Evolution of Non-Terrestrial Networks From 5G to 6G: A Survey (2107.06881v2)

Abstract: Non-terrestrial networks (NTNs) traditionally have certain limited applications. However, the recent technological advancements and manufacturing cost reduction opened up myriad applications of NTNs for 5G and beyond networks, especially when integrated into terrestrial networks (TNs). This article comprehensively surveys the evolution of NTNs highlighting their relevance to 5G networks and essentially, how it will play a pivotal role in the development of 6G ecosystem. We discuss important features of NTNs integration into TNs and the synergies by delving into the new range of services and use cases, various architectures, technological enablers, and higher layer aspects pertinent to NTNs integration. Moreover, we review the corresponding challenges arising from the technical peculiarities and the new approaches being adopted to develop efficient integrated ground-air-space (GAS) networks. Our survey further includes the major progress and outcomes from academic research as well as industrial efforts representing the main industrial trends, field trials, and prototyping towards the 6G networks.

Overview of Non-Terrestrial Networks from 5G to 6G

The paper "Evolution of Non-Terrestrial Networks from 5G to 6G: A Survey" provides a comprehensive examination of the development of Non-Terrestrial Networks (NTNs) and their integration with terrestrial networks as we transition from 5G towards 6G. NTNs encompass satellite systems, unmanned aerial vehicles (UAVs), and high altitude platforms (HAPs), which traditionally have been used for specific applications such as disaster management and remote sensing. However, with advancements in aerial and space technologies and a reduction in manufacturing costs, NTNs are poised to play a more pivotal role in global communications, particularly with the advent of 6G.

Key Contributions

The survey addresses the evolution of NTNs in several contexts:

1. Integration with 5G Ecosystems: The paper explores the roles of NTNs within the existing 5G ecosystem. It highlights that due to their ability to provide pervasive and high-capacity connectivity, NTNs help address the limitations of terrestrial networks, especially in providing coverage for underserved regions such as rural areas, oceans, and during network outages.
2. Operation in mmWave and Emerging Technologies: The paper examines the operation of NTNs in the mmWave spectrum. It discusses the architectural challenges, resource allocation issues, performance evaluations, and interference management strategies pertinent to NTNs working at higher frequency bands. The use of mmWave holds the potential for NTNs to deliver broadband services, yet also presents unique challenges regarding propagation and Doppler effects.
3. Role in IoT Networks: NTNs offer solutions to connectivity challenges in IoT deployments, where terrestrial infrastructure is insufficient. The integration of NTNs in IoT ecosystems allows for applications such as data collection, localization, and wireless power transfer, utilizing the capabilities of satellites, UAVs, and HAPs.
4. Mobile Edge Computing (MEC) in NTNs: The paper explores the use of NTNs in MEC applications, focusing on offloading computation to either aerial platforms or satellite-based nodes. This aspect of NTNs is particularly beneficial for reducing latency and fulfilling the computational needs of high-mobility devices and those located in challenging environments.
5. Machine Learning Applications: The deployment of ML across NTNs for tasks such as resource allocation, beamforming, spectrum management, and channel estimation is thoroughly covered. The document explores how ML helps enhance the operational efficiency of NTNs from a network management perspective.

Implications for 6G

Looking forward to 6G, the paper identifies how NTNs can support new use cases by enabling enhanced mobile broadband (eMBB+), massive machine-type communications (mMTC+), and ultra-reliable low-latency communications (uRLLC+). The integration of AI, advanced spectrum management, and multi-segment communication channels are viewed as pivotal technological enablers. 6G networks aim to leverage NTNs for immersive applications such as pervasive intelligence and interactive telepresence.

Challenges and Future Directions

The survey identifies several challenges that need to be addressed for the efficient functioning of NTNs. These include:

• Propagation Delays and Doppler Shifts: The inherent limitations imposed by the physics of satellite communication, particularly in terms of latency and frequency shifts.
• Seamless Vertical Integration: Ensuring that NTNs can integrate smoothly with terrestrial networks, both from a hardware and protocol perspective.
• Resource Allocation and Energy Constraints: Efficiently managing the limited resources of aerial and satellite platforms.
• Beamforming and Frequency Reuse: Addressing the beam coverage and interference issues in densely populated and dynamically changing settings.

Conclusion

The paper offers a detailed overview of the current and potential future of NTNs. It is evident that NTNs will play an increasingly integral role as global communications systems transition to 6G and beyond, provided that the technical, regulatory, and logistical challenges can be effectively managed. The insight into the evolution from 5G to 6G presented in the paper serves to inform both current practices and future developments in the field of NT networking technology.