  Empowering cellular networks with augmented sensing capabilities is one of
the key research areas in 6G communication systems. Recently, we have witnessed
a plethora of efforts to devise solutions that integrate sensing capabilities
into communication systems, i.e., joint communication and sensing (JCAS).
However, most prior works do not consider the impact of reconfigurable
intelligent surfaces (RISs) on JCAS systems, especially at millimeter-wave
(mmWave) bands. Given that RISs are expected to become an integral part of
cellular systems, it is important to investigate their potential in cellular
networks beyond communication goals. In this paper, we study mmWave orthogonal
frequency-division multiplexing (OFDM) JCAS systems in the presence of RISs.
Specifically, we jointly design the hybrid beamforming and RIS phase shifts to
guarantee the sensing functionalities via minimizing a chordal-distance metric,
subject to signal-to-interference-plus-noise (SINR) and power constraints. The
non-convexity of the investigated problem poses a challenge which we address by
proposing a solution based on the penalty method and manifold-based alternating
direction method of multipliers (ADMM). Simulation results demonstrate that
under various settings both sensing and communication experience improved
performance when the RIS is adequately designed. In addition, we discuss the
tradeoff between sensing and communication.
