  Integrated sensing and communication (ISAC) is recognized as one of the key
enabling technologies for sixth-generation (6G) wireless communication
networks, facilitating diverse emerging applications and services in an energy
and cost-efficient manner. This paper proposes a multi-user multi-target ISAC
system to enable full-space coverage for communication and sensing tasks. The
proposed system employs a hybrid simultaneous transmission and reflection
reconfigurable intelligent surface (STAR-RIS) comprising active transmissive
and passive reflective elements. In the proposed scheme, the passive reflective
elements support communication and sensing links for nearby communication users
and sensing targets, while low-power active transmissive elements are deployed
to improve sensing performance and overcome high path attenuation due to
multi-hop transmission for remote targets. Moreover, to optimize the
transmissive/reflective coefficients of the hybrid STAR-RIS, a semi-definite
relaxation (SDR)-based algorithm is proposed. Furthermore, to evaluate sensing
performance, signal-to-interference-noise ratio (SINR) and Cramer-Rao bound
(CRB) metrics have been derived and investigated via conducting extensive
computer simulations.
