PhD Oral Exam - Zahra Taheri, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The global drive toward next-generation wireless networks has placed spectral efficiency, interference resilience, security, and sustainability at the forefront of physical-layer design. This thesis addresses these imperatives through the theory and design of Non-Reciprocal Reconfigurable Intelligent Surfaces (NR-RIS) for full-duplex (FD) wireless communications. Full-duplex communication enables simultaneous transmission and reception on the same frequency band, theoretically doubling spectral efficiency. Its deployment is constrained by self-interference, where the transmitted signal can exceed the desired signal by orders of magnitude. Conventional reciprocal Reconfigurable Intelligent Surfaces (RIS) apply identical uplink and downlink responses due to the Lorentz reciprocity theorem, limiting interference suppression freedom. This thesis investigates NR-RIS as an enabler for interference-resilient FD communications. By incorporating active or time-modulated elements that break electromagnetic reciprocity, an NR-RIS independently configures its response for each propagation direction. Four contributions are presented. First, a channel modeling framework derives a path loss model, independent phase shift matrices, and closed-form SINR expressions for dual-RIS FD systems. Second, a throughput maximization framework introduces the Effective Rank Criterion and Maximum Singular Value Criterion for multi-user MISO FD systems, solved via a BFGS algorithm with provable convergence. Third, a secrecy-aware alternating optimization framework based on Simultaneous Perturbation Stochastic Approximation jointly optimizes RIS phase configurations and artificial noise. Fourth, a sub-RIS on/off mechanism achieves 27% energy efficiency improvement, and the role of NR-RIS in net-zero 6G is analyzed. These contributions establish NR-RIS as a versatile technology advancing spectral efficiency, interference management, security, and sustainability.