Dear Vijay,
Physical random access procedure is triggered upon request of a PRACH transmission by higher layers or by a PDCCH order. A configuration by higher layers for a PRACH transmission includes the following:
- A configuration for PRACH transmission.
- A preamble index, a preamble SCS, PPRACH, target , a corresponding RA-RNTI, and a PRACH resource.
However below you can find some more details for PRACH 5G SA.
PRACH message sequence is as follows:-
- RA MSG1 contains the Random Access preamble and is sent by the UE on the P-PRACH channel in UL.
- RA MSG2 contains the MAC Random Access Response (RAR). It will be scheduled and sent to the UE using PDCCH (DCI1_0) and PDSCH (MAC RAR).
- RA MSG3 carries the RRC Setup Request message from the UE. After RA MSG3 has been received, gNB will send the Contention Resolution Identity (CRI) message via PDSCH scheduled in PDCCH using DCI 1_0.
- RA MSG4 carries the RRC Setup message from the gNB sent on PDSCH and scheduled in PDCCH using DCI 1_0.
- RA MSG5 carries the RRC Setup Complete message. DCI 0_1 grants are periodically provided for RA MSG5.
Below is the signalling diagram for PRACH NR_SA
• Preamble formats
NR PRACH uses 2 classes for formats. which support long LRA = 839 and short preamble LRA = 139 sequences.
• Preamble sequence
Format 0 to 3 use long preamble sequences
A0 to A3, B1 to B4 and C0 to C2 formats use short sequence
Important to node is that, The “current focus is on the short sequence, format A3 for high band and format B4 for mid band”.
More information on this can be found in 3gpp specs
Available Preamble Formats
• Preamble Sequence generation
The selection N_CS value should be based on the cell size. The configuration to support the maximum cell size is zero correlation zone configuration 0. In such case the zero-correlation zone length is L_RA, though N_CS is 0 in the zero-correlation zone configuration table. The table below illustrate the max cell size with different N_CS.
