PhD Oral Exam - Shreya Khisa, Information and Systems 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 significant increase in wireless traffic and the growing demand for high-speed data transmission have sparked considerable interest in innovative solutions aimed at advancing the upcoming phase of wireless communication, often referred to as the sixth-generation, (6G). In the evolution of 6G, it is imperative to address the escalating need for extremely reliable and low-latency communication (eURLLC), improved mobile broadband (eMBB), and ultra-massive machine-type communication (umMTC). To combat the spectrum crunch and satisfy the rigorous demands of surging broadband usage, the several revolutionary architectures and enabling technologies such as reconfigurable intelligent surface (RIS), next-generation multiple access (NGMA), integrated sensing and communication (ISAC), non-terrestrial network (NTN), full-duplex (FD) communications, movable antenna system (MAS) and connected intelligence are promised to achieve the vision and goals of future networks. Rate-splitting multiple access (RSMA), a member of the NGMA family, has been deemed as a key enabler of multiple access technique for the 6G networks and beyond because of its ability to provide high spectral efficiency, low latency, and massive connectivity. RSMA offers flexible interference management by partially treating the multi-user interference as noise and partially decoding it. This flexible nature assists RSMA in bridging the gap between space-division multiple access (SDMA), which fully treats multi-user interference as noise, and non-orthogonal multiple access (NOMA), which fully decodes the interference.

With the aim of converging towards NGMA, in this thesis, we investigate the interplay between RSMA and other emerging next-generation technologies from the perspective of resource allocation optimization. In particular, our analysis focuses on enhancing the performance of cell-edge users (CEUs) in both downlink and uplink, developing energy-efficient multiple access schemes to support sustainable green communications, and improving the performance of FD communications with the help of advanced technologies such as movable antennas and alleviating the adverse impact of inter-cell interference (ICI). In this regard, as a first contribution of this thesis, in order to improve the signal quality of CEUs and increase the user fairness, we introduce cooperative relaying with the aid of user cooperation from cell-center users (CCU)s, which is called cooperative RSMA (C-RSMA). In doing so, we investigate resource management optimization problem (transmitting beamforming vectors, common stream split ratio, and user power) by proposing a low complexity algorithm. Furthermore, we extend this work by integrating RIS with user cooperation. RIS has been envisioned as a promising technology that can control the wireless channel propagation environment. In this aspect, we propose an optimization framework where we jointly optimize transmitting beamforming vectors, RIS phase shift, common split ratio, and user relaying power. Our results demonstrate that RSMA- and C-RSMA-based strategies exhibit stronger resilience under challenging conditions, including limited BS power budget, residual self-interference (SI) in FD relaying, and significant channel strength disparities. Moreover, we also show that the RIS location plays a pivotal role in achieving lower energy consumption. Realizing the enormous benefits that cooperation and RSMA can bring, we study the performance of C-RSMA also in a multi-cell scenario with the goal of reducing ICI and improving signal quality at the CEUs. In this study, we have found that C-RSMA can tackle ICI better than RSMA, NOMA, and C-NOMA due to RSMA's inherent interference management.

Next, we focus on exploring the potential benefits of C-RSMA in uplink communication. Since the message splitting strategy in uplink RSMA fundamentally differs from that in downlink RSMA, and given the significant literature gap in this area, this study is motivated to address this issue. First, we study a two-user case where the message of CEU is piggybacked by the CCU to the BS to maximize the signal-to-interference-plus-noise-ratio (SINR) at the BS. We look into the resource allocation optimization problem by jointly optimizing receiving beamforming vectors at the BS, user transmission power, while respecting their power budget constraints, by proposing a low-complexity algorithm. Afterward, we extend this model to a multi-user scenario, which is more realistic and practical. In such a framework, we propose a user pairing policy in order to facilitate user cooperation. We investigate different message decoding strategies and also different message splitting strategies. Our results show that different decoding and splitting strategy plays a vital role in achieving a higher data rate in the uplink.

Recently, MAS technology has been receiving a lot of attention due to its flexibility in moving the antenna in the XYZ plane, which helps to reduce the level of interference to some extent. Recognizing this as a motivational fact, we study the potential of MAS in mitigating the residual SI and inter-user/intra-cell interference in the FD-BS scenario. Specifically, we exploit how MAS can benefit in mitigating the interference level in a scenario where the BS is transmitting and receiving at the same time using the same spectrum. Our results reveal that having MAS at the BS is more beneficial in tackling the residual SI level than the fixed position antennas (FPA)s. Another significant finding is that in an FD-BS system, residual SI not only impacts the uplink communication rate, but it also highly impacts the downlink transmission rate.