What is the use of PUCCH format 3 in LTE?

Question

In the LTE network, as per my knowledge only SRB1 and SRB2 two signalling radio bearers were supported for an UE. SRB1 mainly used to RRC messages and optionally NAS messages if SRB2 (optional) is unavailable. If SRB2 is available, NAS messages are transported between UE and MME using SRB2 signalling bearer.
Now I want to know what is the use of another SRB3 ? Is it established along with SRB1 and SRB2 every time or it is optional and established only on specific cases/scenarios ?

Answer 1

Bit tricky question, but as far as i know PUCCH format 3 is being used for CA mode for sending HARQ of secondary cells on primary cells

Comments

PUCCH Format 3 is relatively new format that is mainly used for carrier aggregation which uses 3 or more component carriers.

Answer 2

Two new PUCCH formats are introduced for Carrier Aggregation.
1. Format 1B for upto 4-bit HARQ ACK with "Channel Selection" - Rel 10 (up to 2 Components Carrier)
2. Format 3 can support up to 5 Components Carrier. Rel (13,14)