LTE random access procedure is used by the UEs to initiate a data transfer. The UEs also obtain uplink timing information from the initial handshake.
This sequence diagram describes the tale of three UEs (UE-A, UE-B and UE-C) that are powered on at the same time:
- UEs synchronize with the downlink channel by decoding the PSS and SSS signal. The UEs are synchronized to the downlink frames after completing this procedure.
- The three UEs initiate the random access procedure at exactly the same time. Two of them (UE-A and UE-B) happen to pick the same preamble. This results in a resulting in a collision. UE-C picks a distinct preamble so it succeeds in the random access procedure.
- Contention between UE-A and UE-B is resolved in UE-A’S favor. UE-A proceeds with the RRC connection.
- UE-C times out and retries the random access procedure.