2-Step RACH : MsgB Reception-Part2

In the last article, I have discussed transmission and details related to 2-Step RACH MsgA transmission. In this article, we can explore more details related to the reception of MsgB in the 2-Step RA procedure.

2-Step RACH : MsgA Transmission

In this article we will try to explore and answer below questions

  • What is the content of MsgB ?
  • Is the Random access response (RAR) and msgB is same or are they two different messages?
  • When does the UE expect msgB reception?

For a UE to receive a Random access response from gNB, first it needs to send a RACH preamble, the Specifics of how and when the preamble transmission happens are discussed in Part-1 of the 2-Step RACH procedure.

In the case of 2-step RACH, Random access response and msgB are referred to the same message and there are no separate messages that are sent by gNB as a part of msg2. So RAR and msgB are referred to the same message.

msgB is a combination of Msg2 and Msg4

2-Step-RACH_call flow

In response to a transmission of a PRACH and a PUSCH i.e  msgA transmission, UE expects a Response from gNB containing MsgB information

So the First thing UE needs to know is where I will be receiving  DCI indication for msgB i.e in which CORESET and what will be the RNTI type for msgB ?

The UE does the below few steps

  • The UE starts a timer as indicated by msgB-ResponseWindow after the msgA transmission.
  • UE attempts to detect DCI Format 1_0 with CRC Scrambled with MsB-RNTI or CRNTI during the response window configured i.e. msgB-ResponseWindow.
  • The msgB-ResponseWindow starts at the first symbol where the UE is configured with a CORESET that is scheduled to receive PDCCH for Type1-PDCCH CSS set and ends after the number of slots has elapsed as configured in msgB-ResponseWindow.
    • The CORESET information is configured to the UE via RRC message i.e SIB1 in case of initial access or through an dedicated RRC message.
    • The Earliest CORESET that can be configured to receive after msgA-PUSCH transmission is at least one symbol. 
    • The length of the window in number of slots is provided by msgB-ResponseWindow which is based on the SCS for Type-1 PDCCH CSS set.

PDCCH reception with CRC Scrambled by CRNTI

If the UE detects a DCI Format 1_0 with CRC Scrambled by the corresponding CRNTI within the msgB-ResponseWindow and the received Transport Block is successfully decoded then the UE will

  • Process the received Timing Advance Command if the MAC PDU contains the Absolute Timing Advance Command MAC CE subPDU.
  • Consider the Random Access Response reception successful
  • Stop the msgB-ResponseWindow timer
  • Consider this Random Access procedure successfully completed and
  • Finish the disassembly and demultiplexing of the MAC PDU

PDCCH reception with CRC Scrambled by MsgB-RNTI

If the UE detects a DCI Format 1_0 with CRC Scrambled by the corresponding MsgB-RNTI within the msgB-ResponseWindow, UE does a few checks to see if the PDCCH containing DCI Format 1_0 is scheduled for the UE.

The UE Checks if the

  • LSBs of an SFN Field in the DCI Format 1_0 match with the LSBs of the SFN where the UE transmitted PRACH
  • Does the PDSCH contain a Transport Block?
  • Is the Transport block corresponding to the PDSCH received within the msgB-ResponseWindow?.

If All the above checks are OK , UE passes the transport block to upper layers.

Now the UE Can receive a successful RAR or an indication for fallback RAR, Once the TB is processed, the upper layers indicate that information to the Physical layer.

The UE checks

  • if the MSGB contains a MAC subPDU with Backoff Indicator
    • If Backoff Indicator is received, it sets PREAMBLE_BACKOFF to the value of the BI field of the MAC subPDU and
    • Applies if SCALING_FACTOR_BI is configured.
  • if the MSGB does not contain a Backoff Indicator, it sets set the PREAMBLE_BACKOFF to 0 ms.
  • If the msgB Contains a  fallbackRAR MAC subPDU and a random access preamble identity (RAPID) associated with the PRACH transmission is identified.
    • UE considers Random Access Response reception successful
    • Applies Timing Advance command
    • If the Random Access Preamble transmitted was not selected by the MAC entity i.e gNB indicated RA preamble and it is not from the contention-based Random Access Preamble(s), then UE considers the Random access procedure successful.
  • If the Random Access Preamble transmitted was selected by the MAC entity  and it is from the contention-based Random Access Preamble(s) then
    • It set the TEMPORARY_C-RNTI to the value received in the Random Access Response
    • UE switches to 4 Step RACH process and continues the RACH procedure until successful.
  • If the UE receives Successful RAR, it transmits a PUCCH HARQ-Ack, the PUCCH id to send the HARQ-ACK value is received in the RAR message.
  • If the UE is not able to correctly decode the transport block it sends a HARQ-NACK.

If the UE detects a DCI format 1_0 with CRC scrambled by the corresponding MsgB-RNTI and receives a transport block within the window in a corresponding PDSCH, the UE may assume the same DM-RS antenna port quasi co-location properties as for a SS/PBCH block the UE used for PRACH association, regardless of whether or not the UE is provided TCI-State for the CORESET where the UE receives the PDCCH with the DCI format 1_0

2-Step-RACH-MsgB-Reception

Details of PUCCH HARQ-Ack

  • UE Receives a PUCCH resource id in Successful RAR message, The PUCCH resource id is from a PUCCH resource set that is provided by pucch-ResourceCommon.
  • The slot in which PUCCH is to transmitted is indicated by a t in the Successful RAR message.
  • The PUCCH transmission should be the same active UL BWP as the last PUSCH transmission
  • The UE receives both time and frequency domain information to transmit PUCCH HARQ-Ack in the successRAR message.
  • But how early the PUCCH HARQ-Ack can be transmitted depends upon the PDSCH processing time.

The Minimum duration from the last symbol of PDSCH reception and when an HARQ-ACK can be sent on PUCCH  is given by NT,1 + 0.5 msec

NT,1 = PDSCH processing time for UE processing capability 1

The UE does not  transmits the PUCCH with the HARQ-ACK if

  • If the UE does not detect the DCI format 1_0 with CRC scrambled by the corresponding MsgB-RNTI within the window.
  • If the UE detects the DCI format 1_0 with CRC scrambled by the corresponding MsgB-RNTI within the window and LSBs of a SFN field in the DCI format 1_0 are not same as corresponding LSBs of the SFN where the UE transmitted the PRACH.
  • if the UE does not correctly receive the transport block in the corresponding PDSCH within the window
  • if the higher layers do not identify the RAPID associated with the PRACH transmission from the UE

The contents of msgB are discussed here in this another article.

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