1 What a non-terrestrial network is, in 3GPP's own words

1.1 Why a subject about satellites starts with a glossary

Nowhere in this body of work does 3GPP write "satellite phone". It writes NTN, SAN, service link, feeder link, transparent payload, regenerative payload — and every one of those sits in a numbered definitions clause of a published document.

Learning them in the right order is not pedantry. Two of them, the transparent and the regenerative payload, decide where the base station physically is, and almost every later argument in this course follows from that one choice.

So this chapter is the vocabulary and nothing else. Each word is quoted from the document that defines it, and each is attributed, because five documents contribute here and they were written years apart.

1.1.1 The five documents the words come from

• TS 38.108, the radio rules for the satellite access node. It defines the network, the satellite, the access node, the feeder link, the ground station and the geostationary orbit, and it spells out the short names. TS 38.108

• TR 38.863, the study on radio frequencies and living beside other users of them. It defines the service link and the transparent payload. TR 38.863

• TR 38.742, the Release 20 study on working when the satellite-navigation fix goes away. It defines the regenerative payload and the satellite beam. TR 38.742

• TR 22.887, the phase 4 study of what the system must do. It defines satellite access, the very small aperture terminal, and a phrase it defines only for itself, resilient notification. TR 22.887

• TR 23.700-19, the phase 4 study of the architecture. It defines the serving satellite, phone-to-satellite-to-phone traffic, and the gateway under a second name. TR 23.700-19

1.2 The network, and what makes it non-terrestrial

The founding word is the network itself:

Three parts of that sentence are doing work.

"or segments of networks." A network does not have to be all satellite to be an NTN. One piece of it is enough, which is why this subject keeps meeting ordinary ground networks at its edges.

"airborne or space-borne." Orbit is not required by the definition. A vehicle in the air qualifies, and the Release 17 radio work says in its own objective that it kept implicit compatibility with high altitude platform stations and air-to-ground use ^[11].

"a transmission equipment relay node or SAN." Two shapes of thing may be carried up there, and the second of them is the one this course spends most of its time on.

The short name is the one everybody uses: TS 38.108 spells NTN out as non-terrestrial network in its abbreviations clause ^[12].

1.3 The vehicle

Notice that the definition of the satellite already contains the two payload kinds. They are not an afterthought; they are half of what the word means here.

The two orbits it names are spelled out in the same document as low earth orbiting and geostationary earth orbiting ^{[13] [14]}.

Medium earth orbit is not in this definition — it turns up later, in the objectives of the Release 18 and Release 19 radio work, and NR over satellite, release by release is where that happens.

Geostationary gets a definition of its own, and it is the most physical sentence in the whole vocabulary:

A satellite that appears motionless can be pointed at once and then left alone. One that does not stay put has to be handed over from, and that difference runs through the whole radio half of this subject.

1.4 The base station of this world

Here is the word that surprises people, because it is not the satellite:

Four names in that sentence need unpacking, and none of them is optional.

• User plane is the traffic a person actually sends. Control plane is the signalling that sets it up and keeps it going. Terminating both means this node is the far end of the radio protocols, not a repeater.

• UE is the device — a phone, a tracker, a terminal. NG is the interface from a 5G radio network to the 5GC, the 5G core network. gNB is the 5G base station.

• SAN is the short name, spelled out as Satellite Access Node in TS 38.108's abbreviations clause ^[15].

The last sentence is the one to remember. A SAN encompasses a payload on board the platform together with satellite-gateway and gNB functions. It is not a box in orbit. It is a span, from the thing flying to the equipment on the ground, treated as one node by everything that talks to it.

TS 38.108's own scope says what it then does with that node: it sets the minimum radio characteristics and minimum performance requirements for NR and NB-IoT operation in an in-band satellite access node ^[16].

1.5 The two hops

A signal from a phone to a core network crosses two radio hops, and each has its own name in its own document.

The service link is the one a person is standing under. The feeder link is the one nobody sees. Two documents define them — TR 38.863 the first, TS 38.108 the second — which is a small sign of how this vocabulary grew: by accumulation, document by document, rather than in one sitting.

The satellite's own antenna throws the service link somewhere in particular, and that patch has a name too: a satellite beam is a beam generated by an antenna on board a satellite ^[17].

