1. Introduction
At the time of registration for instance and before the UE would be able to perform data transfer or make/receive voice calls, the network needs to understand UE’s capabilities so that it can configure the UE accordingly.
The network requests for UE capabilities typically during registration procedure and stores it locally. This means that the UE doesn’t need to send UE capabilities every time RRC connection is established or re-established. However, the network can request the UE to send its capabilities at any time during RRC connected state.
Additionally, if the UE changes its radio capabilities, it will initiate a Tracking Area Updating procedure and include the IE UE radio capability information update needed in the TAU REQUEST message. The network would then request for new UE capabilities from the UE.
UE capability Information message size concerns
When LTE was introduced, the size of the UE capability Information message used to be fairly manageable. With the introduction of CA and LTE-Advanced features, the size of this message has grown to enormously, which is mainly due to multiple CA bands and UE capabilities defined per band and per band combination.
With the introduction of 5G NR and with NSA, the LTE UE capability message size would grow further as the UEs have to indicate EN-DC capabilities too.
Several optimisations have been introduced in 3GPP LTE standards to reduce the size of UE capability message (a couple of those are given below).
- Within the UE capability enquiry message, the network may include the IE requestedFrequencyBands to request the supported CA band combinations and non-CA bands specifically for a set of bands.
- Within the UE capability enquiry message, the network may include the maximum number of componentcarriers for which it needs to know the supported CA band combinations and non-CA bands. The IE requestedMaxCCsDL is corresponds to downlink and requestedMaxCCsUL corresponds to uplink.
- For UEs supporting (NG)EN-DC or NE-DC, the network may provide a list of NR and/or E-UTRA frequency bands for which the UE is requested to provide its supported NR CA and/or MR-DC band combinations. The IE requestedFreqBandsNR-MRDC is used for this purpose.
These kinds of optimizations are also proposed and implemented in NR. For instance, the network mandatorily includes frequency band list for which it needs capabilities from the UE. Here, the network needs a filtered UE capability in which the UE is requested to provide supported bands and band combinations. This procedure is optional in LTE.
Another important optimization introduced in NR and LTE Release-15 is the concept of Feature Sets. Feature sets concept is thoroughly discussed in the following section.
2 Concept of Feature Sets and Feature Set Combinations
For UEs supporting (NG) EN-DC or NE-DC, the size of UE capability information would be enormous if the UE has to report capabilities separately per each reported band in the corresponding band combination.
5G NR and LTE (from Release 15) onwards introduced so-called Feature Sets to overcome this problem.
- A Feature Set stores a set of UE capabilities and features.
- An ID is given to each Feature Set.
- Using Feature Set ID, a Feature Set can be linked/associated with one or more bands within the corresponding band combination.
- Using Feature Set ID, a Feature Set can be linked/associated within more than one band combinations.
- The Feature Sets mechanism makes sure that each Feature Set is reported only once to avoid duplication.
- A Feature Set contains a pair of feature sets for UL and DL separately.
- A Feature Set can either be NR feature set or E-UTRA feature set.
Since a set of capabilities/features using Feature Set are reported only once, and any Feature Set can be re-used with any band(s) in each band combination and also with multiple band combinations, the size of the UE capability Information message is drastically reduced.
Feature Set Combination is a two-dimensional matrix containing Feature Set entries. A Feature Set Combination refers to the IDs of the feature set(s) that the UE supports in that Feature Set Combination.
Finally, each Band Combination entry in the Band Combination List is linked/associated with a Feature Set Combination. This is done by indicating the ID of the Feature Set Combination that the UE supports for that band combination.
This is discussed in detail below with an example illustrations.
Step-1: Define Feature Sets
- The IE FeatureSets is used to provide pools of downlink and uplink features sets. The IE structure is given below.
FeatureSets | |
featureSetsDownlink | A list of FeatureSetDownlink for up to 1024 DL FeatureSets |
featureSetsDownlinkPerCC | A list of up to 1024 CC-specific DL FeatureSets |
featureSetsUplink | A list of FeatureSetDownlink(s) for up to 1024 UL FeatureSets |
featureSetsUplinkPerCC | A list of up to 1024 CC-specific UL FeatureSets |
. . . | |
- The IE FeatureSetDownlink or FeatureSetUplink indicates a set of features that the UE supports on the carriers corresponding to one band entry in a band combination. This IE also contains a respective sub-IE structure (featureSetListPerDownlinkCC or featureSetListPerUplinkCC) indicating which features the UE supports on the individual DL/UL carriers of the feature set.
