In this article we will discuss about 5G NR RACH Procedure Overview 2024 – CBRA & CFRA. When we want to connect a UE to a 5G network, we need to synchronize it with uplink as well as downlink. After SSB is successfully decoded, downlink synchronization is available. In order to establish uplink synchronization and RRC connection, UE must access RACH randomly. You can get all information of these searches: Rach procedure in lte 2024, 5g msg1 msg2, msg3 msg4, Rach occasion 5g, Contention resolution in 5g, Contention-free random access, zerocorrelationzoneconfig 5g, 2-step rach cbra and cfra in lte.
5G NR RACH Procedure Overview 2024 – CBRA & CFRA
We are going to share 5g NR RACH Procedure overview – CBRA & CGRA in this article. If you are here to know about it you have to read this article carefully. Hope you will get the right information from our website below:
Types of RACH Procedure
1. Contention-based Random Access (CBRA):
2. Non Contention or Contention Free Random Access (CFRA)
Contention-based Random Access (CBRA): 5G NR RACH Procedure
In Contention Based Random Access, the UE randomly selects a proposal from a pool of suggestions shared with other UEs. This means that UEs have the potential risk of choosing the same proposal as other UEs and may later encounter conflicts or disputes. gNodeB uses a debate resolution method to handle such access requests. In this method, the results are random and not all random access is successful. CBRA is also known as the four step RACH method.
Step 1: Random Access Preamble Transmission (Msg1):
GNB transmits UE on proposal to the appropriate beam.
Step 2: Random access response (Msg2)
After receiving a proposal, gNodeB applied for a TC-RNTI and uplink and downlink scheduling resources. Then, gNodeB sends an RA response to the PDSCH. Responses include RA-prediction identifiers, timing alignment information, initial uplink grants, and temporary C-RNTI. A PDSCH can carry RA responses in multiple UEs. After the UE sends a proposal, it monitors PDCCH and waits for an RA response in an RA response window:
If the UE receives a response containing a RA-proposed identifier similar to the identifier contained in the transmitted RA proposal, the response is successful. UE then sends uplink schedule information.
If UE RA does not receive a response in the response window or fails to verify the response, the response fails. In this case, if the number of RA attempts is less than the upper limit (10), UE tries RA again. Otherwise, RA fails.
Step 3: Scheduled UL transmission (Msg3)
UE sends uplink scheduling information over PUSCH. The signal message and information sent by UE varies in different RA situations and some examples are listed here
- Initial RRC Connection Setup: The RRCSetupRequest message (carrying NAS UE_ID) is transmitted through the General Control Channel (CCCH) at the RLC level TM. The message is not split.
- RRC Connection Restore: RRC restoration request message (does not carry NSA message) is transmitted via CCCH at RLC level TM. The message is not split.
- Handover: Content-based RA, instead of non-content-based RA, is triggered if UE accesses the target cell and no dedicated offerings are found during the transfer. The RRC handover is transmitted via confirmation message and C-RNTI Dedicated Control Channel (DCCH). If necessary, a Buffer Status Report (BSR) is also carried.
- In other cases: At least C-RNTI of UE is transmitted.
Step 4: Dispute Resolution (Msg4)
After sending UE Msg3, a debate resolution timer of 4 ms starts. GNodeB assists UE in resolution resolution using C-RNTI in PDCCH or UE Content Resolution Identity in PDSCH using IE.
UE monitors the PDCCH before the timer expires and considers the resolution of the debate to be successful and stops the timer if any one of the following conditions is met: UE gets C-RNTI on PDCCH.
UE gets temporary C-RNTI on PDCCH and MAC PDU is successfully decoded. In particular, the UE Content Resolution Identity obtained on PDSCH is the same as that carried by IE UE in Msg3.
If the timer of the resolution of the dispute has expired, the resolution of the dispute has failed considering UE. Then, UE performs RA again if the number of RA attempts does not reach the upper limit (10). If the number of RA attempts reaches its upper limit, the RA method fails.
Contention Free Random Access (CFRA): 5G NR RACH Procedure Overview
In non-conflict based random access, the proposal is assigned by gNodeB and such suggestions are known as dedicated random access suggestions. Dedicated feedback is provided to the UE via RRC signaling (recommendations can be assigned in RRC messages) or PHY layer signaling (DCI in PDCCH). Therefore, there is no proposal dispute. When dedicated resources are insufficient, gNodeB instructs the UE to initiate debate-based RA. CFRA is also known as the three step RACH method.
Step 1: Random Access Preamble Assignment:
gNodeB assigns a RA prompt to the UE and sends it using RRC Msg or DCI. Some situations are listed here.
Handover: MobilityControlInfo IE Carries Proposed Proposals Posted by GNodeB Source
Arrival of DL data: When downlink data arrives at gNodeB, GNodeB instructs the UE to initiate RA via PDCCH via DCI, which carries the assigned proposition.
NSA Networking: When NR cells are added to the NSA, GNODB instructs the UE to initiate RA via PDCCH, which carries the assigned proposition.
Step 2: Random Access Preblem Transmission (Msg1)
Step 3: Random access response (Msg2)
gNodeB sends an RA response.
- Handover: RA responses must include time alignment information and initial uplink grants.
- Arrival of DL Data: When the downlink data reaches gNodeB, the RA response must contain timing alignment information and RA Recommendation Identifier (RAPID).
- NSA Networking: When NR cells are added to the NSA, the RA response must contain timing alignment information and the RA Recommendation Identifier (RAPID).
RACH Scenarios and RACH Type: 5G NR RACH Procedure Overview – CBRA & CFRA
- Case 1 : Initial RRC connection setup
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- Contention Based Random Access
- Case 2: RRC connection re-establishment
- Contention Based Random Access
- Case 3: Handover
- First choice is to choose Non-Contention Based Random Access
- CBRA can be used if all dedicated parables are already being used
- Case 4: Downlink data arrival (PDCCH Order)
- First choice is to choose Non-Contention Based Random Access.
- CBRA can be used if all dedicated parables are already being used
- Case 5: Uplink data arrival
- Contention Based Random Access
- Case 6: Uplink L data arrival during RRC_CONNECTED when no PUCCH resources for SR available
- Contention Based Random Access
- Case 7: Scheduling Request Failure
- Contention Based Random Access
- Case 8: Transition form RRC_INACTIVE to RRC_CONNECTED
- First choice is to choose Non-Contention Based Random Access
- CBRA can be used if all dedicated parables are already being used
- Case 9: Request for Specific System Information (On Demand SI)
- Non-Contention Based Random Access
- Case 10: NR cell addition for NSA networking
- Non-Contention Based Random Access
- Case 10: Beam Recovery
- Non-Contention Based Random Access
- CBRA can be used if all dedicated parables are already being used
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References:
- 3GPP TS 38.211 NR; Physical channels and modulation
- 3GPP TS 38.221 NR; Medium Access Control (MAC) protocol specification
- 3GPP TS 38.300 NR; NR and NG-RAN Overall description; Stage-2