A STAR-RIS-Segmented Symbiotic AmBC System

This paper proposes a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) segmented symbiotic ambient backscatter system. The elements of the STAR-RIS are segmented into an enhancing primary-signal (EP) zone and a backscatter-device (BD) zone, which are adjusted to enhance receivers’ reception of the primary signal from S and to convey STAR-RIS’s information, respectively. With the aid of the proposed Max-Min rule, the optimal reflection coefficients to achieve the maximum sum ergodic capacity are derived. A simplified yet realistic setup of the reflection coefficients is then developed to balance the decoding performance of the two receivers, where the optimal reflection coefficients are shown to counteract the channel gain variation adaptively. Moreover, it is demonstrated that the ergodic capacity (EC) of the backscatter channel, under high signal-to-noise ratio (SNR) conditions, is primarily influenced by the number of STAR-RIS elements in the BD zone. Whereas the EC of the primary channel is determined by the ratio of STAR-RIS elements in the EP region to those in the BD region.