- The paper demonstrates that NOMA significantly enhances spectral efficiency and user connectivity by enabling simultaneous access for multiple users using advanced power allocation techniques.
- It employs extensive simulation and comparative analysis to showcase the benefits of power-domain, multi-carrier, and MIMO-NOMA schemes over traditional OMA methods.
- The survey identifies key research challenges such as CSI imperfections and resource allocation complexities while outlining promising future trends in 5G and next-generation networks.
A Survey on Non-Orthogonal Multiple Access for 5G Networks: Research Challenges and Future Trends
Abstract
This paper provides a comprehensive survey on Non-Orthogonal Multiple Access (NOMA), a pivotal technology in the field of fifth-generation (5G) wireless networks aimed at meeting diverse requirements such as low latency, high reliability, massive connectivity, improved fairness, and high throughput. The central premise of NOMA is to facilitate simultaneous service for multiple users within the same resource block -- this can include time slots, subcarriers, or spreading codes. This survey meticulously examines the latest NOMA research and its application prospects, juxtaposes it with existing literature, and outlines future research challenges for NOMA in 5G and future wireless networks.
Introduction
NOMA represents a significant evolution in multiple access techniques for 5G networks, transcending traditional Orthogonal Multiple Access (OMA) methods like Time Division Multiple Access (TDMA) and Orthogonal Frequency Division Multiple Access (OFDMA). Unlike OMA techniques, which allocate unique resource blocks to individual users, NOMA serves multiple users within the same orthogonal resource block by leveraging power domain considerations and coding differences. This paradigm shift is driven by the necessity to optimize spectrum efficiency and accommodate a growing user base.
Single-Carrier NOMA
The implementation of NOMA over a single orthogonal resource block, often referred to as power-domain NOMA, is critical to its spectral efficiency. Power-domain NOMA differentiates users by allocating distinct power levels, ensuring that users with poor channel conditions are still serviced while maintaining higher overall system throughput compared to OMA.
Power-domain NOMA
Power-domain NOMA pairs users and allocates power asymmetrically to optimize system capacity. The stronger user, characterized by better channel conditions, decodes its signal through Successive Interference Cancellation (SIC) after the weaker user's signal has been decoded. The key insight here is that NOMA significantly enhances system performance when users' channel conditions differ greatly. Numerical simulations demonstrate that NOMA can enhance sum-rate performance even in high Signal-to-Noise Ratio (SNR) scenarios.
Cognitive Radio Inspired NOMA (CR-NOMA)
CR-NOMA addresses user requirements by integrating principles of cognitive radio where users' Quality of Service (QoS) targets dictate power distribution. This approach adapts the power allocation to meet stringent QoS constraints, ensuring fairness and efficiency. The application of CR-NOMA in scenarios with diverse data rate requirements, such as IoT and broadband users, is particularly effective.
Multi-Carrier NOMA
Given the foundational role of OFDMA in modern wireless communications, the merger of NOMA onto multiple carriers (subcarriers) is of paramount interest.
General Principles
Multi-carrier NOMA can be viewed as a hybrid form where users are grouped and served on shared subcarriers while different groups are allocated distinct subcarriers. This facilitates massive connectivity and reduces complexity by leveraging group-based user management.
Specific Schemes: LDS, SCMA, and PDMA
Sparse Code Multiple Access (SCMA) and Pattern Division Multiple Access (PDMA) represent multi-carrier NOMA schemes that distribute user codes across fewer subcarriers to ensure manageability and alleviate the burden on system resources. These paradigms underscore the adaptability of NOMA schemes to diverse system configurations.
MIMO-NOMA
Combining NOMA with Multiple-Input Multiple-Output (MIMO) systems is essential for leveraging spatial dimensions and maximizing throughput in 5G networks. Several methods including quasi-degradation, beamforming, and signal alignment have been developed to implement MIMO-NOMA effectively.
Cooperative NOMA
Cooperation among NOMA users introduces significant benefits in terms of coverage and reliability. Utilizing strong users as relays for weaker ones, and incorporating dedicated relays, enhances system performance by improving signal quality and extending reach. Additionally, full-duplex transmission can drastically augment spectral efficiency.
mmWave-NOMA
The integration of millimeter-wave (mmWave) technology with NOMA can potentially manage the expansive bandwidth in mmWave bands for high data rate transmissions. Considering the high directionality of mmWave channels, NOMA helps efficiently manage user grouping based on correlated channel states, thus optimizing beamforming and utilization.
Practical Implementation Challenges
Coding and Modulation
Advanced coding and modulation techniques are vital for NOMA to achieve practical gains. Techniques like LPMA, which use lattice coding, and PAM with turbo codes, effectively enhance robustness against interference.
Imperfect CSI
Addressing channel state information (CSI) imperfections is crucial. Schemes for dealing with channel estimation errors, partial CSI, and limited feedback mechanisms are critical to ensure robust NOMA performance under practical constraints.
Resource Allocation
Efficient resource allocation integrating power allocation, beamforming, user clustering, and subcarrier assignment remains a complex yet essential aspect for operationalizing NOMA in heterogeneous 5G environments.
Future Research Directions
The future research avenues for NOMA include its combination with SWIPT for energy-efficient communications, the integration with cognitive radio paradigms, focusing on security to address new potential vulnerabilities, its application to evolving 5G settings and beyond, including heterogeneous networks, and the development of NOMA for emerging areas like VLC and integrated satellite communications.
Conclusion
NOMA stands as a crucial innovation in the pursuit of meeting 5G’s rigorous performance targets. The extensive studies and their promising results underscore NOMA's potential for significantly enhanced system throughput, connectivity, and low latency. Ongoing and future research efforts are expected to continue refining its applications and exploring new horizons beyond the fifth generation of wireless networks.