STAR-RIS-Assisted Communication Radar Coexistence: Analysis and Optimization

Integrated sensing and communication (ISAC) is ex-
pected to play a prominent role among emerging technologies in
future wireless communications. In particular, a communication
radar coexistence system is degraded significantly by mutual
interference. In this work, given the advantages of promising
reconfigurable intelligent surface (RIS), we propose a simultane-
ously transmitting and reflecting RIS (STAR-RIS)-assisted radar
coexistence system where a STAR-RIS is introduced to improve
the communication performance while suppressing the mutual
interference and providing full space coverage. Based on the
realistic conditions of correlated fading, and the presence of
multiple user equipments (UEs) at both sides of the RIS, we
derive the achievable rates at the radar and the communication
receiver side in closed forms in terms of statistical channel state
information (CSI). Next, we perform alternating optimization
(AO) for optimizing the STAR-RIS and the radar beamforming.
Regarding the former, we optimize the amplitudes and phase
shifts of the STAR-RIS through a projected gradient ascent
algorithm (PGAM) simultaneously with respect to the amplitudes
and phase shifts of the surface for both energy splitting (ES)
and mode switching (MS) operation protocols. The proposed
optimization saves enough overhead since it can be performed
every several coherence intervals. This property is particularly
beneficial compared to reflecting-only RIS because a STAR-
RIS includes the double number of variables, which require
increased overhead. Finally, simulation results illustrate how
the proposed architecture outperforms the conventional RIS
counterpart, and show how the various parameters affect the
performance. Moreover, a benchmark full instantaneous CSI (I-
CSI) based design is provided and shown to result in higher
sum-rate but also in large overhead associated with complexity.