This paper proposes a general optimization framework for rate splitting
multiple access (RSMA) in beyond diagonal (BD) reconfigurable intelligent
surface (RIS) assisted ultra-reliable low-latency communications (URLLC)
systems. This framework can solve a large family of optimization problems in
which the objective and/or constraints are linear functions of the rates and/or
energy efficiency (EE) of users. Using this framework, we show that RSMA and
RIS can be mutually beneficial tools when the system is overloaded, i.e., when
the number of users per cell is higher than the number of base station (BS)
antennas. Additionally, we show that the benefits of RSMA increase when the
packets are shorter and/or the reliability constraint is more stringent.
Furthermore, we show that the RSMA benefits increase with the number of users
per cell and decrease with the number of BS antennas. Finally, we show that RIS
(either diagonal or BD) can highly improve the system performance, and BD-RIS
outperforms regular RIS.Comment: submitted to at IEEE journa