Non-Orthogonal Multiple Access: Common Myths and Critical Questions

Abstract: Non-orthogonal multiple access (NOMA) has received tremendous attention for the design of radio access techniques for fifth generation (5G) wireless networks and beyond. The basic concept behind NOMA is to serve more than one user in the same resource block, e.g., a time slot, subcarrier, spreading code, or space. With this, NOMA promotes massive connectivity, lowers latency, improves user fairness and spectral efficiency, and increases reliability compared to orthogonal multiple access (OMA) techniques. While NOMA has gained significant attention from the communications community, it has also been subject to several widespread misunderstandings, such as $``\textit{NOMA is based on allocating higher power to users with worse channel conditions. As such, cell-edge users receive more power in NOMA and due to this biased power allocation toward cell-edge users inter-cell interference is more severe in NOMA compared to OMA. NOMA also compromises security for spectral efficiency.}''$ The above statements are actually false, and this paper aims at identifying such common myths about NOMA and clarifying why they are not true. We also pose critical questions that are important for the effective adoption of NOMA in 5G and beyond and identify promising research directions for NOMA, which will require intense investigation in the future.

• The paper critically examines common myths surrounding Non-Orthogonal Multiple Access (NOMA) in 5G and future networks, clarifying issues like power allocation, decoding order, and interference impact.
• It refutes misconceptions by explaining that NOMA's primary goal includes enhanced connectivity and fairness, not solely spectral efficiency, and discusses compatibility with techniques like FFR and complexity mitigation.
• The paper highlights critical research questions regarding NOMA's practical performance under non-ideal conditions, the potential role of machine learning, and the influence of various real-world constraints on its deployment.

Non-Orthogonal Multiple Access: Common Myths and Critical Questions

The paper "Non-Orthogonal Multiple Access: Common Myths and Critical Questions" tackles the crucial subject of non-orthogonal multiple access (NOMA) in the context of modern cellular systems, particularly its application in 5G networks and beyond. NOMA has been proposed as a promising solution to the challenges posed by conventional orthogonal multiple access (OMA) techniques, especially with the massive connectivity requirements of future wireless networks.

Overview of Non-Orthogonal Multiple Access

The core concept of NOMA is to allow multiple users to share the same resource block, such as time slots or frequency bands, by employing strategies like power-domain multiplexing. This approach differs fundamentally from OMA techniques, which allocate orthogonal resources to each user to avoid interference. Distinct from the orthogonal principles of previous generations, NOMA seeks to enhance spectral efficiency, user fairness, and reduce latency while supporting a massive number of devices characterized by diverse data rates and latency needs.

Addressing Myths Surrounding NOMA

The paper systematically addresses several prevalent misconceptions about NOMA:

1. Power Allocation Myth: It clarifies that NOMA does not inherently require higher power allocation to users with weaker channels. Power allocation is context-dependent and should be adapted based on the specific rates targeted within the achievable capacity region.
2. Decoding Order: Contrary to some beliefs, the decoding order in successive interference cancellation (SIC) does not change with varying power allocation. Rather, it depends on the relative channel conditions.
3. Interference Impact: The misunderstanding that NOMA inherently leads to worse interference conditions for weaker users is refuted. While the weaker users do treat other users’ signals as interference, efficient NOMA design considers this in the power allocation process.
4. Spectral Efficiency Premise: The paper argues that the primary motivation for NOMA is not merely spectral efficiency but also significant enhancements in connectivity and fairness.
5. Inter-cell Interference (ICI): The concern that NOMA increases ICI due to biased power control is alleviated by explaining that the total transmission power remains constant, whether OMA or NOMA is used.
6. FFR Compatibility: The compatibility of NOMA with fractional frequency reuse (FFR) is explored, explaining how NOMA can be integrated into FFR frameworks effectively.
7. Complexity and Errors: The complexity of implementation in user equipment (UEs) and issues with error propagation are recognized, but advancements in UE capability and interference management strategies are showed to mitigate these concerns.

Future Implications and Research Directions

The implications of successfully implementing NOMA are profound. It opens pathways to enhance the throughput and performance of future wireless networks considerably. However, the paper points to several critical questions that need addressing to unlock the full potential of NOMA:

• Understanding the practical benefits of NOMA under realistic conditions with imperfect SIC and without ideal channel state information (CSI).
• The role of machine learning and deep learning in optimizing user clustering, power allocation, and decoding strategies in NOMA deployments.
• The impact of various real-world constraints and conditions influencing the performance and efficiency of NOMA systems.

Given the challenges and opportunities outlined, future research has ample fertile ground to explore innovations in NOMA design, leveraging advanced signal processing and machine learning techniques to overcome practical hurdles. The results could be pivotal for the evolution of next-generation, highly connected, and efficient wireless networks.