Passive Reflection Techniques for Wireless Communications

Demand for wireless connectivity is increasing exponentially, with billions of devices expected to be connected in the coming few years. Traditional techniques used to support the increasing number of devices and associated traffic has caused the complexity and energy consumption of wireless networks to increase substantially over the years. Simple, energy-efficient technologies are needed to sustain the increasing demand for wireless connectivity. Use cases in future networks will range from very low-rate short-range services, e.g., internet of things (IoT) sensors, to high-rate broadband services, e.g., virtual reality (VR) headsets. Passive reflection radio techniques offer energy-efficient solutions for both use cases. For low-rate communications, ambient backscatter allows the devices to piggyback existing RF transmission, while for high-rate communications, reconfigurable intelligent surfaces (RIS) can be used to control the propagation to support higher rates between devices. In this dissertation, we propose passive reflection techniques for both use-cases and analyze their performance. In the first part of this dissertation, we propose two novel techniques for ambient backscatter over OFDM carriers. In Chapter 2, we propose an energy-detection based ambient backscatter modulation scheme over OFDM signals, investigate its detector design, and analyze its error performance. In Chapter 3, we propose a frequency shift keying scheme over ambient OFDM signals, investigate its transceiver design, and analyze its error performance. Both techniques allow noncoherent detection and exploit the null subcarriers to alleviate direct link interference. In the second part of this dissertation, we propose three techniques for exploiting RISs in various communications scenarios. In Chapter 4, we investigate the use of RISs to facilitate orthogonal spatial multiplexing in uplink multi-user scenarios and propose a design criterion that achieves this goal. In Chapter 5, we investigate the use of RISs to allow simultaneous communications between multiple user pairs in interference channels and propose a Riemannian manifold optimization approach to solve the problem of configuring the RIS passive reflection coefficients to minimize the total interference under constant modulus constraints. Finally, in Chapter 6, we investigate the use of RISs to maximize the dirty-paper coding rate of an RIS-assisted broadcast channel and propose an alternating optimization technique to optimize both the transmit covariances and the RIS reflections coefficients.