1 What a non-terrestrial network is

1.1 The word, and the document that defines it

Every trade has a phrase it uses instead of the obvious one. 3GPP's phrase for a network whose transmitter is flying is a non-terrestrial network, short form NTN ^[5].

The definition sits in TS 38.108, the specification that says what a satellite base station has to transmit and receive TS 38.108.

Two phrases in that sentence carry the weight. "Or segments of networks" means a network can be terrestrial in one place and not in another. "Airborne" means the vehicle need not be in orbit at all.

The satellite gets its own line, and it is narrower than the everyday word: a space-borne vehicle carrying a transparent or a regenerative transmitter, in low earth orbit or geostationary orbit ^[6].

1.2 The base station that is partly in orbit

The thing a phone talks to is not called a base station here. It is a satellite access node, SAN for short ^[7].

Read the last sentence twice. The node is not the satellite. It takes in the payload flying overhead and the equipment on the ground, and the two together do the job one mast does on land.

1.3 What the satellite does to the signal

There are two answers, and the whole architecture turns on which one is meant.

A transparent payload changes the frequency carrier of the signal, filters it and amplifies it, and sends it on ^[8]. The intelligence stays on the ground; the satellite is a mirror with a power supply.

A regenerative payload transforms the signal before amplifying it, and that transformation is digital processing — demodulation, decoding, re-encoding, re-modulation, filtering ^[9]. Part of the base station is then in orbit.

The first definition is from TR 38.863, the report holding the radio-frequency and coexistence work for this whole subject TR 38.863. The second is from TR 38.742, a Release 20 study TR 38.742.

1.4 The two hops, and what each is called

The radio path is in two pieces and each has its own name.

The service link is the radio link between satellite and phone ^[11]. The feeder link is the wireless link between the satellite-gateway and the satellite ^[12].

The ground station at the far end of the feeder link is a satellite-gateway in TS 38.108: an earth station at the surface of the Earth with enough transmit power and enough receive sensitivity to reach the satellite ^[13].

The system side calls the same kind of box an NTN Gateway, an earth station providing connectivity to the payload over the feeder link ^[14]. Two documents, two spellings, one job.

1.5 Orbits, and the beam that comes down

Two orbit abbreviations turn up in nearly every paper. GEO is geostationary earth orbiting ^[15] and LEO is low earth orbiting ^[16].

A geostationary orbit is the circular one 35,786 km above the equator, going the way the Earth turns, so the satellite keeps its place in the sky ^[17].

A low orbit does not. A satellite in one covers a given area only for a limited time, which the architecture study says outright when it defines a serving satellite ^[18].

What arrives at the ground is a satellite beam — a beam generated by an antenna on board a satellite ^[19]. That is the whole definition, and it is deliberately plain.

1.6 One more word, and it is not about phones

The requirements side defines a term for the other kind of satellite equipment, the sort that sits on a roof rather than in a pocket.

It is worth knowing because it marks how wide the subject has got. TR 22.887 defines it in the same clause as satellite access and resilient notification TR 22.887, so the phase 4 study is looking at equipment on a roof as well as equipment in a pocket.

1.7 The words Release 20 had to add

The newest study reports define words the earlier documents did not need.

TR 22.887, the phase 4 requirements study TR 22.887, defines satellite access as direct connectivity between the device and the satellite ^[20] — no mast anywhere in the sentence.

The same report defines resilient notification: a highly reliable way of getting a notice to a device such as a smartphone when conditions on the satellite link are poor ^[21].

TR 23.700-19, the phase 4 architecture study TR 23.700-19, adds two more. A serving satellite is one providing the satellite access to a device, and, depending on the orbit, it covers a given area only for a limited time ^[18].

The second names something the ground never sees at all:

Two phones, one satellite, and nothing on the ground in between. It is defined in a study report, so the phrase has a meaning; the record says nothing about whether anything will be built to it.

1.8 Where these words turn up next

Every one of them is a term or an abbreviation quoted out of a published 3GPP document, and the register names the document and the clause for each. Nothing in this chapter is a paraphrase of somebody's slide.

The vocabulary is split across four documents, and which one a word comes from tells you who was arguing about it.

The radio side owns the hardware words, in TS 38.108 TS 38.108 and TR 38.863 TR 38.863. The requirements and architecture sides own the newer service words, in TR 22.887 TR 22.887 and TR 23.700-19 TR 23.700-19.

A word defined in a study report is a word somebody needed in order to describe a problem, not a word anything is built to yet. Every term in the section above comes from a report of exactly that kind.

Why anyone wanted it takes the reasons the work was proposed, in the words of the proposals themselves. How the work is cut up, and who carries it explains the paperwork these documents sit in, and The radio half, release by release follows the radio line release by release.