Question: Under what condition(s) is support for PUCCH (Physical Uplink Control Channel) format 4, as specified in TS 36.213, mandatory for a User Equipment (UE) in the context of carrier aggregation?
Option 1: For all UEs supporting any form of carrier aggregation.
Option 2: Only for UEs utilizing TDD (Time Division Duplexing) with up to 3 DL (Downlink) component carriers.
Option 3: Exclusively for FDD (Frequency Division Duplexing) UEs with a single DL component carrier.
Option 4: For UEs supporting TDD carrier aggregation with more than 5 DL component carriers, and potentially for FDD carrier aggregation under specific conditions.
Explanation: The correct answer is Option 4 because, according to the specifications, PUCCH format 4 support is mandatory for UEs of this release if they support TDD carrier aggregation with more than 5 DL component carriers. Additionally, there's a mention of a condition for FDD (though the exact number of DL component carriers is left unspecified as [FFS], implying further specification is needed), which aligns with the "potentially for FDD under specific conditions" part of Option 4, making it the most comprehensive and accurate choice.