User Detection in RIS-based mmWave JCAS: Concept and Demonstration

In this article, joint communication and sensing (JCAS) is combined with reconfigurable intelligent surfaces (RISs), which validates a novel RIS-based JCAS (RIS-JCAS) system.The proposed system performs monostatic sensing to detect a potential user, after which communication with the user is established.The corresponding RIS-cascaded channel is formulated, based on which an algorithm is created, to handle radar target detection and communication channel estimation while multiple RISs operate simultaneously.A joint beam training process by utilizing the identical beamforming pattern on multiple RISs is created, which increases the time efficiency significantly.The detection accuracy is subsequently further improved through a novel fine-tuning method.To practically validate the aforementioned algorithm, a RIS-JCAS system is demonstrated using a mmWave testbed with a bandwidth of 1 GHz centralized at 26.2 GHz.Besides combining the widely adopted orthogonal frequency-division multiplexing (OFDM) scheme with high-directional RIS beams, the system also allows the use of variant RIS flat-top beam shaping combined with orthogonal chirp-division multiplexing radar (OCDM) waveforms.This idea keeps the sensitivity of the RIS-JCAS system while broader beams with wider coverage but lower gain are utilized.Overall, accurate 4-dimensional sensing results are derived, which strongly assists the communication channel estimation and realizes the core benefits of the proposed RIS-JCAS system.