  Joint radar-communications (JRC) benefits from multi-functionality of radar
and communication operations using same hardware and radio frequency (RF)
spectrum resources. Thus JRC systems possess very high potential to be employed
into the sixth generation (6G) standards. This paper designs a flexible
beamformer for multiple-input multiple output (MIMO) JRC with maximized
spectral efficiency (SE). Hybrid beamforming is implemented which constitutes
lesser number of RF chains than number of transmitter antennas. We jointly
express JRC rate with communication and radar entities including a weighting
factor which depicts the dominance of one operation over the other. The
joint-SE based proposed method optimally selects the number of RF chains with
flexible hynrid beamforming design. Furthermore, when the communication
operation takes place the proposed method takes into account the interference
occurring from the radar operation and vice-versa. Fractional programming based
selection procedure is used for flexible beamforming and optimal number of RF
chains while considering interference of each operation. Simulation results are
presented and compared with different baselines to show effectiveness of the
proposed flexible hybrid beamforming method.
