Physical Layer Techniques for the Protection of Positioning Signals and Communication Systems

Wireless communications have become critical for a variety of applications, including navigation, timing, and communication services. However, unlike wired communication, the wireless channel is inherently a broadcast medium, making it more susceptible to a variety of threats. Malicious users can easily disrupt communications through jamming attacks, intercept signals to access confidential information or identify the transmitter, or carry out spoofing attacks by either generating counterfeit signals or tampering with legitimate transmissions. Securing communication systems at the physical layer is becoming crucial, especially given the limitations of traditional cryptographic methods, which can be computationally expensive and vulnerable to emerging threats such as quantum computing. Physical layer authentication (PLA) techniques provide an alternative by leveraging the inherent physical characteristics of wireless signals to verify the authenticity of transmitted data. Additionally, PLA mechanisms offer unconditional security, as they do not rely on the computational hardness of some problems. This thesis examines PLA mechanisms in two prominent emerging contexts, namely global navigation satellite system (GNSS) and 5G cellular systems. In the context of GNSS, the research tackles the growing threat of spoofing attacks, where malicious entities transmit counterfeit signals to mislead GNSSs receivers into calculating incorrect positions or times. The thesis proposes a comprehensive model for analyzing these attacks and develops optimal defense strategies that enhance signal-level authentication, making GNSS systems more resilient to such threats. It also examines authentication cross-checks that leverage the combination of signals from multiple GNSS constellations, outlining both their advantages and limitations. Finally, a novel PLA technique is proposed to enhance existing spreading code authentication (SCA) mechanisms. In the context of cellular networks, we extend PLA techniques to 5G positioning services, investigating how the 5G positioning reference signal (5G PRS) can be exploited by attackers. Furthermore, this second part of the thesis explores hybrid systems that combine GNSS and 5G signals. It proposes a framework for the design of a multi-layer positioning and PVT assurance system that integrates both GNSS and 5G. Our research aims to improve the overall robustness of wireless communication systems, ensuring that critical services relying on accurate positioning and timing can withstand sophisticated attacks. Finally, a 5G communication system protected by a challenge response (CR)-PLA mechanism is developed, which utilizes a partially controllable channel enabled by the presence of an intelligent reflecting surface (IRS). Our work explores the trade-off between communication and security performance and introduces worst-case attack models across various scenarios, along with the optimal probability distribution for the random IRS configuration to mitigate these threats.