1 A tag with no battery

1.1 The problem, written down first

3GPP writes the standards that mobile networks are built from. It works by naming a piece of work, agreeing to do it, and then filing documents against that name. The Ambient IoT story starts with one such proposal in 2022.

The proposal did not begin by describing a new gadget. It began by saying that some real situations had no answer in anything 3GPP had already built.

The same sentence names three of them. ^[1]

• Extreme conditions — high pressure, extreme heat, extreme cold.

• Devices nobody will ever service — something a millimetre thick that has to keep working without anyone opening it to change a cell.

• Places where an ordinary battery cannot be used at all.

Notice what those three have in common. Not one of them is fixed by making an existing radio cheaper or smaller. They are fixed, if at all, by taking the battery out of the design altogether.

1.2 What the device is

The definition is short, and it was written into that first proposal.

Years later the finished architecture document said the same thing in one line.

1.3 Where the energy comes from

The first study said which sources it had in mind, and then deliberately left the list open.

Radio waves are the interesting one. If a device can be powered by the radio signal that is also talking to it, then the network is both the caller and the power supply, and that is a radio engineering problem rather than a battery problem.

1.4 How small is small

The radio side put a number on its target when it worked out what could be built.

One microwatt at peak. That is the figure to remember, because every later argument — how a message is coded, how long a device may listen, how much it can remember — is an argument about staying under it.

1.5 One limit that gets forgotten

It is tempting to read all this as "5G now supports battery-less tags everywhere". The published architecture document is narrower, and says so in its own opening clause. ^[7]

In this release, it states, the Ambient IoT system is defined as an isolated private network that does not interact with a public network. That is a real fence around what has been specified so far.

1.6 Where to read this yourself

The proposal that opened all of it is SP-220085, and the report it produced is TR 22.840. SP-220085 TR 22.840

The requirements that came out of that report are TS 22.369, and the architecture built on them is TS 23.369. TS 22.369 TS 23.369

This school does not carry the text of those documents. It gives you the number and says what it is for; the words themselves are on 3gpp.org, and every number above links there.