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.
- Network detects UE is Out of Sync
- Network sends PDCCH order to UE with dedicated RACH premable
- PDCCH order is sent via DCI Format 1_0 with C-RNTI CRC scrambled , containing SSB index, ra-PreambleIndex and PRACH Mask Index
- In the Case of 2-step RACH procedure PDCCH order contains, containing SSB index, ra-PreambleIndex and msgA-SSB-SharedRO-MaskIndex.
- UE triggers RACH procedure with the information received
- If no dedicated RACH premamble was sent by network then UE will trigger CFRA procedure and send C-RNTI MAC CE in msg3 for contention resolution
- UE Completes RACH procedure
- Network Reconfigures all the IE’s to UE.
DCI Format 1_0 is typically used for scheduling PDSCH to a UE in a cell, DCI format 1_0 can be CRC scrambled by C-RNTI or CSRNTI or MCS-C-RNTI or P-RNTI or SI-RNTI or RA-RNTI. For DCI Format 1_0 CRC Scrambled by C-RNTI this can be used for normal PDSCH Scheduling by the UE. When PDCCH order is triggered with C-RNTI the network sets the Field “Frequency-domain resource assignment” in DCI with all ones.
When UE decodes the DCI Format1_0 with CRC scrambled by C-RNTI and the “Frequency domain resource assignment” field are of all ones, the DCI format 1_0 is for random access procedure initiated by a PDCCH order
UE then decodes below DCI Format 1_0 Fields
Random Access Preamble index: This field contains 6 bits. It indicates which Random access preamble to use in case of Contention Free Random Access(CFRA) or the value 000000 in the case of Contention based Random Access(CBRA) procedure. If the Preamble index bit is set as ‘0’ then the UE will trigger a contention-based random access procedure or else if the Preamble index is >0 then UE will trigger a Contention-free Random access procedure. Non-zero values are used to allocate the dedicated Prach index (0 to 63) to the UE.
UL/SUL indicator: This field contains 1 bit. If the UE is configured with supplementaryUplink in ServingCellConfig in the cell, this field indicates which UL carrier in the cell to transmit the PRACH according. The Below table indicates the Value of UL/SUL indicator.
SS/PBCH index: This field contains 6 bits. This field indicates the SS/PBCH that shall be used to determine the RACH occasion for the PRACH transmission.
PRACH Mask index: This field contains 4 bits. If the value of the “Random Access Preamble index” is not all zeros, this field indicates the RACH occasion associated with the SS/PBCH indicated by “SS/PBCH index” for the PRACH transmission. PRACH Mask index is applicable if the preamble index bit is not ‘0’ i.e a dedicated Preamble index has been assigned by the network to the UE.
The Below table indicates the PRACH Mask index values.
- A value of 0 indicates all PRACH ocassions are available
- A value of 1 to 8 indeicates PRACH ocassions 1 to 8 are available
- A value of 9 indicates even PRACH ocassions are available
- A value of 10 indicates even PRACH ocassions are available
Reserved bits: 12 bits for operation in a cell with shared spectrum channel access; otherwise 10 bits.
- 3GPP TS 38.321 NR; Medium Access Control (MAC) protocol specification
- 3GPP TS 38.214 NR; Physical layer procedures for data