Random access procedureV15.11.0

Prior to initiation of the physical random access procedure, Layer 1:

  • receives from higher layers a set of SS/PBCH block indexes and provides to higher layers a corresponding set of RSRP measurements
  • receives the following information from the higher layers
    • Configuration of physical random access channel (PRACH) transmission parameters (PRACH preamble format, time resources, and frequency resources for PRACH transmission).
    • Parameters for determining the root sequences and their cyclic shifts in the PRACH preamble sequence set (index to logical root sequence table, cyclic shift ( CS N ), and set type (unrestricted, restricted set A, or restricted set B)).

From the physical layer perspective, the L1 random access procedure includes:

  1. transmission of random access preamble (MSG1) in a PRACH
  2. random access response (RAR) message with a PDCCH/PDSCH (MSG2)
  3. when applicable, transmission of a PUSCH scheduled by a RAR UL grant
  4. when applicable, PDSCH for contention resolution

If a random access procedure is initiated by a PDCCH order to the UE, a PRACH transmission is with a same SCS as a PRACH transmission initiated by higher layers.

If a UE is configured with two UL carriers for a serving cell and the UE detects a PDCCH order, the UE uses the UL/SUL indicator field value from the detected PDCCH order to determine the UL carrier for the corresponding PRACH transmission.


Contention-based vs Contention-free Random Access

There are two categories of Random Access procedure:

  • Contention Based Random Access (CBRA): allows the UE to select a random access preamble from a pool shared with other UEs. There are risks that multiple UEs may select the same preamble. What happens when multiple UEs select the same preamble:
    • these UEs will decode the same content from RAR (MSG2)
    • these UEs will transmit MSG3 using the same set of RBs and symbols
    • BS will decode one of those MSG3, and will complete contention resolution:
      • if MSG3 included a CCCH message: contention resolution achieved based on MAC CE within MSG4
      • if MSG3 indluded a DCCH message or DTCH data: conteition resolution achieved addressing the UE on the PDCCH by its C-RNTI
    • One UE will complete the Random Access procedure, remaining UEs will continue the procedure by selecting another preamble
  • Contention Free Random Access (CFRA): base station allocates a dedicated random access preamble beforehand, thus, ensures different UEs use different preambles. The preamble is provided using either RRC signaling by ra-PreambleIndex, or Layer 1 signaling (within DCI on the PDCCH)
RA contention based
RA contention free

Table below sumarizes the possible cases of random access, and the choices of contention resolution types. Note that, Contention-based Random Access can be used in all the cases.

Possible casesContention-basedContention-freeComments
Initial access from RRC Idle state-UE: send RRCSetupRequest in MSG3
Transition from RRC Inactive to RRC Connected-UE: send RRCResumeRequest in MSG3
RRC Connection re-establishment-UE: send RRCReestablishmentRequest in MSG3
HandoverBS: provide preamble using RACH-ConfigDedicated within RRCReconfiguration
UE: send RRCReconfigurationComplete in MSG3
Downlink data arrival while UE is out of syncBS: signal a PDCCH-Order using DCI format 1_0, with (contention-free) or without (contention-based) preamble index
UE: for contention resolution, include a C-RNTI MAC CE in MSG3
Uplink data arrival while UE is out of sync-UE: for contention resolution, include a C-RNTI MAC CE in MSG3
Uplink data arrival for UE without PUSCH resource allocation-UE: for contention resolution, include a C-RNTI MAC CE in MSG3. MSG3 may include a BSR MAC CE to request additional uplink resource
On-demand system information✓ (MSG3 based)✓ (MSG1 based)
Beam failure recoverContention-based: UE selects a PRACH preamble corresponding to the SSB beam for recovery
Contention-free: BS provide UE with one PRACH preamble index for each beam available for recovery
Scheduling request failure-UE use Random Access if BS does not provide an uplink grant after sr-TransMax (4 to 64) Scheduling Requests
Synchronous reconfigurationBS: trigger synchronous reconfiguration by including reconfigurationWithSync within RRCReconfiguration
UE: perform contention-based (ra-PreambleIndex excluded from reconfigurationWithSync) or contention-free (ra-PreambleIndex included in reconfigurationWithSync) Random Access
Establishing time alignment during Scell additionused to initialize Timing Advance of a newly added SCell belong to a new Timing Advance Group (TAG).
Contention-based on contention-free depends on if ra-PreambleIndex is excluded from reconfigurationWithSync