Question: In a UTRAN TDD system, upon receiving an RRC HANDOVER FROM UTRAN COMMAND with an activation time set to "now", how soon must the UE be ready to start transmitting on the new uplink PRACH channel, considering the maximum RRC procedure delay is 50 ms and interruption time is part of the overall handover delay (Dhandover)?
Option 1: Immediately after receiving the RRC command
Option 2: Within the interruption time stated in section 5.3b.2.2
Option 3: After the maximum RRC procedure delay (50 ms) but without adding any interruption time
Option 4: Within Dhandover seconds (sum of maximum RRC procedure delay plus interruption time) from the end of the last TTI containing the RRC command
Explanation: According to the specifications for handover in UTRAN TDD systems, when a UE receives a handover command with an immediate activation ("now"), it must be prepared to initiate transmission on the new uplink PRACH channel within a timeframe that accounts for both the processing of the RRC procedure (maximum delay of 50 ms) and the additional interruption time before the actual handover. This cumulative delay is defined as Dhandover, ensuring a seamless transition while respecting both protocol processing times and physical layer constraints.