  In this paper, we firstly overview the application scenarios and the research
progress in the area of communication and radar spectrum sharing (CRSS). We
then propose a novel transceiver architecture and frame structure for a
dual-functional radar-communication (DFRC) base station (BS) operating in the
millimeter wave (mmWave) band, using the hybrid analog-digital (HAD)
beamforming technique. We assume that the BS is serving a multi-antenna aided
user equipment (UE) operating in a mmWave channel, which in the meantime
actively detects multiple targets. Note that part of the targets also play the
role of scatterers for the communication signal. Given this framework, we
propose a novel scheme for joint target search and communication channel
estimation relying on the omni-directional pilot signals generated by the HAD
structure. Given a fully-digital communication precoder and a desired radar
transmit beampattern, we propose to design the analog and digital precoders
under non-convex constant-modulus (CM) and power constraints, such that the BS
can formulate narrow beams towards all the targets, while pre-equalizing the
impact of the communication channel. Furthermore, we design an HAD receiver
that can simultaneously process signals from the UE and echo waves from the
targets. By tracking the angular variation of the targets, we show that it is
possible to recover the target echoes and mitigate the potential interference
imposed on the UE signals by invoking the successive interference cancellation
(SIC) technique, even when the radar and communication signals share the
equivalent signal-to-noise ratio (SNR). The feasibility and the efficiency of
the proposed approaches in realizing DFRC are verified via numerical
simulations. Finally, our discussions are summarized by overviewing the open
problems in the research field of CRSS.
