  This work studies the optimization of rate-splitting multiple access (RSMA)
transmission technique for a cloud radio access network (C-RAN) downlink
system. Main idea of RSMA is to split the message for each user equipment (UE)
to private and common messages and perform superposition coding at transmitters
so as to enable flexible decoding at receivers. It is challenging to implement
ideal RSMA scheme particularly when there are many UEs, since the number of
common signals exponentially increases with the number of UEs. An efficient
RSMA scheme is hence proposed that uses a linearly increasing number of common
signals whose decoding UEs are selected using hierarchical clustering. Via
numerical results, we show the performance gains of the proposed RSMA scheme
over conventional space-division multiple access (SDMA) and nonorthogonal
multiple access (NOMA) schemes as well as over a conventional RSMA scheme that
uses a single common signal.
