UE capability — What this particular device can do, and who is allowed to remember it
The device — identity, state, mobility
Where it sits
What it is
What it is. What this particular device can actually do — which bands, which numerologies, how many MIMO layers, which release features. TS 38.306 defines the parameters; TS 38.331 carries them in an RRC UE Capability Information message.
Why they are signalled at all. The specification defines a very large set of features and makes most of them optional. A device implements a subset chosen by its designers, its silicon, its price point and its target markets, and the network has no way to guess which. So before the network can configure anything, it must ask — and until it has asked, it must assume only the mandatory baseline.
Capabilities appear throughout this folder, whether or not they are named as such. The
tXrY grammar of SRS antenna switching — t1r2, t1r4, t2r4 — is a
capability, supportedSRS-TxPortSwitch. Which
modulation table the device supports is a capability. The
peak data rate formula of TS 38.306 §4.1.2 is computed from capabilities: the
bands, the layers, the modulation order and the bandwidths this device can actually use.
So a "5G phone" has no single speed. The number quoted is the output of that formula for that device's declared capability set, which is why two phones on the same network differ.
Capability signalling grew big enough to need a function of its own. A full capability report is large, and it is the same every time the device registers, and it is fetched again on every AMF change. TS 23.501 §6.2.21 therefore defines the UCMF — UE radio Capability Management Function — to store and dictionary-encode capability sets so that an identifier can be passed around instead of the whole structure.
A network function whose entire purpose is to stop re-signalling something that never changes. That is a good measure of how much optionality the standard carries.
Where they are stored. The AMF holds the device's radio capabilities and gives them to the base station, so a device does not re-report them on every handover. They are part of the context that makes registration worth doing once rather than continuously.
Read on
This concept is read as part of one argument in ref-ue-state, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: