Radar sensing and communication aided by reconfigurable intelligent surfaces

As wireless communication systems evolve toward the sixth generation (6G), there is a growing need for reliable high-speed connectivity, accurate sensing, and intelligent network adaptability. Meeting these demands introduces several challenges, especially at higher frequency bands, where signal attenuation and limited coverage become significant obstacles. Reconfigurable Intelligent Surface (RIS) have emerged as a promising solution to address these limitations by enabling programmable control over the wireless propagation environment. By adjusting the phase, amplitude, and direction of electromagnetic waves, RIS can improve coverage, enhance link performance, and support new functionalities such as sensing and localization. This thesis investigates the design and application of advanced RIS architectures, with a focus on low-complexity systems that enhance sensing capabilities in 6G networks. Both active and passive RIS technologies are explored and evaluated in terms of their ability to support radar detection, target tracking, and communication tasks under practical constraints. The first part of the study focuses on a monostatic radar system assisted by an active RIS. The amplification provided by the RIS helps mitigate path loss in indirect propagation paths. A dual-path signal model is developed, and joint detection and localization are performed using a Generalized Likelihood Ratio Test (GLRT). Simulation results show clear improvements over passive RIS configurations. The second part presents a pulse-Doppler radar system using a Simultaneous Transmitting and Reflecting Reconfigurable Intelligent Surface (STAR-RIS). This architecture enables sensing in both reflective and transmissive directions using collocated receive antennas. Direction-dependent slow-time modulation is applied to distinguish echoes from each side, and a model selection method based on the Generalized Information Criterion (GIC) is used for target detection and velocity estimation. Both simultaneous and sequential scanning strategies are evaluated in terms of performance and complexity. The final part introduces a radar-centric Integrated Sensing And Communication (ISAC) transceiver, combining a passive STAR-RIS with a single channel radar receiver using a Passive Electronically Scanned Array (PESA). Through joint space-time coding and beamforming design, the system supports transmit sensing and receive data transmission while maintaining low implementation complexity and minimal interference between functions. These three contributions demonstrate how RIS technology, including active and STAR-RIS configurations, can significantly enhance the functionality, efficiency, and adaptability of future wireless systems. The proposed architectures offer practical and scalable solutions for next-generation networks, particularly in power- and hardware-constrained environments.