5G NR MSG3-PUS* According to the 5G NR CBRA (Contention Based Ransom Access) procedure, four messages are exchanged between the UE and the gNB as follows :
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MSG1
- UE randomly selects a Preamble within the pool of preambles signalled by the gNB and calculates the RA-RNTI (Random Access Radio Network Temporary Identity) based upon the information provided and signalled by the gNB for a selected RO(RACH Occasion). The UE also calculates the initial power and finally sends the preamble as MSG1 via PRACH. The Same preamble can be selected by multiple UEs in for a particular RO.
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MSG2
- The gNB first schedules the PDSCH including the RAR by PDCCH (Physical Downlink Control Channel) scrambled by RA-RNTI and then sends the RAR(Random Access Response) message using PDSCH to the UE(s).
- In the RAR message, the gNB prepares the Timing Advance, UL-Grant and TC-RNTI (Temporary Cell Radio Network Temporary Identity). The UL-Grant includes Frequency hopping flag, Msg3 PUSCH frequency resource allocation, Msg3 PUSCH time resource allocation, MCS (Modulation and Coding Scheme), TPC (Transmit Power Control) command for Msg3 PUSCH and CSI (Channel State Information) request.
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MSG3
- The RRC layer of the UE(s) prepares the RRC Setup Request message(48 bits) containing the UE ID and then passes it down to the MAC sub-layer through the RLC sub-layer. The MAC sublayer prepares a TB(Transport Block) based upon the UL Grant information received from the RAR message and then sends it to the physical layer through the transport channel for further processing. Finally, the MSG3 is sent to the gNB using PUSCH in the 5G NR air interface.
- Note: The PUSCH for MSG3 is scrambled using T(C)-RNTI
- The RRC layer of the UE(s) prepares the RRC Setup Request message(48 bits) containing the UE ID and then passes it down to the MAC sub-layer through the RLC sub-layer. The MAC sublayer prepares a TB(Transport Block) based upon the UL Grant information received from the RAR message and then sends it to the physical layer through the transport channel for further processing. Finally, the MSG3 is sent to the gNB using PUSCH in the 5G NR air interface.
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MSG4
- The gNB prepares the MSG4 containing the UE Contention Resolution Identity and sends it to the UE(s) after scheduling it using PDCCH scrambled by TC-RNTI.
Here I try to depict how the MSG3 is prepared and sent using PUSCH through 5G NR air interface from the perspective of the protocol stack.CH scheduled Transmission by RAR UL grant in the CBRA procedure.
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