Abstract
Non-orthogonal multiple access (NOMA) is a promising technology for next-generation wireless networks with emerging demands on low latency, high throughput, and massive connectivity. Unlike orthogonal multiple access, NOMA allows multiple users to share the same radio resources, which significantly improves spectral efficiency (SE). To achieve green wireless communications for numerous networked devices, NOMA helps reduce energy consumption while satisfying rate fairness and quality-of-experience requirements. The goal of this paper is to introduce the innovative approaches for NOMA in terms of the SE and energy efficiency, and discuss emerging technologies involved with NOMA. Further, its challenges and future research directions are highlighted.
Highlights
The rapid growth in massive connectivity, where billions of devices are connected to a dense network (i.e., Internet of Things (IoT)) with a high demand for quality-of-experience (QoE) [1], requires innovative technologies to overcome the scarcity of radio resources
We present the recent advances in improving the spectral efficiency (SE) and EE of the non-orthogonal multiple access (NOMA) systems, and we discuss user equipments (UEs)/devices association and cooperative NOMA that are based on cooperation among devices
We further describe the principle of NOMA when it is applied to UE groups
Summary
The rapid growth in massive connectivity, where billions of devices are connected to a dense network (i.e., Internet of Things (IoT)) with a high demand for quality-of-experience (QoE) [1], requires innovative technologies to overcome the scarcity of radio resources. (i) satisfying the QoE with a low-power consumption and improving energy efficiency (EE), and (ii) enabling to simultaneously serve multiple UEs while providing fairness among the UEs. Some previous surveys introduced the latest works on NOMA [9,10,11], in which different performance metrics were used to show the benefits of NOMA in combination with a wide range of technologies. Recent NOMA-integrated innovations for the SE and EE improvement need to be comprehensively exposed, including both the processing methods and system-level approaches based on emerging technologies These motivate us to conduct a review of NOMA and its advancements, which helps recognize the research challenges, promising solutions, and future vision targeted at enhancing the SE and EE for future wireless communications. The challenges of forthcoming networks are discussed, and we introduce potential research directions, including terahertz (THz) wave, intelligent reflecting surface (IRS), and learning-based methods, which may help to further improve the SE and EE
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