Novel applications in ris identification and user localization for next-generation wireless systems

Reconfigurable intelligent surface (RIS)-empowered communication is one of the promising physical layer technologies for the next generation wireless networks due to its unprecedented ability to dynamically shape the wireless propagation environment. The current RIS literature assumes that the availability of base station (BS)-RIS and user equipment (UE)-RIS links is always guaranteed as a given in the system design. However, due to the mobility of UEs and/or obstacles, these links can appear/disappear instantly. Therefore, the BS needs first to be aware of over which RIS or RISs the UE has the UE-RIS-BS link available before it can start optimizing its phase shifts to serve that UE. Chapters 3 and 4 propose two novel methods to address this critical need and improve resource allocation in RIS-assisted systems. In Chapter 3, we first introduce a novel system-level problem where the BS aims to detect and uniquely identify RISs that are reachable by a specific UE (UE-RIS-BS link is available). Next, to tackle this problem, we propose a novel RIS detection and identification (RIS-ID) scheme that enables the BS to pair UEs with their corresponding reachable RISs in a given time slot without requiring hardware modifications at the RIS, UE or BS side. Specifically, RIS-ID scheme employs binary phase shift keying (BPSK) modulation by exploiting phase changes at the RIS. Reachable RISs encode their unique phase shift reflection patterns (PSRPs) onto the unmodulated signal transmitted from the UE. The base station subsequently evaluates the correlation between the received signal and the possible reachable RISs' PSRPs to determine if the corresponding RIS is reachable by the UE. To evaluate the proposed RIS-ID scheme, we analytically derive the false and miss-detection probabilities as the main performance metrics. These probabilities are verified through computer simulations, demonstrating the effectiveness of the proposed RIS-ID scheme across various operating scenarios. In Chapter 4, we propose a new and simpler modulation method for the RIS-ID scheme. In this method, reachable RISs modulate the signal hitting their surface by changing the amplitude of the reflected signals, exploiting the amplitude-phase correlation in RIS elements. The proposed method is validated through a real-world experimental setup with different operating scenarios and system configurations. Conventional fingerprinting localization techniques often rely on measurements from multiple access points (APs), increasing deployment costs and infrastructure requirements. RISs have recently been proposed as a solution to enhance fingerprint diversity, enabling reliable localization with fewer APs. However, many existing RIS-based methods depend on phase shift optimization, which limits their real-world applicability. To address this limitation, in Chapter 5, we propose a novel RIS-assisted fingerprint-based localization method that avoids the need for both multiple APs and RIS phase shift optimization. In the proposed method, multiple RISs reflect the unmodulated signal transmitted by a radio frequency (RF) source using their unique PSRPs. These modulated reflections enable the UE to extract fingerprint features by correlating the received signal with the known PSRPs of the RISs. During the offline phase, a fingerprint database is constructed by recording the correlation metrics corresponding to all RISs at predefined reference points. In the online phase, the UE computes correlation metrics at its unknown location, compares them with the fingerprint database, and uses the weighted k-nearest neighbor (KNN) algorithm to estimate its position. Computer simulation results confirm that the proposed method can achieve submeter-level localization accuracy even under moderate transmission power conditions.