PDCCH Order will be initiated by network to trigger a Random Access procedure.
This is initiated when there is an upstream loss.
When gNB has downstream data to send, PDCCH order will be used to initiate RACH and get up-stream synchronization.
PDCCH order uses PDCCH DCI 1_0 on SSB index UE is camped along with PRACH preamble and RACH occasion to configure the UE.
PDCCH order will be triggered by the network, if the network detects there is a DL data to be sent but there is an issue with UL synchronization because of Time Alignment timer expiry.
If there is on-going random access procedure that is triggered by PDCCH, and UE receives another PDCCH order with the same Random Access Preamble, PRACH mask index, and uplink carrier, then it is considered as the same and will not be initialized again.
PDCCH Order in 5G NR
When the UE in RRC_CONNECTED mode, and network detects that UE goes out of sync.
Network sends PDCCH order DCI 1_0 to UE with dedicated RACH preamble.
DCI Format 1_0 contains C-RNTI CRC scrambled, containing SSB index, ra-PreambleIndex and PRACH Mask Index
UE triggers RACH procedure and completes RACH procedure.
Then RRCConnectionReconfiguration message will be sent to UE.
DCI Format 1_0 will be used for PDCCH order
DCI format 1_0 is used for allocating downlink resources for PDSCH.
DCI format 1_0 with CRC scrambled by C-RNTI for PDCCH order:
When UE decodes the DCI format 1_0 with CRC scrambled by C-RNTI and the “Frequency domain resource assignment” are all 1s, then the random access procedure initiated by PDCCH order.