PDCCH Order Triggered RACH

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What is PDCCH Order ?

PDCCH order is a technique used by base station eNB/gNB to force UE to trigger the RACH procedure. There are many reasons, why UE can initiate a RACH procedure and PDCCH Order is one out of those. Base station takes PDCCH Order decision when it founds the UE is out-of-sync in Downlink and Downlink (DL) data is available for the UE.

Under this out-of-sync condition, the base station initiate 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 information. If the gNB find that there is DL Data to be sent to UE in its MAC Layer Buffer and there is a Uplink (UL) synchronization because of the Time Alignment (TA) timer expiry at UE, then the gNB triggers a PDCCH order in order to UL re-synchronize with the UE.

Key Bullets

  • Both 5G and 4G have PDCCH order technique for UL synchronization
  • In 5G, DCI Format 1_0 is used by gNB to triggers PDCCH order on the SSB beam index UE is camped
  • RACH trigger by PDCCH Order use dedicated RACH preamble to enable Non-Contention Base RACH
  • In a condition of an ongoing RACH procedure triggered by a PDCCH order while the UE receives another PDCCH order indicating the same RACH Preamble, PRACH mask index, and uplink carrier, the RACH procedure is considered as the same RACH procedure as the ongoing one and not initialized again
  • When UE is with two or more Uplink secondary Carriers, 3GPP specifies that the Random Access procedure on an SCell shall only be initiated by a PDCCH order with ra-PreambleIndex different from 0b000000

RACH by PDCCH Order Call Flow

How PDCCH order call flow works in 5G

    • UE Out-of-Sync is detected at gNB
    • gNB sends PDCCH order to UE with dedicated RACH preamble
    • PDCCH order is sent via DCI Format 1_0 with C-RNTI CRC scrambled, containing SSB index, ra-PreambleIndex  and  PRACH Mask Index
    • When using 2-step RACH procedure, PDCCH order contains, containing SSB index, ra-PreambleIndex  and msgA-SSB-SharedRO-MaskIndex
    • UE triggers RACH procedure as per the received information
    • If no dedicated RACH preamble 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 and gNB Reconfigures all the IE’s to UE

PDCCH Order DCI 1_0 Details

  • DCI Format 1_0: This DCI used to schedule PDSCH to the UE, 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. 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 Format 1_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

DCI 1_0 Content for PDCCH Order

    • Random Access Preamble index: It contains 6 bits which indicates which RACH 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 RACH. Non-zero values are used to allocate the dedicated Prach index from 1 to 63 to the UE.
    • UL/SUL indicator: This field contains 1 bit. If the UE is configured with supplementary Uplink  in ServingCellConfig in the cell, this field indicates which UL carrier in the cell to transmit the PRACH according. Following table indicates the Value of UL/SUL indicator.
  • PDCCH Order Supplementary DL -UL
    • SS/PBCH index: It contains 6 bits which indicates the SS/PBCH that shall be used to determine the RACH occasion for the PRACH transmission.
    • PRACH Mask index: It contains 4 bits. In the case “Random Access Preamble index” value 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’ means a dedicated Preamble index has been assigned by the network to the UE.

PDCCH Order RACH Configuration related to SSB

    • Reserved bits: 12 bits reversed when a cell is operational with shared spectrum channel access; otherwise 10 bits.

References 

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

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