In this chapter we shall see in detail of 5G NR MAC PDU for RACH.
After sending Msg1, UE will monitor PDCCH window and then PDCCH will indicate the location of PDSCH and PDSCH will contain RAR.
Msg2 is a RAR message.
We will discuss MAC PDU for 2 step RACH in this chapter only.
MAC PDU for Random Access Response [4 step RACH]
A MAC PDU consists of one or more MAC subPDUs and optionally padding. Each MAC subPDU consists one of the
following:
– a MAC subheader with Backoff Indicator only;
– a MAC subheader with RAPID only (i.e. acknowledgment for SI request);
– a MAC subheader with RAPID and MAC RAR
It means MAC subPDU or only one BI subhearder, or only one RAPID subheader, or both RAPID subheader and RAR.
BI subheader may or may not be present, as it is used to indicate the backoff time after Msg2 reception fails.
Every MAC PDU follows below rules:
1. A MSC subPDU with BI is placed at the beginning of the MAC PDU if included.
2. Padding is placed at the end of the MAC PDU if present.
3. MAC subPDU with RAPID only and MAC subPDU(s) with RAPID and MAC RAR can be placed anywhere between MAC subPDU with BI and padding.
Below image indicates BI subheader and RAPID subheader
A MAC subheader with Backoff Indicator consists of five header fields E/T/R/R/BI
A MAC subheader with RAPID consists of three header fields E/T/RAPID as described in Figure 6.1.5-2.
E(Extension): Used to indicate whether this MAC subPDU is the last one in a string.
– set to “1” to indicate at least another MAC subPDU follows.
– set to “0” to indicate that this MAC subPDUis the last MAC subPDU;
T(Type): Used to indicate whether the MAC subheader is RAPID subheader or BI subheader.
– set to “0” to indicate the presence of a BI field in the subheader (BI).
– set to “1” to indicate the presence of a RAPID field in the subheader (RAPID);
R: reserved bit, set to “0”;
BI: Backoff Indicator. The size of the BI field is 4 bits;
RAPID: The Random Access Preamble IDentifier. The size of the RAPID field is 6 bits.
MAC payload for Random Access Response
R: reserved bit, set to “0”;
Timing Advance Command: This is the timing advance instruction, which tells the UE the timing advance information, a total of 12 bits;
UL Grant: Uplink scheduling indication, indicating the PUSCH information occupied by subsequent Msg3 transmissions, a total of 27 bits;
Temporary C-RNTI: The temporary C-RNTI allocated to the UE by the network. The UE that is not in the CONNECTED state does not have a C-RNTI. Therefore, the network needs to allocate a temporary C-RNTI to the UE. This temporary subsequent random access is successful. It will be upgraded to a real C-RNTI, a total of 16 bits.
UL grant:
RAR UL grant is used to schedule the transmission of Msg3 PUSCH, including the following:
TS 38.213:
Frequency hopping flag: If 1 then UE has to perform frequency hopping, else 0.
Msg3 PUSCH frequency resource allocation
Msg3 PUSCH time resource allocation
MCS: It indicates the modulation and coding strategy (refer to 38214 for details).
TPC command for Msg3 PUSCH: It indicates the transmission power of PUSCH. For details, please refer to [TS 38.213]
CSI request; It is reserved.
MAC PDU for MSGB [2 step RACH]
A MAC PDU consists of one or more MAC subPDUs and optionally padding. 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
Text and Image source TS 38.321