Architectures and Key Technical Challenges for 5G Systems Incorporating Satellites (1806.02088v1)

Abstract: Satellite Communication systems are a promising solution to extend and complement terrestrial networks in unserved or under-served areas. This aspect is reflected by recent commercial and standardisation endeavours. In particular, 3GPP recently initiated a Study Item for New Radio-based, i.e., 5G, Non-Terrestrial Networks aimed at deploying satellite systems either as a stand-alone solution or as an integration to terrestrial networks in mobile broadband and machine-type communication scenarios. However, typical satellite channel impairments, as large path losses, delays, and Doppler shifts, pose severe challenges to the realisation of a satellite-based NR network. In this paper, based on the architecture options currently being discussed in the standardisation fora, we discuss and assess the impact of the satellite channel characteristics on the physical and Medium Access Control layers, both in terms of transmitted waveforms and procedures for enhanced Mobile BroadBand (eMBB) and NarrowBand-Internet of Things (NB-IoT) applications. The proposed analysis shows that the main technical challenges are related to the PHY/MAC procedures, in particular Random Access (RA), Timing Advance (TA), and Hybrid Automatic Repeat reQuest (HARQ) and, depending on the considered service and architecture, different solutions are proposed.

• The paper explores architectural scenarios for integrating satellites into 5G systems (transparent vs. regenerative payloads, direct vs. Relay Node access) and identifies key technical challenges at the Physical and MAC layers.
• It analyzes challenges for 5G enhanced Mobile Broadband (eMBB), including waveform sensitivity to non-linearities and significant delays impacting HARQ and Timing Advance, proposing modifications to address large Round Trip Times.
• The study investigates challenges for NarrowBand-Internet of Things (NB-IoT) in satellite links, focusing on Doppler shift management, timing synchronization, and adapting random access procedures to accommodate delays.

Architectures and Key Technical Challenges for 5G Systems Incorporating Satellites

The paper "Architectures and Key Technical Challenges for 5G Systems Incorporating Satellites" explores the integration of satellite communications within the 5G framework, addressing pivotal technical obstacles and architectural considerations peculiar to satellite-enabled networks. With the 3GPP's initiation of the New Radio (NR)-based paper item for Non-Terrestrial Networks (NTN), the integration of satellite systems is crucial, both as standalone solutions and as supplements to terrestrial networks. The research primarily tackles the core technical challenges posed by satellite channel impairments and their effects on 5G infrastructure at the Physical (PHY) and Medium Access Control (MAC) layers.

Key Findings and Technical Challenges

The paper systematically categorizes architectural scenarios based on satellite payload types—transparent and regenerative—and the access link configurations, which could be directly from satellites to users or involve ground-based Relay Nodes (RNs). The emphasis on direct and RN-mediated access links aligns with the specific requirements of enhanced Mobile Broadband (eMBB) and NarrowBand-Internet of Things (NB-IoT) services.

Enhanced Mobile Broadband (eMBB) Challenges

1. Waveform Performance: The paper contrasts CP-OFDM with filtered-OFDM (f-OFDM) in terms of sensitivity to non-linear distortions and spectral performance. While f-OFDM offers reduced out-of-band emissions crucial for satellite links, it also exhibits higher sensitivity to non-linearities and PAPR, particularly in satellite transponders utilizing TWTA.
2. Propagation Delays: GEO-based eMBB scenarios experience substantial signal delays, impacting Timing Advance (TA) and HARQ operations. The paper suggests methods to address these delays, including the possibility of tailored modulation and coding schemes to optimize performance amidst large path losses and channel delays.
3. HARQ Processes: The maximum number of HARQ processes for acknowledging packets significantly exceeds terrestrial norms due to increased Round Trip Times (RTTs). Proposed solutions to manage this include modifying the bit-width of the DCI fields or possibly increasing buffer sizes, which could affect system throughput.

NarrowBand-Internet of Things (NB-IoT) Challenges

1. Doppler Shift and Frequency Offset: The primary concern in LEO-based NB-IoT systems is the management of doppler shifts and frequency offsets caused by satellite velocity relative to Earth. This affects resource alignment in SC-FDMA, essential for maintaining reliable uplink communications. The paper proposes potential mitigation strategies, such as frequency advance techniques.
2. Timing Considerations: Similar to eMBB systems, NB-IoT networks must address scheduling and TA adjustments to mitigate timing misalignments in uplink transmissions due to satellite channel delays. The work discusses how extended timing windows might offer viable solutions without necessitating fundamental changes to existing standards.
3. Random Access Procedures: The NB-IoT standard allows for extensive RA time windows and repetitions, which helps to accommodate satellite delay characteristics. This presents a viable adaptation of the terrestrial standard to satellite conditions without additional enhancements.

Practical and Theoretical Implications

The integration of satellites into 5G networks promises extended coverage, improved reliability, and enhanced service continuity in sparsely connected areas. It poses practical benefits for applications in IoT, remote monitoring, and global connectivity. Theoretically, these implementations necessitate profound modifications to the classical NR waveform and MAC procedures to account for distinct satellite channel impairments.

Future Directions

The paper paves the way for further exploration into waveform optimizations for non-linear satellite channels and enhanced MAC protocols that can dynamically adapt to variable RTT and doppler conditions. It suggests the necessity to standardize the use of satellites in NR beyond the current adaptability discussions.

Overall, this work adds substantial understanding to the feasibility and design considerations for incorporating satellites into 5G frameworks, reflecting the critical role such systems can play in shaping future global communication landscapes.