Question: Upon receiving MsgB containing a fallbackRAR MAC subPDU in a 5G NR network, what action must the User Equipment (UE) take regarding the Random Access (RA) procedure?
Option 1: The UE continues with the current 2-step RA type, retransmitting msg3 with an updated payload.
Option 2: The UE falls back to the 4-step RA type, transmitting msg3 containing the payload of MsgA PUSCH.
Option 3: The UE skips msg3 transmission and directly proceeds to establish a dedicated connection.
Option 4: The UE restarts the entire RA process from the initial preamble transmission step.
Explanation: When the UE receives MsgB with a fallbackRAR MAC subPDU, it indicates a need to switch back to a more reliable, though slower, 4-step Random Access procedure. This ensures a stable connection establishment, hence the UE must fallback to the 4-step RA type and transmit msg3 with the payload of MsgA PUSCH as specified. This action prioritizes reliability over speed in the initial access phase, crucial for maintaining network stability and user experience.