PDCCH order in 5G NR

What is a PDCCH Order

PDCCH order in 5G NR is a way of network instructing the UE to trigger a Random access Procedure. Typically RACH is triggered by UE and there are a number of reasons why RACH can be triggered by UE. But the network can force the UE to trigger RACH when it detects that UE is out-of-sync in Downlink. So in that scenario, the network triggers a PDCCH order by sending a DCI Format 1_0 on the SSB beam index UE is camped along with a PRACH preamble and RACH occasion.

PDCCH order is one of the reasons for RACH trigger in both LTE and 5G-NR. If the network detects that there is DL Data to be sent to UE in its MAC Buffer and there is a UL synchronization issue due to the expiry of the Time Alignment timer at UE, then the network triggers a PDCCH order in order to re-synchronize with the UE.

If UE is configured with two or more Uplink secondary Carriers then 3GPP specifies that the Random Access procedure on an SCell shall only be initiated by a PDCCH order with ra-PreambleIndex different from 0b000000.

If there was an ongoing Random Access procedure that is triggered by a PDCCH order while the UE receives another PDCCH order indicating the same Random Access Preamble, PRACH mask index, and uplink carrier, the Random Access procedure is considered as the same Random Access procedure as the ongoing one and not initialized again.

PDCCH Order Process Flow

1. Network detects UE is Out of Sync.
2. Network sends PDCCH order to UE with dedicated RACH preamble.
3. PDCCH order is sent via DCI Format 1_0 with C-RNTI CRC scrambled, containing SSB index, ra-PreambleIndex, and PRACH Mask Index.
4. In the case of 2-step RACH procedure, the PDCCH order contains the SSB index, ra-PreambleIndex, and msgA-SSB-SharedRO-MaskIndex.
5. UE triggers RACH procedure with the information received.
6. If no dedicated RACH preamble was sent by the network, then UE will trigger a CBRA procedure and send C-RNTI MAC CE in msg3 for contention resolution.
7. UE Completes RACH procedure.
8. Network Reconfigures all the IEs to UE.

DCI Format 1_0 for PDCCH Order

DCI Format 1_0 is typically used for scheduling PDSCH to a UE in a cell. It can be CRC scrambled by C-RNTI, CS-RNTI, MCS-C-RNTI, P-RNTI, SI-RNTI, or RA-RNTI. When a PDCCH order is triggered with C-RNTI, the network sets the field "Frequency-domain resource assignment" in the DCI to all ones.

When the UE decodes DCI Format 1_0 with CRC scrambled by C-RNTI and finds the "Frequency domain resource assignment" field consists of all ones, it recognizes the DCI as a random access procedure initiation (PDCCH order).

Key DCI Fields for PDCCH Order

• Random Access Preamble index (6 bits): Indicates which preamble to use for Contention Free Random Access (CFRA). A value of 000000 triggers a Contention-Based Random Access (CBRA) procedure. Non-zero values (1–63) allocate a dedicated PRACH index.
• UL/SUL indicator (1 bit): If the UE is configured with supplementaryUplink, this field indicates which UL carrier to use.

• SS/PBCH index (6 bits): Indicates the SS/PBCH block used to determine the RACH occasion.
• PRACH Mask index (4 bits): If the preamble index is non-zero, this field indicates the specific RACH occasion associated with the SSB index.

• Reserved bits: 12 bits for shared spectrum (NR-U); otherwise 10 bits.

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FAQs

What is the PDCCH order in 5G NR? It is a command sent by the gNB to the UE to initiate a Random Access Procedure, typically to restore uplink synchronization.

When is it used? When the network has DL data but the UE's Time Alignment timer has expired, or when moving a UE to an SCell.

What happens if a RACH is already ongoing? If the new PDCCH order matches the preamble and carrier of the ongoing RACH, the UE continues the existing process without restarting.

References:

• 3GPP TS 38.321: NR Medium Access Control (MAC) protocol specification
• 3GPP TS 38.214: NR Physical layer procedures for data

Further Reading

• RACH Failure Reasons in 5G NR
• 2-Step RACH in Release 16