WHAT IS THE CONTENTION RESOLUTION (RANDOM ACCESS PHASE 4)?

PHASE 4

By the time Random Access Phase 4 takes place Uplink synchronization is already established by the UE with eNodeB. In this phase, only a UE terminal which finds match between the identity received in the fourth step (Phase 4) and the identity transmitted as part of third step (Phase 3) will declare the random access procedure successful.

Hybrid ARQ (HARQ) is applied to the downlink signaling as in this step 4 (albeit with some special rules for the feedback signaling). The terminal which is successful in phase 4 of random access procedure also transmits a HARQ acknowledge in the uplink.

Special Note: For non-initial access, (i.e. when the UE is already RRC Connected), in Phase 4, the UE identity is reflected on the L1/L2 control channel. If the UE has not yet been assigned a C-RNTI, the temporary identity from the second step (Phase 2 ) is promoted to the C-RNTI, otherwise the UE keeps its already assigned C-RNTI.

WHAT IF A UE DOES NOT FIND ITS IDENTITY IN PHASE 4? OR WHAT IF CONTENTION RESOLUTION IS FAILED FOR A UE?

User terminals (UEs) which do not find a match between the identity received in Phase 4 and the respective identity transmitted as part of Phase 3 are considered to have failed the random access procedure and need to restart the random access procedure with Phase 1. They select new random access parameters for the preamble sequence and the RA frequency band.

No HARQ feedback is transmitted from these UEs (terminals).