  In this paper, we aim to achieve energy efficient design with minimum
hardware requirement for hybrid precoding, which enables a large number of
antennas with minimal number of RF chains, and sub-arrayed multiple-input
multiple-output (MIMO) radar based joint radar-communication (JRC) systems. A
dynamic active RF chain selection mechanism is implemented in the baseband
processing and the energy efficiency (EE) maximization problem is solved using
fractional programming to obtain the optimal number of RF chains at the current
channel state. Subsequently hybrid precoders are computed employing a
sub-arrayed MIMO structure for EE maximization with weighted formulation of the
communication and radar metrics, and the solution is based on alternating
minimization. The simulation results show that the proposed method with minimum
hardware achieves the best EE while maintaining the rate performance, and an
efficient trade-off between sensing and communication.
