Communication by Sensing: A Coherent Backscatter Framework with Space–Time RIS

This paper proposes a sensing-centric framework for integrated sensing and communication (ISAC) based on coherent backscatter from a space-time reconfigurable intelligent surface (ST-RIS). The system enables uplink communication through the echoes of a standard sensing waveform, such as FMCW radar, without an active transmitter. By dynamically modulating the reflection phase, the ST-RIS embeds data symbols into the radar backscatter while preserving sensing functionality. A pilot-aided receiver estimates delay and Doppler to separate the sensing and communication components with minimal accuracy loss. Analytical bounds for range estimation (CRB) and data detection SNR are derived, and numerical results confirm reliable data recovery with high sensing precision. The proposed approach demonstrates the feasibility of RIS-enabled coherent backscatter for future low-power ISAC systems.