Localization and Sensing Through Meta-surface Structures

In light of future 6G wireless networks, the integration of sensing and communication (ISAC) has emerged as a roundbreaking innovation, motivating the development of a wide variety of location-based applications. The demand for spatially aware networks has sparked great interest, both from the academia and the industry. Unlike past and current wireless networks that strongly prioritized the communication performance, future wireless networks are envisioned as integrated infrastructure, where sensing, localization and communication tasks coexist, sharing network resources and hardware. ISAC transforms the traditional network design, necessitating novel algorithmic approaches that support the practical implementation of localization solutions. A key challenge in spatially aware networks is the ability to detect, locate and/or track mobile terminals, within the network. This network quality envelops sensing capabilities for non-cooperative or non-connected devices within the network’s topography. Such capacity requires a robust system design, as it is directly affected by the quality of the propagation links among the devices. Additionally, within the ISAC framework, radar functionalities are facilitated under shared spectrum with conventional communication tasks, thereby improving the spectral efficiency in the expansion of mobile and sensor networks. In parallel, Reconfigurable Intelligent Surfaces (RISs) are being extensively studied as fundamental pieces of the evolving 6G ecosystem. RISs are planar structures with non-conventional reflective properties, often viewed as the evolutionary step of massive MIMO technology. RISs appear as promising hardware developments that shall revolutionize the structure of wireless networks. In particular, the reconfigurability of RISs enables for a dynamically adjustable propagation environment, in real-time. This adaptability not only enhances network performance under varying conditions but also allows RISs to achieve data rates comparable to those of traditional multi-antenna arrays—albeit with significantly reduced energy requirements. These unique capabilities position RISs as one of the most promising candidate technologies for future wireless networks, enabling high-quality service delivery with a low energy footprint. This thesis explores the integration of RISs into ISAC networks, particularly for advanced localization purposes in 6G environments. By leveraging the steady development of the RIS technology, 6G networks can achieve unprecedented localization accuracy. However, new challenges emerge in detection and estimation, since appropriate signal processing frameworks need to be developed for RIS-aided localization and sensing. To address these challenges, this research proposes innovative design strategies, to enhance the detection reliability and accuracy of the considered systems. The aim is to establish RIS-aided networks capable of performing communication, localization, and tracking functions within a unified hardware and software infrastructure, targeting sub-6 GHz and sub-millimeter wave frequency bands. In particular, the problem of estimating the position and orientation of a mobile station (MS) is investigated. The considered downlink localization scheme relies on the pilot symbols emitted by the serving base station (BS), which are received via a direct link and an indirect link provided by an RIS. To counterbalance the multiplicative pathloss in the indirect link, an active RIS is employed, which is able to reflect and amplify the incident signal. The maximum-likelihood estimators of the MS position and orientation are derived along with the corresponding Cramér Rao Lower Bounds under three levels of system cognition at the MS. Suboptimal solutions are also proposed. A numerical analysis shows the merits of the proposed estimators, highlighting the achievable gains granted by the use of an active RIS (as compared to a passive one), and investigates the impact of the main system parameters, including the BS-RIS distance and the amplification gain at the RIS. In addition, the problem of detecting active user equipment devices (UEs) and localization in the near field is studied, wherein a BS is unaware of the number of active (or inactive) UEs and their positions. The considered system is equipped with multiple RISs, which provide a lowcomplexity solution for detection and localization with additional degrees of freedom due to the additional inspection points. Specifically, an iterative detection procedure is proposed, allowing the BS to assign pilots to detected UEs, and thereby providing a structured channel access. Also, the problem of multiple access interference is explored as a limiting performance factor. The results show that, with a proper configuration of the RISs, the proposed scheme can detect/localize UEs, augmenting benchmark detection schemes to spatially aware UE detection. Finally, the problem of radar-like detection of small non-cooperative airborne targets is explored within the RIS-aided framework. RISs provide an alternative point of inspection, better suited for the considered area. The presence of communicating users leads to beamforming restrictions, such that the interference of the sensing application is minimized. Since the energy of the received echoes drastically depends on the RIS’s beampattern, the aforementioned constrains lead to a low signal-to-noise ratio (SNR) sensing scenario. Track-before-detect (TBD) is a scheme that increases the probability of detection as more inspections are jointly considered for a target to be detected. Numerical results show that a TBD procedure is vital in complex ISAC scenarios, achieving remarkable sensing reliability with minimal degradation of the communication performance.