Question: In an inter-RAT handover scenario from E-UTRAN to NR, what is the primary factor that determines when a UE should start transmitting on the uplink PRACH in NR after receiving an RRC message?
Option 1: Immediately after decoding the RRC message, regardless of any specified delays.
Option 2: Based solely on `Tinterruption`, the time until the UE starts PRACH transmission after the last TTI in E-UTRAN.
Option 3: After a delay of `Dhandover` seconds, which is the sum of `TRRC_procedure_delay` (50 ms) and `Tinterruption`, following the end of the last TTI containing the RRC command.
Option 4: Once the UE completes its current uplink transmission in E-UTRAN, without any specific time constraint from the RRC message reception.
Explanation: The correct answer is Option 3 because, according to handover delay specifications for inter-RAT handovers, the UE must wait for `Dhandover` seconds before starting PRACH transmission in NR. This `Dhandover` period is specifically defined as the sum of `TRRC_procedure_delay` (a fixed 50 ms delay for RRC procedure completion) and `Tinterruption`, ensuring a coordinated handover process between radio access technologies.