The technical requirements for Non-Terrestrial Networks (NTNs) are critical to ensuring their seamless integration and efficient operation alongside terrestrial communication systems. One of the foremost challenges is managing latency, particularly for satellites in higher orbits like Geostationary Orbit (GEO). To address this, systems need advanced techniques for reducing transmission delays, such as optimizing signal routing and using Low Earth Orbit (LEO) satellites for time-sensitive applications. Another essential requirement is spectrum coordination, as NTNs share frequencies with terrestrial networks, making efficient spectrum allocation and interference management vital to prevent signal degradation.

Interoperability between NTNs and terrestrial systems is also crucial, demanding sophisticated network architecture and protocols that enable seamless handovers and consistent service quality. This includes designing ground stations and user terminals equipped to handle dual connectivity, switching between satellite and terrestrial networks without service disruption. Power efficiency is another significant consideration, particularly for user devices relying on NTN links, as satellite communication can require higher transmission power. Advanced modulation and coding schemes, such as those defined in 3GPP standards for 5G NTNs, are essential to maximize spectral efficiency while minimizing power consumption.

Additionally, NTNs must be robust against environmental and atmospheric disruptions, such as signal attenuation caused by rain, clouds, or other atmospheric conditions. To address this, technologies like adaptive beamforming and error correction mechanisms are essential to maintain link reliability. Scalability is another key technical requirement, as NTNs often serve millions of users simultaneously in diverse and dynamic environments. This necessitates the use of dynamic resource allocation and advanced traffic management algorithms to optimize network performance.

Security is also a critical concern, requiring encrypted communication, secure satellite control systems, and mechanisms to prevent unauthorized access or jamming. Finally, the ground infrastructure supporting NTNs, including gateways and user terminals, must be designed to handle the unique characteristics of non-terrestrial communication, ensuring high reliability and low operational costs. These technical requirements collectively shape the development of NTNs, enabling them to provide resilient and expansive connectivity in harmony with terrestrial networks.

Delay Requirements

The total Delay Requirement specified in TS 22.261 is roughly the following value + 5 ms (delay caused by 5G protocol)

< 22.261 (Rel 18) - Table 7.4.1-1: UE to satellite propagation delay >

NOTE :  Even the smallest propagation is greater than the max delay that can be covered by TA field of RAR.(Around 2 ms is covered by RAR TA in SCS 15Khz, 1 ms in SCS 30Khz).

< TR 38.821 - Table 7.1-1: NTN scenarios versus delay constraints >

Performance Requirements

High lights of Performance Requirements can be summarized as :

• GEO satellite access with up to 285 ms end-to-end latency, including a 5 ms assumed network latency.
• MEO satellite access with up to 95 ms end-to-end latency, plus a 5 ms network latency.
• LEO satellite access with up to 35 ms end-to-end latency, with an additional 5 ms network latency.
• Allow for quality of service negotiation to optimize user experience, considering the latency.
• Provide high uplink and downlink data rates for satellite UEs.
• Ensure communication service availability of at least 99.99%.

More detailed requirement would vary depending on various scenario and UE type which is summarized in following table.

< 22.261 (Rel 18) - Table 7.4.2-1: Performance requirements for satellite access >

The table can be summarized as follows :

• Pedestrian: 1 Mbit/s DL, 100 kbit/s UL, with area traffic capacities of 1.5 Mbit/s/km DL and 150 kbit/s/km UL, at 100 users/km, and an activity factor of 1.5%.
• Public Safety: 3.5 Mbit/s for both DL and UL, other capacities TBD, users moving at 100 km/h.
• Vehicular Connectivity: 50 Mbit/s DL, 25 Mbit/s UL, details TBD, with 50% activity factor, speeds up to 250 km/h.
• Airplanes Connectivity: 360 Mbit/s per plane DL, 180 Mbit/s UL, with users traveling up to 1000 km/h.
• Stationary: 50 Mbit/s DL, 25 Mbit/s UL, activity factor not applicable, stationary users.
• Video Surveillance: 0.5 Mbit/s DL, 3 Mbit/s UL, users stationary or moving up to 120 km/h.
• Narrowband IoT Connectivity: 2 kbit/s DL, 10 kbit/s UL, with area traffic capacities of 8 kbit/s/km DL and 40 kbit/s/km UL, at 400 users/km, activity factor of 1%, at speeds up to 100 km/h.

How 3GPP is updated to get around the problems and meet the requirements listed above ?

To cope with the issues and to meet the requirement mentioned above, some new features are introduced in 3GPP release 17. In summary, those new features can be summarized as below.

• Handling Timing Offset for Long Delay :  Additional Timing Address Information elements (ta-Info-r17) are added in SIB 19.
• Handling the long delay for HARQ due to long distance between UE and gNB : a New Information Elements (DL-DataToUL-ACK-v1700 ) is added to specify long enough K1 value.

NOTE :  To cover all the possible distance between UE and Satellite, the number of HARQ should be very large, but in current extenstion, the max HARQ number is increased only up to 32. Waiting to see the feedback from industry

NOTE : Theoretically increasing K1 is not the only possible solution. We can remove HARQ completely and relay on higher layer for error checking and retransmission. I guess some would be trying this. But removing HARQ completely would be too much impact on protocol since it would impact on signaling message transmission and reception.

• Indicating the Position and Motion of the Satellite : For this purpose, a new Information Elements (ephemerisInfo-r17 ) is added and broadcast in SIB 19.