The maximum number of LTE CA combinations that the UE can report in "UE Capability Information" message is limited to 128. Newer smartphones that are launched in the market are offering the capability to support 2CA, 3CA, 4CA & 5CA in combination with the capability to support 4x4 MIMO. Due to the increasing capability of the UE, the CA combination length goes beyond 128 which limits the network(eNB) from knowing all the CA combinations that the device could support. To overcome this we have a new feature called as "Dynamic UE Capability" which has been introduced in 3GPP release 11. However this method adds additional signalling
The network shall request additional LTE frequency band information from the UE only when UE satisfies the below conditions,
The number of supported CA Band Combination in "supportedBandCombination_r10" ≥ 128
UE should support "freqBandRetrieval-r11" by including this in UE Capability Information message.
Dynamic UE Capability
As per 3GPP TS 36.306 Release 12 the IE "freqBandRetrieval-r11" is a parameter which defines whether the UE supports reception of requestedFrequencyBands as specified in TS 36.331.
When the device is switched ON, it first register to LTE and the UE capabilities are shared to the network. If the IE "freqBandRetrieval-r11" is included and the number of reported CA combinations in "supportedBandCombination_r10" ≥ 128 then the network sends another UE Capability enquiry requesting band information for specific LTE FDD/TDD bands. The device shall include the following band information,
UE first reports all LTE non-CA bands
Followed by 2CA DL + 1UL band combinations for the requested bands
Also includes 3CA/4CA band combinations only for requested bands.
Finally may/may not include 2CA DL + 1UL band combinations for unrequested bands
For more information refer to 3GPP TS 36.331 section "18.104.22.168 Reception of the UECapabilityEnquiry by the UE".
However there are some drawbacks with this feature,
It takes additional ~40 msec for the UE to move from RRC_IDLE to RRC_CONNECTED state.
There is additional signalling which adds overhead on the network
In the below document I have shown an example of how a normal and additional "UE Capability Enquiry" from the network might look like.