Step-2: Define Feature Set Combination (s)
This is an important step in which the Feature Sets created in Step-1 would be used to create a Feature Set Combination.
The IE FeatureSetCombination is a two-dimensional matrix of FeatureSet entries.
A Feature Set Combination is defined such that each entry could be mapped to a band within the corresponding Band Combination.
- Each entry is referred to as FeatureSetsPerBand which contains a list of feature sets applicable to the carrier(s) of one band entry of the associated Band Combination.
- The number of FeatureSetsPerBands in a Feature Set Combination must have an equal number of band entries in an associated Band Combination. The first FeatureSetPerBand applies to the first band entry of the band combination, and so on.
- In case of NR, the actual feature sets for UL and DL are defined in the FeatureSets IE (discussed in Step-1) and referred to by their ID, i.e., their position in the featureSetsUplink/featureSetsDownlink list in the FeatureSet IE.
- In case of E-UTRA, the feature sets referred to from this list are defined in TS 36.331 and conveyed as part of the UE-EUTRA-Capability container.
FeatureSetCombination IE is structure is shown below.
The following example shows the mapping between a Band Combination and Feature Set Combination.
Step-3: Assigning and ID to a Feature Set Combination
The IE FeatureSetCombinationId identifies a FeatureSetCombination.
The FeatureSetCombinationId of a FeatureSetCombination is the position of the FeatureSetCombination in the featureSetCombinations list (in UE-NR-Capability or UE-MRDC-Capability).
- The FeatureSetCombinationId = 0 refers to the first entry in the featureSetCombinations list (in UE-NR-Capability or UE-MRDC-Capability).
The BandCombination entries in the BandCombinationList then indicate the ID of the FeatureSetCombination that the UE supports for that band combination.
Step-4: Linking a Feature Set Combination to a Band Combination
The Band Combination entries in the Band Combination List indicate the ID of the Feature Set Combinationthat the UE supports for that band combination. As shown in the below BandCombination IE structure, FeatureSetCombinationId is assigned to featureSetCombination IE.
BandCombination | |
bandList | List of BandParameters for all bands in this band combination |
featureSetCombination | FeatureSetCombinationId |
ca-ParametersEUTRA | CA-ParametersEUTRA |
ca-ParametersNR | CA-ParametersNR |
mrdc-Parameters | MRDC-Parameters |
. . . | |
Linking of a Feature Set Combination to a Band Combination is illustrated in the below example.
3 UE Radio Capability Size estimation
For LTE, the primary overhead introduced by Rel-15 parameters is FeatureSetsEUTRA-r15. The theoretical overhead of FeatureSetsEUTRA-r15 can exceed 10 kbytes.
In practical deployment, the size of featureSetsEUTRA-r15 is variable depending on the number of DL/UL feature sets per band and the number of DL/UL feature sets per CC, in some cases the size offeatureSetsEUTRA-r15 can be several kbytes, and the total size of UE-EUTRA-Capability can be ~4 kbytes with realistic assumptions.
For NR and EN-DC, based on the newly introduced structure for UE capability in Rel-15, the size of the UE capability is dominated by rf-Parameters, featureSetCombinations and featureSets. The estimated theoretical maximum size of UE radio capabilities for NR or EN-DC (based on the maximum sizes in ASN.1) can approximate to 10000 kbytes,
In practical deployment, the supported bands/band combinations for a device is limited, and the eNB/gNB can for example request the UE to provide UE radio capabilities for a restricted set of band combinations. According to LTE experience and analysis of the NR capability, the estimated practical size of rf-Parametersis about 11~12 kbytes based on ~1024 band combinations and up to 4 bands per band combination.
The size of featureSetCombinations is highly variable depending on the number of feature set combinations and the number of feature sets per band, the size of featureSets is variable depending on the number of DL/UL feature sets per band and the number of DL/UL feature sets per CC. However, in some cases either the size of featureSetCombinations or featureSets can significantly exceed the size limitation of PDCP PDU. For example, with 256 feature set combinations, 8 feature sets per band, and 4 bands per band combination, the feature set combination list occupies ~22 kbytes.
