Cellular Device Capability Exchange with 5G Network

The above message that the Network sends to the UE is an RRC message details of which are standardized in TS 38.331. 

Each of the IE (IE stands for Information Element) in the above message, e.g. RRC-TransactionIdentifier, UECapabilityEnquiry-IEs etc. are also present in TS 38.331.

In response to above reuqest, the UE responds with UECapabilityInformation message. Following is the message detail

The UE responds with the above message with ueCapabilityInformation-IE containing UE's capabilities. Within this IE, ue-CapabilityRAT-ContainerList IE is very important as it contains all the details about the Radio Access Technologies (RAT) that the UE supports (Section 6.3.3, TS 38.331). The gNB keeps and remembers the UE context extracting from this IE. Based on information extracted from this IE, the gNB can work in an optimized fashion. For example, (as we shall see later) a sub-IE within ue-CapabilityRAT-Container  would contain MAC capabilities of UE, e.g. if it supports longDRX and ShortDRX, or what Phy-Paramaters does the UE supports (see the discussion below).

In the above IE, 

• rat-Type contains the Radio Access Technology Type that UE supporsts i.e. either {nr, eutra-nr, eutra, utra-fdd-v1610, ...}

• UE-CapabilityRAT-Container contains the UE's capabilities for indicated RAT-Type.

Following is the container for the supported RAT types by UE

Now, in the above, lets examine if the rat-Type set as NR. Then the encoding of UE capabilities are defined in an IE UE-NR-Capability (again in TS 38.331).

The IE contains a very comprehensive details of the PDCP, RLC, MAC and PHY parameters along with other set of feature parameters. A glimpse of these features is as follows:

Each IE above, e.g. PDCP-parameters, RLC-Parameters, MAC-Parameters etc. would contain respective details of relevant NR features for that particular layer/feature. 

All these details are present in TS 38.331 in respective sections (use search feature in the MS-word or PDF document).

Let's for sake of our understanding see what MAC-Parameters IE would contain. Note that this will contain the information that the UE would provide. 

It is a simple, yet complicated, process of how the UE exchanges its capability with the gNB. At the gNB, it is yet more complicated how the UE context is stored and utilized properly. However, that is a completely different and an in-depth topic of discussion. Hopefully, with this blog, it is clear how UE's capability exchange takes place with the network.

Till next time...