Contention Resolution Identity (MSG4) in 5G NR RACH

Contention Resolution Identity (MSG4) in 5G NR RACH

What is Contention Resolution in 5G NR RACH?

The Contention Resolution Identity (MSG4) is a crucial part of the Random Access procedure in 5G NR. Contention resolution in 5G occurs when multiple UEs attempt to access the network simultaneously, causing potential conflicts in the allocation of resources. To prevent these conflicts, a mechanism is in place to identify and resolve contention between UEs. The process relies heavily on the MAC Control Element (MAC CE) within the Random Access procedure.

During this process, the UE continuously monitors the CORESET (Control Resource Set) within the Type 1 Common Search Space, as defined by the ra-SearchSpace, while the ra-ContentionResolutionTimer is running. This timer is a crucial part of the process, and the UE must manage it carefully to successfully complete the procedure.

The Role of MSG3 and MSG4

The MSG3 transmission is a pivotal part of the contention resolution process. If the UE used MSG3 to transfer a CCCH message (Common Control Channel), the UE will then attempt to decode a PDCCH (Physical Downlink Control Channel) transmission, with the CRC (Cyclic Redundancy Check) bits scrambled by the Temporary C-RNTI (C-Radio Network Temporary Identifier). This scrambling process ensures that the communication is secure and specific to the UE.

In response, the UE may receive one of two possible DCI (Downlink Control Information) formats:

• DCI Format 0_0: Allocates PUSCH (Physical Uplink Shared Channel) resources for the retransmission of MSG3.
• DCI Format 1_0: Allocates PDSCH (Physical Downlink Shared Channel) resources for MSG4.

The MSG4 transmission contains the UE Contention Resolution Identity MAC CE, a critical component in determining whether the Random Access procedure has been successful.

Contention Resolution Identity

The UE Contention Resolution Identity MAC CE plays a vital role in ensuring the success of the Random Access procedure. This MAC CE includes the first 48 bits of the uplink CCCH SDU (Service Data Unit) within MSG3, allowing the UE to compare the content of the MAC CE with the original CCCH SDU it transmitted.

Successful Contention Resolution

If the content of the MAC CE matches the original CCCH SDU sent by the UE, the Random Access procedure is considered successful. The Temporary C-RNTI becomes the allocated C-RNTI, and the UE proceeds to transmit a HARQ (Hybrid Automatic Repeat Request) acknowledgment for MSG4.

Unsuccessful Contention Resolution

If the content does not match, it indicates that contention has occurred. The MAC CE is intended for a different UE, and the current UE must revert to transmitting PRACH (Physical Random Access Channel) preambles to attempt a new connection.

Once the contention resolution is successful, the UE uses the PUCCH (Physical Uplink Control Channel) to transmit a HARQ acknowledgment for MSG4. However, at this stage, the UE does not have dedicated PUCCH resources. Instead, it relies on common resources defined by PUCCH-ConfigCommon, which are provided either via SIB1 (System Information Block 1) or through dedicated signaling.

The process of contention resolution can vary depending on the type of message transmitted in MSG3. For example:

• If MSG3 contains a DCCH (Dedicated Control Channel) or DTCH (Dedicated Traffic Channel) message, the UE will attempt to decode a PDCCH transmission scrambled by the C-RNTI included in MSG3. In this case, contention resolution is successful if the UE receives a PDCCH transmission scrambled by the correct C-RNTI.

Once MSG3 is transmitted, the MAC entity on the UE side initiates the ra-ContentionResolutionTimer. This timer is restarted after each HARQ retransmission in the first symbol following the end of the MSG3 transmission. The MAC entity must carefully manage the timer to ensure the correct handling of retransmissions and avoid contention.

UE Contention Resolution Identity MAC CE

The UE Contention Resolution Identity MAC CE is identified by a MAC subheader with LCID: 62.

It has a fixed 48-bit size and consists of a single field:

• UE Contention Resolution Identity: This field contains the UL CCCH SDU. If the UL CCCH SDU is longer than 48 bits, this field contains the first 48 bits of the UL CCCH SDU.

MSG4 and UE Contention Resolution Identity Process

General Process after Msg3 Transmission:

1. Start and restart the ra-ContentionResolutionTimer with each HARQ retransmission after Msg3.
2. Monitor the PDCCH while the timer is running, even during measurement gaps.

If PDCCH Transmission is Received:

• If C-RNTI MAC CE was included in Msg3:

  • Check specific cases (e.g., SpCell beam failure recovery, PDCCH order, new UL grant).
  • If conditions are met:
    • Mark contention resolution as successful.
    • Stop the timer and discard TEMPORARY_C-RNTI.
    • Consider Random Access procedure complete.

• If CCCH SDU was included in Msg3:

  • If PDCCH is addressed to TEMPORARY_C-RNTI and MAC PDU is decoded:
    • Stop the timer.
    • Verify UE Contention Resolution Identity.
    • If match: Successful contention resolution and Random Access completion.
    • If mismatch: Discard TEMPORARY_C-RNTI and consider resolution unsuccessful.

If ra-ContentionResolutionTimer Expires:

• Discard TEMPORARY_C-RNTI.
• Mark contention resolution as unsuccessful.

If Contention Resolution is Unsuccessful:

1. Flush HARQ buffer for Msg3.
2. Increment PREAMBLE_TRANSMISSION_COUNTER.
3. If counter exceeds preambleTransMax, report Random Access problem.
4. Handle unfinished Random Access procedures:
   • For 4-step RA: Select backoff time and perform resource selection.
   • For 2-step RA: Handle msgA-TransMax or select resources post-backoff.

References

• 3GPP TS 38.213: 5G NR; Physical layer procedures for control

Further Reading

• 5G NR RACH Process
• RNTI Values and Usage
• 2-Step RACH in Release 16