1.6 The ground end, under two names

TS 38.108 calls the ground station a satellite-gateway:

TR 23.700-19, written for the architecture side years later, calls the same place an NTN Gateway:

The second definition ends with three letters this register does not expand anywhere, so this course does not expand them either. What the sentence does say plainly is that the gateway is a node of the transport underneath, not a part of the radio network above it.

Two names, one place, two documents. Both are correct, and which one you write depends on whose specification you are reading.

1.7 The payload, which is where the design splits

Everything above is geometry. This is the choice.

UL and DL are uplink and downlink, which the regenerative definition below writes out in words. Read the verbs: changes, filters, amplifies. Nothing in that list understands what it is carrying. The satellite is a mirror with a power supply.

Now read those verbs: demodulation, decoding, re-encoding, re-modulation. The thing in orbit takes the signal apart and puts it back together, which means part of the base station is up there with it.

	Transparent	Regenerative
What the payload does to the signal	changes the carrier, filters, amplifies	transforms and amplifies, the transformation being digital processing
Whether it decodes	the definition names no decoding	may demodulate, decode, re-encode, re-modulate
Which document defines it	TR 38.863 TR 38.863	TR 38.742 TR 38.742

The definition of a satellite allows either ^[2].

The definition of a satellite access node names only the transparent one ^[4] — which is a fact about what TS 38.108 was written for, and NR over satellite, release by release is where the releases that assumed it are laid out.

1.8 The words the newer studies added

The definitions so far are mostly out of the radio documents. The requirements and architecture studies of Release 20 added five more, and they are worth knowing because they are the language of the arguments still running.

Satellite access is the plainest sentence in the whole vocabulary: direct connectivity between the UE and the satellite ^[18]. No mast, no relay, no intermediate anything.

A very small aperture terminal, VSAT, is a small-sized earth station used to transmit and receive data, voice and video over a satellite communication network, excluding broadcast television ^[19]. So not every device in this subject is a phone in a pocket.

A serving satellite is one providing the satellite access to a device — and the definition adds, in its own second sentence, that depending on the orbit it covers a given geographic area only for a limited period of time ^[20]. That clause is the whole problem of non-stationary orbits, written into a glossary entry.

UE-satellite-UE communication is traffic between devices under the coverage of one or more serving satellites, using satellite access, without the user traffic transiting through the ground segment ^[21]. Phone to satellite to phone, and the ground never sees it.

Resilient notification is a word TR 22.887 made up for itself, and says so: in the context of this study, a highly reliable mechanism that delivers notifications directly to user devices, such as smartphones, when in poor conditions in satellite access ^[22].

1.9 The picture all of that draws

Put the definitions end to end and one chain falls out. Reading from the person outwards:

• The device — a phone, a tracker, or a small earth station ^[19].

• The service link, the radio hop up to the satellite ^[5], thrown by a satellite beam ^[17].

• The satellite, carrying a payload of one of the two kinds, in low earth or geostationary orbit ^[2].

• The feeder link, the radio hop down to the ground station ^[6].

• The gateway on the ground, called a satellite-gateway in the radio documents and an NTN Gateway in the architecture ones ^{[7] [8]}.

• The core network, reached over the NG interface ^[4].

The satellite access node is not one link in that chain. It is a bracket drawn around the payload, the gateway and the base station functions together ^[4].

And in the phone-to-satellite-to-phone case the chain is cut short on purpose: the traffic turns round at the satellite and never reaches the feeder link at all ^[21].

1.10 Where these words are used in the rest of the course

Everything after this chapter leans on the vocabulary above.

Why anyone wanted it, and who said so takes the justifications the groups wrote for themselves, where transparent payload, power class and geostationary orbit turn up as reasons rather than definitions.

How the work is cut up, and who carries it is the machinery: what counts as satellite work at all, how one short name becomes three work plan rows, and which groups carry the weight.

NR over satellite, release by release and The same idea on the older radio, for things that are not phones are the two radio lines, and the payload split defined above is what separates the releases inside each of them.

To read the definitions in place rather than quoted, the two shortest ways in are TS 38.108 for the radio words TS 38.108 and TR 23.700-19 for the architecture ones TR 23.700-19.