2-Step RACH: msgB Contents information

In 2-Step RACH the msg2 i.e msgB Consists of a Random Access response and Contention resolution identity. It is a combination of msg2+msg4 from a 4-step RACH perspective.

The details of msgA transmission and msgB reception are discussed in another article mentioned below

msgB Contains a MAC PDU for Random Access response and Contention Resolution Identity MAC CE.

The Below image illustrates the MAC PDU Structure in 5G-NR. A 5G-NR MAC PDU contains one or more MAC SubPDU’s and each MAC SubPDU will contain its own header and Payload. The Header information indicates the type of MAC SubPDU and the payload information it carries.

5G-NR MAC PDU structure 2-Step RACH
5G-NR MAC PDU structure

Each MAC subPDU consists one of the following:

  • a MAC subheader with Backoff Indicator only;
  • a MAC subheader and fallbackRAR;
  • a MAC subheader and successRAR;
  • a MAC subheader and MAC SDU for CCCH or DCCH;
  • a MAC subheader and padding.

In the scope of this article, the MAC SubPDU which are related to 2-Step RACH random access reponse are discussed below.

MAC subheader with Backoff Indicator
  • A MAC subheader with Backoff Indicator consists of five header fields E/T1/T2/R/BI as shown in the figure below.
  • A MAC subPDU with Backoff Indicator only is always placed at the beginning of the MAC PDU if its present.
5G-NR 2-Step RACH MAC SubPDU Header for Back off indicator
5G-NR MAC SubPDU Header for Back off indicator
MAC subheader and fallbackRAR
  • A MAC subheader for fallbackRAR consists of three header fields E/T1/RAPID as shown in figure below.
5G-NR MAC SubPDU Header for Fall back RAR
5G-NR MAC SubPDU Header for Fall back RAR
MAC subheader and successRAR
  • A MAC subheader for successRAR consists of eight header fields E/T1/T2/S/R/R/R/R   as shown in figure below.
  • At most one ‘MAC subPDU for success RAR’ indicating presence of ‘MAC subPDU(s) for MAC SDU’ is included in a MAC PDU.
5G-NR 2-Step RACH MAC Subheader SucessRAR
MAC subheader for 2 Step RACH MSGB
E:
  • The Extension field is a flag indicating if the MAC subPDU including this MAC subheader is the last MAC subPDU.
  • The E field is set to “1” to indicate at least another MAC subPDU follows.
  • The E field is set to “0” to indicate that the MAC subPDU including this MAC subheader is the last MAC subPDU in this MAC Sub PDU.
T1
  • The T1 field is a flag indicating whether the MAC subheader contains a Random Access Preamble ID or T2
  • The T1 field is set to “1” to indicate the presence of a Random Access Preamble ID field in the subheader (RAPID)
  • The T1 field is set to “0” to indicate the presence of T2 field in the subheader
T2
  • The T2 field is a flag indicating whether the MAC subheader contains a Backoff Indicator (BI) or a MAC SDU indicator (S).
  • The T2 field is set to “0” to indicate the presence of a Backoff Indicator field in the subheader
  • The T2 field is set to “1” to indicate the presence of the S field in the subheader
S
  • This field indicates whether ‘MAC subPDU(s) for MAC SDU’ follow the MAC subPDU including this MAC subheader or not
  • The S field is set to “1” to indicate presence of ‘MAC subPDU(s) for MAC SDU
  • The S field is set to “0” to indicate absence of ‘MAC subPDU(s) for MAC SDU
R
  • Reserved bit, set to “0”
BI:
  • The Backoff Indicator field identifies the overload condition in the cell.
  • The size of the BI field is 4 bits
RAPID:
  • The Random Access Preamble IDentifier field identifies the transmitted Random Access Preamble ).
  • The size of the RAPID field is 6 bits.

An example of a msgB MAC PDU along with other MAC SDU’s is shown in the image below. One MAC PDU can contain multiple MAC SDU’s and also MAC SubPDU’s carrying msgB information.

2 Step RACH msgB MAC PDU

MAC payload for 2 Step RACH MSGB

The Random access response in 2-step RACH can contain a SucessRAR or a Fall Back RAR.

