  Dual-Functional Radar-Communication (DFRC) system is an essential and
promising technique for beyond 5G. In this work, we propose a powerful and
unified multi-antenna DFRC transmission framework, where an additional radar
sequence is transmitted apart from communication streams to enhance radar
beampattern matching capability, and Rate-Splitting Multiple Access (RSMA) is
adopted to better manage the interference. RSMA relies on multi-antenna
Rate-Splitting (RS) with Successive Interference Cancellation (SIC) receivers,
and the split and encoding of messages into common and private streams. We
design the message split and the precoders of the radar sequence and
communication streams to jointly maximize the Weighted Sum Rate (WSR) and
minimize the radar beampattern approximation Mean Square Error (MSE) subject to
the per antenna power constraint. An iterative algorithm based on Alternating
Direction Method of Multipliers (ADMM) is developed to solve the problem.
Numerical results first show that RSMA-assisted DFRC achieves a better tradeoff
between WSR and beampattern approximation than Space-Division Multiple Access
(SDMA)-assisted DFRC with or without radar sequence, and other simpler
radar-communication strategies using orthogonal resources. We also show that
the RSMA-assisted DFRC frameworks with and without radar sequence achieve the
same tradeoff performance. This is because that the common stream is better
exploited in the proposed framework. The common stream of RSMA fulfils the
triple function of managing interference among communication users, managing
interference between communication and radar, and beampattern approximation.
Therefore, by enabling RSMA in DFRC, the system performance is enhanced while
the system architecture is simplified since there is no need to use additional
radar sequence and SIC. We conclude that RSMA is a more powerful multiple
access for DFRC.
