PDCCH order 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 network can force the UE to trigger RACH when it detects that UE is out-of-sync in Downlink. So in that scenario UE 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.
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.
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 case of Contention based Random Access(CBRA) procedure.
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.Tthis 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. The Below table indicates the PRACH Mask index values.
Reserved bits – 12 bits for operation in a cell with shared spectrum channel access; otherwise 10 bits.
- Network detects UE is Out of Sync
- Network sends PDCCH order to UE
- 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
- UE Completed RACH procedure
- Network Reconfigures all the IE’s to UE.
