This book has presented a comprehensive investigation into advanced NOMA techniques for HCNs, offering both theoretical insights and practical perspectives. The study addressed critical challenges such as non-linear distortions in HPAs, i-SIC, and i-CSI, demonstrating that NOMA maintains significant performance advantages over OMA even under non-ideal conditions. Furthermore, the robustness of NOMA against impulsive noise was established, highlighting its suitability for industrial IoT applications and ultra-reliable low-latency communication scenarios. The integration of SWIPT with cooperative NOMA was explored, revealing substantial improvements in energy efficiency and performance for cell-edge users, while providing a new paradigm for interference exploitation in energy harvesting systems. In addition, the role of RISs, particularly active RIS, was analyzed as a transformative enabler for dynamically reconfiguring the propagation environment and enhancing spectral efficiency, coverage, and fairness among users. Analytical evaluations further underscored the potential of RIS to reshape the trade-offs between performance, power consumption, and scalability in next-generation networks.