Active RIS-Empowered Backscatter Communication Based on Single-Carrier Frequency-Domain Equalization

Backscatter communication (BC) is a promising technology to support massive Internet-of-Things (IoT) due to its low cost and high energy efficiency. To compensate for the severe double and multipath fading effects, in this paper, we consider an active reconfigurable intelligent surface (RIS)-empowered BC system, where the active RIS acts as an IoT device to communicate with the receiver (Rx). Thanks to the capability of signal amplification, the active RIS modulates its transmitted symbols over the incident single-tone signal through enhanced reflections. In addition, single-carrier frequency-domain equalization (SC-FDE) with cyclic delay diversity (CDD) technique is adopted for the active RIS transmission to further improve the reliability. During each RIS symbol block, reflecting elements (REs) are employed by the active RIS, each of which reflects a cyclically delayed version of the original symbol block. The FDE is used at the Rx to recover the RIS information, which obtains a diversity gain provided by the multiple REs. Based on the derived matched filter bound (MFB), we specifically design the system configurations, including the location of the active RIS and the power splitting factor between the active RIS and the Tx, to achieve the maximum received signal-to-noise ratio (SNR). Simulation results demonstrate that our proposed design enhances the BER performance with an increased diversity gain, and the optimal RIS position and power splitting factor derived by our analysis closely match the actual optimal values.