The Payload for Fallback RAR Contains the following fields:

  • R: Reserved bit, set to “0”;
  • Timing Advance Command: The Timing Advance Command field indicates the index value TA used to control the amount of timing adjustment that the MAC entity has to apply. The size of the Timing Advance Command field is 12 bits;
  • UL Grant: The Uplink Grant field indicates the resources to be used on the uplink. The size of the UL Grant field is 27 bits;
  • Temporary C-RNTI: The Temporary C-RNTI field indicates the temporary identity that is used by the MAC entity during Random Access. The size of the Temporary C-RNTI field is 16 bits.
5G-NR 2 Step RACH Fallback RAR  MAC Payload

The Payload for SuccessRAR Contains the following fields:

  • UE Contention Resolution Identity: This field contains the UL CCCH SDU. If the UL CCCH SDU is longer than 48 bits, this field contains the first 48 bits of the UL CCCH SDU.
  • R: Reserved bit, set to “0”;
  • ChannelAccess-Cpext: The field is only present when the MSGB HARQ feedback is to be transmitted with shared spectrum channel access.
    • The channel access type and CP extension for the PUCCH resource containing the HARQ feedback for MSGB in shared spectrum channel access
    • If its not a shared spectrum then , the field is not present and R bits are present instead.
    • . The size of the ChannelAccess-CPext field is 2 bits;
  • TPC: The TPC command for the PUCCH resource containing HARQ feedback for MSGB. The size of the TPC field is 2 bits;
  • HARQ Feedback Timing Indicator: The PDSCH-to-HARQ feedback timing indicator field for MSGB HARQ feedback. The size of the HARQ Feedback Timing Indicator field is 3 bits;
  • PUCCH Resource Indicator: The PUCCH resource indicator for HARQ feedback for MSGB. The size of the PUCCH resource Indicator field is 4 bits;
  • Timing Advance Command: The Timing Advance Command field indicates the index value TA used to control the amount of timing adjustment that the MAC entity has to apply. The size of the Timing Advance Command field is 12 bits;
  • C-RNTI: The C-RNTI field indicates the identity that is used by the MAC entity upon completion of Random Access. The size of the C-RNTI field is 16 bits.
5G-NR 2-Step RACH Successful RAR  MAC Payload
UE Contention Resolution Identity MAC CE
  • The UE Contention Resolution Identity MAC CE is identified by MAC subheader with LCID ’62’ as specified in 3GPP TS 38.321 Table 6.2.1-1 Which is shown below.
  • It has a fixed 48-bit size and consists of a single field i.e UE Contention Resolution Identity.
  • The UE Contention Resolution Identity contains the UL CCCH SDU
  • If the UL CCCH SDU is longer than 48 bits, this field contains the first 48 bits of the UL CCCH SDU
5G-NR 2-Step RACH
Timing Advance Command MAC CE
  • The Timing Advance Command MAC CE is identified by MAC subheader with LCID ’61’ as specified in 3GPP TS 38.321 Table 6.2.1-1 Which is shown above.
  • It has a fixed size and consists of a single octet.The Timing Advance Command MAC CE contains
Timing Advance Command MAC CE 2-Step RACH

TAG Identity (TAG ID):

  • This field indicates the TAG Identity of the addressed TAG.
  • The TAG containing the SpCell has the TAG Identity 0.
  • The length of the field is 2 bits;

Timing Advance Command:

  • This field indicates the index value TA (0, 1, 2… 63) used to control the amount of timing adjustment that MAC entity has to apply.
  • The length of the field is 6 bits.
  • Upon reception of a timing advance command for a TAG, the UE adjusts uplink timing for PUSCH/SRS/PUCCH transmission on all the serving cells in the TAG based on a  Timing advance value provided.
CRNTI MAC CE
  • The C-RNTI MAC CE is identified by MAC subheader with LCID a ’68’ as specified in 3GPP TS 38.321 Table 6.2.1-2.
  • It has a fixed size and consists of a single field i.e CRNTI
  • This field contains the C-RNTI of the MAC entity.
  • The length of the field is 16 bits.

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