2-Step RACH MsgB Reception (Part 2)

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

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:

1. The UE starts a timer as indicated by msgB-ResponseWindow after the msgA transmission.
2. UE attempts to detect DCI Format 1_0 with CRC Scrambled with MsB-RNTI or C-RNTI during the response window configured i.e. msgB-ResponseWindow.
3. 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.
4. 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.
5. The Earliest CORESET that can be configured to receive after msgA-PUSCH transmission is at least one symbol.
6. 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 C-RNTI

If the UE detects a DCI Format 1_0 with CRC Scrambled by the corresponding C-RNTI 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.

Further Reading

• 2-Step RACH in Release 16 of 5G NR (Part 1)
• Common Causes of MSG2 RACH Failure
• RACH Failure Reasons in 5G NR
• 5G NR RACH Trigger Reasons with Details