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In the context of future ultra-dense mobile networks, spectrum and energy efficiencies (SE and EE) are critical measures in designing efficient systems for the sixth-generation (6G) of wireless networks. Recognized for their benefits in increasing SE and EE, non-orthogonal multiple access (NOMA) and device-to-device (D2D) communications are combined in this work to present a new NOMA-based D2D scheme increasing the performance in terms of SE and EE. The users in the proposed scheme are split into coalitions. Coalition heads are served in NOMA directly from the base stations, while the other users within the coalitions get the service through D2D links. We investigate the system’s SE and EE for different mobility patterns, and we discuss optimal system configurations with the help of Monte Carlo simulations. The obtained results show that the proposed system exhibits a better performance compared to conventional OMA and NOMA models, especially in low mobility contexts.