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NOMA in 5G Systems: Exciting Possibilities for Enhancing Spectral Efficiency
S. M. Riazul Islam, Ming Zeng, Octavia A. Dobre
TL;DR
Power-domain NOMA is considered for 5G because it serves multiple users on the same time and frequency resources. This article surveys its concepts, solutions, and standardization alongside its limitations and research challenges. It concludes that NOMA is promising for spectral efficiency, while practical use requires addressing issues including error propagation and multi-cell interference.
Problem
Method
The article surveys NOMA concepts and benefits, existing solutions, standardization activity, limitations, and research challenges.
Source: §1 · p. 1, §3 · pp. 2–3, §4 · p. 3Results
The article concludes that NOMA’s spectral efficiency is highly promising for 5G radio access and that its potential extends to MIMO, millimeter-wave, and visible-light systems.
Source: §7 · p. 5Takeaways & Limitations
Making NOMA more practical requires addressing limitations including error propagation and inter-cell interference in multi-cell networks.
Source: §7 · p. 5NOMA adds receiver complexity and energy consumption, risks SIC error propagation, and requires considerable channel-gain differences for its claimed power-domain benefits.
Source: §5 · p. 4
Abstract
from arXiv · showhide
This article provides an overview of power-domain non-orthogonal multiple access for 5G systems. The basic concepts and benefits are briefly presented, along with current solutions and standardization activities. In addition, limitations and research challenges are discussed.
1. Introduction
Power-domain NOMA serves multiple users on the same time and frequency resources by multiplexing them in the power domain.
1. Introduction
- Power-domain NOMA multiplexes users in the power domain, whereas code-domain NOMA multiplexes them in the code domain.The article focuses on power-domain NOMA and henceforth calls it NOMA.Source: §1 · p. 1
- NOMA superposes users’ signals at the transmitter and uses successive interference cancellation (SIC) at the receiver.Source: §1 · pp. 1–2
- The strong user first cancels the weak user’s signal, while the weak user treats the strong user’s signal as noise and receives more power for fairness.Source: §1 · p. 2
2. Benefits of NOMA
NOMA’s simultaneous use of time and frequency resources is associated with spectral-efficiency, connectivity, latency, and fairness benefits.
2. Benefits of NOMA
- Serving multiple users on the same time and frequency resources improves spectral efficiency through SIC-based interference mitigation.Source: §2 · p. 2
- Simultaneous service increases the number of users supported and can support massive connectivity.Source: §2 · p. 2
- Because users transmit simultaneously rather than waiting for a scheduled time slot, NOMA offers lower latency.Source: §2 · p. 2
- Flexible power control between strong and weak users supports fairness and diverse quality of service, with more power for weak users improving cell-edge throughput.Source: §2 · p. 2
3. Existing NOMA Solutions
Existing NOMA solutions address power allocation and fairness in SISO systems, extend NOMA to MIMO and multi-cell settings, and explore other communication systems.
3. Existing NOMA Solutions
- SISO work treats power allocation and fairness jointly, since maximizing sum rate alone assigns all power to the strong user and offers no gain over OMA.Dynamic allocation in guarantees both users higher individual rates than OMA; studies max-min data rate and min-max outage probability with optimal low-complexity polynomial algorithms.Source: §3 · p. 2
- MIMO-NOMA clusters users, with random pairing and a channel-based greedy algorithm [8] proposed to reduce the burden of exhaustive pairing.Users in a cluster share a precoding vector, creating parallel SISO channels; the cited work reports NOMA’s superiority over OMA.Source: §3 · pp. 2–3
- MIMO-NOMA approaches include zero-forcing and signal-alignment methods assuming perfect instantaneous CSI, and power allocation using statistical CSI.The perfect instantaneous CSI assumption may be impractical for MIMO scenarios; work also considers massive MIMO with limited feedback.Source: §3 · p. 3
- Multi-cell NOMA must contend with inter-cell interference, addressed in a two-cell MIMO-NOMA setting by coordinated beamforming.Source: §3 · p. 3
- NOMA is also being investigated for millimeter-wave and visible-light communications [14].Source: §3 · p. 3
4. Industry Trends and Standardization Status
Industry activity around NOMA spans 5G white papers, joint research, and 3GPP study items across LTE Release 13 and 5G NR Releases 14 and 15.
4. Industry Trends and Standardization Status
- NOMA appeared in 5G white papers from ZTE, SK Telecom, and METIS, and DOCOMO and MediaTek were researching a combination of NOMA and multi-user interference cancellation.Source: §4 · p. 3
- 3GPP included downlink multi-user superposition transmission in the LTE Release 13 study item.Source: §4 · p. 3
- The Release 14 5G NR study recognized uplink NOMA as a consideration, especially for massive machine-type communications, and study continued in Release 15.Source: §4 · p. 3
5. Limitations of NOMA
NOMA’s implementation faces receiver, error-propagation, pairing, and channel-information constraints. These limitations qualify the benefits of power-domain multiplexing.
5. Limitations of NOMA
- SIC requires each user to decode information from all same-cluster users with worse channel gains before its own, increasing receiver complexity and energy use versus OMA.The passage identifies this decoding burden as an implementation limitation.Source: §5 · p. 4
- An SIC error can make subsequent decoding of other users’ information erroneous, motivating reasonably small user clusters.The passage identifies limiting cluster size as a way to reduce the effect of error propagation.Source: §5 · p. 4
- Power-domain multiplexing requires considerable channel-gain differences between strong and weak users, restricting effective user pairs and reducing NOMA’s sum-rate gain.Source: §5 · p. 4
- Users must report channel-gain information to the base station, and NOMA is inherently sensitive to measurement uncertainty.Source: §5 · p. 4
6. Research Challenges
The article identifies overcoming NOMA’s limitations and addressing open issues as research challenges. These include interference, carrier aggregation, resource allocation, integration with other technologies, and physical-layer security.
6. Research Challenges
- As base-station and device density rises, inter-cell interference may become severe, especially challenging to mitigate in heterogeneous networks.The article calls for combining interference cancellation and management approaches with NOMA.Source: §6 · p. 4
- The suitable carrier-aggregation type for NOMA remains undetermined, although integrating carrier aggregation can further increase targeted users’ data rates.Source: §6 · p. 4
- Low-complexity resource allocation algorithms are important for NOMA performance optimization, including user pairing, power allocation, and subband assignment.Source: §6 · p. 4
- Further work is needed on NOMA solutions for massive MIMO and mmWave networks; [18] presents an initial study in this direction.Source: §6 · p. 4
- Physical-layer security for NOMA remains an open research direction, investigated in large-scale networks in.Source: §6 · p. 4
7. Conclusion
The article concludes that NOMA’s spectral-efficiency potential is promising for 5G radio access and spans multiple communication settings. Making it more practical requires addressing limitations including error propagation and multi-cell interference.
7. Conclusion
- NOMA’s superior spectral efficiency is presented as highly promising for 5G radio access.Source: §7 · p. 5
- NOMA research includes resource allocation and fairness, and its capacity can be further increased by applying it in MIMO systems.Source: §7 · p. 5
- NOMA applications extend to mmWave and visible-light communication systems, while practical deployment requires overcoming error propagation and multi-cell interference.Source: §7 · p. 5