A phone communicates directly with a satellite. Where is the base station, and how does the traffic reach the mobile operator? These questions matter because the satellite can either relay the radio waveform or perform base-station processing onboard.
Start by separating three things: the physical route, the radio protocol endpoints and the operator arrangement. A gateway is not automatically a eNB. A satellite operator is not automatically the subscriber's home operator. A direct access link does not remove the core network.
This page focuses on conventional cellular handset access. The diagrams compare conceptual processing locations using LTE/EPC labels for consistency. They are not complete diagrams of any one vendor's implementation. See D2C versus standardized NTN for the scope distinction.
- The End-to-End Path
- Transparent Payload: Base Station on the Ground
- Regenerative Payload: Base Station on the Satellite
- How Does the Mobile Operator Fit In ?
- What Changes as the Satellite Moves ?
- A Simple Propagation Example
- What to Put on an Architecture Diagram
- References
The End-to-End Path
Regardless of payload type, an ordinary interactive service needs a path from UE to radio access, core-network functions and the destination service. The satellite radio segment is part of this path, rather than the complete service.
Figure 1 shows the physical route without assuming where the base-station processing sits. A deployed system can add inter-satellite transport or use multiple gateways. The arrows indicate connectivity in both directions, not a specific signalling exchange.
Figure 1. Direct refers to the UE-to-satellite service link; gateway, core and service functions remain in the path.
Service Link, Feeder Link and Inter-Satellite Link
The service link connects UE to the satellite payload. The feeder link connects the payload to a ground gateway. Their frequencies, antennas and link budgets can differ. The phone needs to support the service-link band, not the gateway's feeder-link band.
An inter-satellite link connects satellites within the space network. It may provide a route toward another satellite with access to a suitable gateway. The original serving satellite then need not have the same immediate ground path as the destination gateway.
Inter-satellite transport is optional. It does not mean the mobile core is in orbit, and it does not eliminate the eventual connection to terrestrial services for the examples here.
Gateway, RAN and Core Network
A gateway provides the ground endpoint of the feeder connection and access to the ground network. Depending on architecture, that ground network leads to a base station or transports an onboard base station's interfaces.
The core handles mobility management, authentication coordination and session connectivity. In the LTE/EPC examples here, MME handles access signalling and the Serving/PDN Gateways carry user traffic. Subscriber databases and service platforms remain separate functions.
Applications and service platforms sit beyond these access functions. Voice can require IMS integration; messaging needs its corresponding service path. A working radio bearer alone does not prove that either service is provisioned.
Transparent Payload: Base Station on the Ground
A transparent payload forwards the received waveform between service and feeder links. It can amplify and convert frequency, but it does not terminate the UE's Uu radio protocol as a base station in this model.
Figure 2 illustrates a conceptual ground-base-station arrangement using LTE labels. The eNB is beyond the gateway, and the logical LTE Uu connection reaches it through the satellite relay. This model explains processing placement; it does not establish a vendor's exact functional split.
Figure 2. With a transparent payload, the radio protocol endpoint is the ground eNB, not the satellite relay.
Where Scheduling and Radio Processing Happen
The ground eNB performs the base-station radio functions, including scheduling and the applicable radio protocol processing. The satellite's signal processing should not be confused with termination of the UE's cellular connection.
For a simple collocated gateway/eNB model, the UE-to-eNB propagation path includes the service and feeder segments. Timing configuration must accommodate that arrangement. The gateway and eNB can also be separated by transport, so collocation should be stated as an assumption.
The network must accommodate the resulting delay while keeping the handset within its supported LTE timing behavior. A geometric delay calculation alone does not prove that random access, HARQ and all other timers can operate correctly.
What the Feeder Path Changes
The serving radio connection depends on the relay path reaching the ground eNB. Changes in gateway connectivity can affect delay and continuity even when UE still sees the same satellite.
Noise from the two hops also matters to a transparent radio path. A strong satellite-to-phone downlink alone is insufficient to establish end-to-end signal quality. Use the appropriate combined budget rather than treating the payload as an ideal decoder.
This model is useful when drawing a clear boundary between satellite transport and ground RAN processing. It does not establish the exact functional placement of every commercial D2C deployment.
Regenerative Payload: Base Station on the Satellite
A regenerative payload can decode and regenerate traffic onboard. In the full onboard-eNB example, UE's LTE radio endpoint is on the satellite. Transport then connects that base station to the ground EPC.
Figure 3 separates the onboard LTE radio endpoint from the ground core. It is a simplified full-eNB model, motivated by the published onboard LTE base-station example; the drawing does not assert the exact internal split of that implementation.
Figure 3. An onboard eNB terminates Uu, while core-network connectivity still uses the space/ground transport path.
Radio Endpoint and Transport Endpoint
With a full onboard eNB, radio scheduling and base-station protocol handling move onto the satellite. The logical S1-MME connection reaches MME, while S1-U carries user traffic toward the Serving Gateway through the transport network.
Moving the radio endpoint can shorten the propagation path for radio interactions that terminate there. It does not make the application round trip equal to only the UE-satellite delay. Core-network and service traffic still traverses the remaining route.
An inter-satellite link can carry backhaul toward a satellite with a usable ground connection. It is a physical transport path, not a substitute name for a cellular RAN or core interface.
