Store and Forward Satellite Operation

Description

Store and Forward (S&F) Satellite operation is an operation mode defined in 3GPP Release 19 for non-terrestrial networks (NTN) that provides communication service to a UE in periods of time and/or geographical areas in which the serving satellite is not simultaneously connected to the ground network via a feeder link or inter-satellite link (ISL). In this mode, the satellite acts as a store-and-forward relay, buffering user data on-board until a ground network connection becomes available.

For the uplink (UL) direction, "store" refers to the on-board storage of UL information received from the UE, and "forward" refers to the subsequent transmission of that stored UL information to the ground network when connectivity is restored. For the downlink (DL) direction, "store" refers to the on-board storage of DL information received from the ground network, and "forward" refers to the delivery of that stored DL information to the UE when the UE is within coverage. This dual-direction buffering allows the satellite to serve UEs even when the satellite itself is out of contact with ground infrastructure.

Architecturally, S&F satellite operation is implemented in the satellite payload, which must include sufficient on-board memory and processing capability to buffer user data, manage storage queues, and execute forwarding decisions. The satellite access node (SAN) or the satellite itself must support the necessary protocols for data storage, retrieval, and forwarding. The 5G core network must also be aware of the store-and-forward mode to manage session continuity, QoS, and charging appropriately. Key interfaces include the feeder link (between satellite and ground station) and the service link (between satellite and UE).

The operation mode is particularly relevant for LEO satellite constellations, where satellites orbit the Earth in approximately 90-120 minutes and may have intermittent visibility of ground stations. By storing data on-board, the satellite can provide service to UEs in remote or oceanic regions where no ground station is available, and then forward the data when it passes over a ground station. This enables global coverage for IoT, messaging, and delay-tolerant applications.

S&F satellite operation is specified in 3GPP TR 23.700 (Study on architecture enhancements for 5G satellite access) and TS 28.874 (Study on management aspects of satellite access). The concept builds on earlier NTN studies in Release 17 and 18, which focused on transparent and regenerative payloads with continuous feeder link availability. Release 19 extends this to intermittent connectivity scenarios, introducing new challenges for mobility management, session management, and QoS enforcement.

In terms of protocol stack, the satellite must implement a store-and-forward function that interacts with the NR protocol layers (PHY, MAC, RLC, PDCP, SDAP) and the 5G core network protocols (NAS, NGAP). The storage and forwarding decisions may be based on factors such as storage capacity, data priority, remaining orbital time, and predicted feeder link availability. The UE may need to be aware of the store-and-forward mode to adjust its own timers and retransmission strategies.

Purpose & Motivation

S&F Satellite operation was created to overcome the limitations of continuous connectivity assumptions in earlier NTN designs. In Release 17 and 18, 5G satellite access assumed that the satellite always had a feeder link to the ground network, either directly or via inter-satellite links. This assumption restricted coverage to areas within range of ground stations and required complex ISL constellations for global coverage. S&F operation removes this restriction by allowing the satellite to function autonomously, storing data when disconnected and forwarding it later.

The primary problem it solves is the coverage gap in remote, oceanic, and polar regions where deploying ground stations is impractical or impossible. By enabling on-board storage, a single satellite can provide intermittent but reliable service to UEs anywhere on Earth, without the need for a continuous backhaul link. This is particularly valuable for IoT devices that generate small, delay-tolerant data, such as environmental sensors, asset trackers, and emergency beacons.

Historically, store-and-forward operation has been used in proprietary satellite systems (e.g., Orbcomm, Argos) for decades, but 3GPP standardization brings it into the 5G ecosystem, ensuring interoperability, security, and integration with terrestrial networks. The motivation for Release 19 was to extend 5G service continuity to non-terrestrial environments and to support new use cases such as global IoT, disaster recovery, and remote area communications. The challenge was to design a standardized architecture that handles intermittent connectivity while maintaining 5G QoS, security, and mobility principles.