  The polarization domain provides an extra degree of freedom (DoF) for
improving the performance of multiple-input multiple-output (MIMO) systems.
This paper takes advantage of this additional DoF to alleviate practical issues
of successive interference cancellation (SIC) in rate-splitting multiple access
(RSMA) schemes. Specifically, we propose three dual-polarized downlink
transmission approaches for a massive MIMO-RSMA network under the effects of
polarization interference and residual errors of imperfect SIC. The first
approach implements polarization multiplexing for transmitting the users' data
messages, which removes the need to execute SIC in the reception. The second
approach transmits replicas of users' messages in the two polarizations, which
enables users to exploit diversity through the polarization domain. The third
approach, in its turn, employs the original SIC-based RSMA technique per
polarization, and this allows the BS to transmit two independent superimposed
data streams simultaneously. An in-depth theoretical analysis is carried out,
in which we derive tight closed-form approximations for the outage
probabilities of the three proposed approaches. Accurate approximations for the
ergodic sum-rates of the two first schemes are also derived. Simulation results
validate the theoretical analysis and confirm the effectiveness of the proposed
schemes. For instance, under low to moderate cross-polar interference, the
results show that, even under high levels of residual SIC error, our
dual-polarized MIMO-RSMA strategies outperform the conventional
single-polarized MIMO-RSMA counterpart. It is also shown that the performance
of all RSMA schemes is impressively higher than that of single and
dual-polarized massive MIMO systems employing non-orthogonal multiple access
(NOMA) and orthogonal multiple access (OMA) techniques.
