DESIGN AND ANALYSIS OF MULTIPLE ACCESS TECHNIQUES FOR NEXT-GENERATION WIRELESS SYSTEMS

Author

• Mayushi Amaya Jayasinghe, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2026

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

First Advisor

Aruma Baduge, Gayan Amarasuriya

Abstract

Next-generation wireless networks are expected to support unprecedented levels of connectivity, spectral efficiency, energy efficiency, and quality-of-service (QoS) to accommodate the rapid growth of mobile data traffic, Internet-of-Things (IoT) devices, machine-type communications (MTC), and emerging applications such as augmented reality, virtual reality, and integrated sensing and communication (ISAC). Conventional orthogonal multiple access (OMA) techniques face significant challenges in meeting these stringent requirements due to limited spectral resources and inefficient utilization of available bandwidth. Consequently, advanced multiple access techniques, including non-orthogonal multiple access (NOMA) and rate-splitting multiple access (RSMA), have emerged as promising solutions for beyond fifth generation (B5G) and sixth generation (6G) wireless systems. This dissertation investigates the design, analysis, and optimization of multiple access techniques for next-generation wireless communication systems, with a particular focus on OMA, NOMA, and RSMA in massive multiple-input multiple-output (MIMO), cell-free massive MIMO, and ISAC-enabled wireless networks. The dissertation begins with the performance analysis of downlink NOMA systems operating under practical wireless channel conditions. Fundamental performance metrics, including achievable rate, spectral efficiency, and energy efficiency, are derived and analyzed. The impacts of inter user/cluster interference, power allocation, channel estimation errors, and hardware impairments are quantified, providing valuable insights into the trade-offs between system complexity and performance gains. The study demonstrates that NOMA can significantly improve spectral efficiency, cater massive connectivity, and enhance user fairness compared to conventional OMA schemes, particularly in scenarios with heterogeneous channel conditions. Building upon these findings, the dissertation investigates RSMA as a flexible multiple access framework capable of bridging and generalizing OMA and NOMA. Analytical expressions and optimization frameworks are developed to evaluate the achievable rate performance of RSMA-enabled wireless networks. Resource allocation algorithms are proposed for optimizing transmit power and rate-splitting parameters, enabling efficient interference management while maintaining fairness among users. The results demonstrate that RSMA achieves superior rate performance and robustness to channel uncertainty compared to both OMA and NOMA, particularly in interference-limited environments. Additionally, the dissertation investigates the effects of practical impairments, including hardware non-idealities, imperfect CSI, spatial correlation, and user mobility, on the performance of multiple access techniques. Advanced optimization frameworks based on convex optimization, fractional programming, and federated learning are developed to address these challenges while maintaining scalable and energy-efficient network operation. Overall, this dissertation provides a comprehensive analytical framework and innovative optimization techniques for the design and performance evaluation of OMA-, NOMA-, and RSMA-aided wireless communication systems. The findings establish important theoretical insights and practical design guidelines for enabling reliable, spectrally efficient, and intelligent multiple access solutions in future B5G and 6G wireless networks.