  In a superimposed unicast and multicast transmission system, one layer of
Successive Interference Cancellation (SIC) is required at each receiver to
remove the multicast stream before decoding the unicast stream. In this paper,
we show that a linearly-precoded Rate-Splitting (RS) strategy at the
transmitter can efficiently exploit this existing SIC receiver architecture. By
splitting the unicast message into common and private parts and encoding the
common parts along with the multicast message into a super-common stream
decoded by all users, the SIC is used for the dual purpose of separating the
unicast and multicast streams as well as better managing the multi-user
interference between the unicast streams. The precoders are designed with the
objective of maximizing the Weighted Sum Rate (WSR) of the unicast messages
subject to a Quality of Service (QoS) requirement of the multicast message and
a sum power constraint. Numerical results show that RS outperforms existing
Multi-User Linear-Precoding (MU-LP) and power-domain Non-Orthogonal Multiple
Access (NOMA) in a wide range of user deployments (with a diversity of channel
directions and channel strengths). Moreover, since one layer of SIC is required
to separate the unicast and multicast streams, the performance gain of RS comes
without any increase in the receiver complexity compared with MU-LP. Hence, in
such non-orthogonal unicast and multicast transmissions, RS provides rate and
QoS enhancements at no extra cost for the receivers.