Published Implementation Examples
A January 2024 implementation description identifies an onboard LTE modem acting as a base station, with laser backhaul and mobile-operator integration. This establishes a practical example of moving cellular processing into space.
A separate 2022 annual report describes a planned system using large satellite phased arrays and Q/V-band feeder links to in-country gateways connected to mobile-operator infrastructure. This describes the transport arrangement, but does not by itself locate every cellular protocol function onboard or on the ground.
Before mapping a product onto either conceptual diagram, identify where PHY, MAC, RLC, PDCP and RRC terminate. A large satellite antenna or a cellular 5G demonstration alone does not answer that question.
How Does the Mobile Operator Fit In ?
The satellite route must connect to subscriber and service handling. A phone still needs an identity, permitted network selection and successful authentication before ordinary subscribed service can proceed. Direct radio access does not bypass these functions.
Home Operator, Satellite Operator and Serving Network
The home operator maintains the subscriber relationship. A satellite operator supplies some combination of space infrastructure, radio access and transport. The serving network and commercial integration determine how the phone uses those resources.
One arrangement integrates satellite access into an operator network. Another uses a partner network with roaming-style integration. These are architectural possibilities, not universal descriptions of PLMN identity, roaming agreements or user-plane routing.
Do not draw a mandatory "roaming interface" between two generic clouds without identifying the actual core architecture. Different cellular core architectures have different functions and interfaces. Home routing versus local breakout is also a deployment choice.
Control Plane and User Plane
In the LTE/EPC example, UE NAS signalling reaches MME through the eNB. User packets travel through the eNB and Serving Gateway toward the PDN Gateway and the destination network.
MME does not forward every user packet. Likewise, a satellite gateway does not authenticate subscribers merely because the radio traffic passes through its antennas. Separate protocol roles from physical boxes.
These LTE labels make the example concrete. A conventional cellular 5G implementation needs a drawing matching its actual core and deployment mode; a 5G handset connection alone does not establish that it uses a standalone 5G core.
What Changes as the Satellite Moves ?
The physical path changes even when UE is stationary. A beam can move, a satellite can leave its service interval, or the space network can change its route to the ground. These events do not all require the same UE procedure.
Change | Architectural question |
|---|---|
Serving beam or cell | Does the logical cell change? Does UE need reselection, handover or another configured transition? |
Serving satellite | Does the same ground RAN endpoint remain, or does UE access a new onboard base station? |
Gateway / feeder route | Can transport preserve the connection, or does the switch affect radio timing and service continuity? |
Inter-satellite route | Does routing change only transport, or also connectivity between RAN/core endpoints? |
No usable end-to-end path | Is service interrupted? Are explicit deferred-delivery features supported, or must UE wait for connectivity? |
A regenerative payload does not automatically provide store-and-forward service. That needs explicit support for receiving, retaining and later delivering traffic with suitable control and application handling. Onboard processing alone is insufficient.
A transport route change can be hidden from UE if the radio connection and required timing remain usable. A change of serving cell may instead require ordinary cellular mobility procedures. The outcome depends on the implementation and the phone's supported behavior.
A Simple Propagation Example
Assume a service-link slant range of 1200 km and a feeder-link range of 1800 km. Use light speed approximately 300,000 km/s, and ignore processing, scheduling, queuing and all other transport for this illustration.
For a transparent payload with ground eNB at the gateway, one-way UE-to-eNB propagation is (1200 + 1800)/300,000 = 10 ms. The corresponding geometric round trip is 20 ms.
For a full onboard eNB, one-way UE-to-eNB propagation is 1200/300,000 = 4 ms, with an 8 ms geometric round trip. A transaction continuing to a core at the gateway still crosses the additional 6 ms feeder segment each way.
This example explains where a radio-loop benefit can arise. It does not predict a complete HARQ timer or application latency. Different gateway ranges, inter-satellite paths and processing locations change the result.
What to Put on an Architecture Diagram
An architecture drawing should let another engineer locate both the radio endpoint and the service endpoint. Label assumptions explicitly, especially when a generic drawing is being used to explain a particular deployment.
- UE and RAT: the conventional cellular RAT, band and actual handset capabilities.
- Links: service, feeder and optional inter-satellite paths; distinguish physical transport from logical interfaces.
- Payload: transparent relay, full onboard base station or a documented functional split.
- Core: EPC or EPC, with control-plane and user-plane roles identified.
- Operators: serving/home relationships and the actual integration model.
- Continuity: what happens when a beam, satellite, gateway or route changes.
Continue with D2C Technology for handset compatibility, antenna gain and network handling of delay and Doppler. These constraints apply to both conceptual processing arrangements.
References
The dated examples below describe conventional cellular handset access. They support the stated implementation examples, rather than claims about current commercial coverage or compatibility with every phone.
- Direct to Cell First Text Update, January 2024 : SpaceX; existing LTE handset access, onboard LTE processing and transport to mobile operators.
- Cellular connectivity demonstration, September 2023 : AST SpaceMobile; an unmodified smartphone using a cellular 5G connection through BlueWalker 3.
- 2022 Annual Report : AST SpaceMobile; described satellite, feeder-link and mobile-operator gateway arrangement.