  To enable non-line-of-sight (NLoS) sensing and communications, dual-function
radar-communications (DFRC) systems have recently proposed employing
reconfigurable intelligent surface (RIS) as a reflector in wireless media.
However, in the dense environment and higher frequencies, severe propagation
and attenuation losses are a hindrance for RIS-aided DFRC systems to utilize
wideband processing. To this end, we propose equipping the transceivers with
the reconfigurable holographic surface (RHS) that, different from RIS, is a
metasurface with an embedded connected feed deployed at the transceiver for
greater control of the radiation amplitude. This surface is crucial for
designing compact low-cost wideband wireless systems, wherein ultra-massive
antenna arrays are required to compensate for the losses incurred by severe
attenuation and diffraction. We consider a novel wideband DFRC system equipped
with an RHS at the transceiver and a RIS reflector in the channel. We jointly
design the digital, holographic, and passive beamformers to maximize the radar
signal-to-interference-plus-noise ratio (SINR) while ensuring the
communications SINR among all users. The resulting nonconvex optimization
problem involves maximin objective, constant modulus, and difference of convex
constraints. We develop an alternating maximization method to decouple and
iteratively solve these subproblems. Numerical experiments demonstrate that the
proposed method achieves better radar performance than non-RIS, random-RHS, and
randomly configured RIS-aided DFRC systems.
