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)

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.