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.
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
- 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
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.