baseline schemes but is more robust to
CSIT inaccuracies, network loads and user deployments.
