  Dual-Functional Radar-Communication systems enhance the benefits of
communications and radar sensing by jointly implementing these on the same
hardware platform and using the common RF resources. An important and latest
concern to be addressed in designing such Dual-Functional Radar-Communication
systems is maximizing the energy-efficiency. In this paper, we consider a
Dual-Functional Radar-Communication system performing simultaneous multi-user
communications and radar sensing, and investigate the energy-efficiency
behaviour with respect to active transmission elements. Specifically, we
formulate a problem to find the optimal precoders and the number of active RF
chains for maximum energy-efficiency by taking into consideration the power
consumption of low-resolution Digital-to-Analog Converters on each RF chain
under communications and radar performance constraints. We consider
Rate-Splitting Multiple Access to perform multi-user communications with
perfect and imperfect Channel State Information at Transmitter. The formulated
non-convex optimization problem is solved by means of a novel algorithm. We
demonstrate by numerical results that Rate Splitting Multiple Access achieves
an improved energy-efficiency by employing a smaller number of RF chains
compared to Space Division Multiple Access, owing to its generalized structure
and improved interference management capabilities.
