The
Random Access procedure in 5G NR from the MAC layer perspective is thoroughly
discussed in the post ‘5G NR: RandomAccess Procedure’. The current post will discuss about MAC PDU structure and
its contents in detail.
MAC PDU for Random Access Response
As
shown in the example figure below, a MAC PDU for random access response consists
of one or more MAC subPDUs and optionally padding. Each MAC subPDU consists one
of the following:
-
a MAC subheader with
Backoff Indicator (BI) only
-
a MAC subheader with Random
Access Preamble IDentifier (RAPID) only
-
a MAC subheader with
RAPID and MAC RAR
A MAC subheader
consists of the following fields in general.
- E: The Extension
field is a flag indicating if the MAC subPDU including this MAC subheader
is the last MAC subPDU or not in the MAC PDU. The E field is set to
"1" to indicate at least one more 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 the MAC PDU
- T: The Type field
is a flag indicating whether the MAC subheader contains a RAPID or a BI (T
= 0 for BI and T = 1 for RAPID)
- R: Reserved bit,
set to "0"
- BI: The BI field
identifies the overload condition in the cell. The size of the BI field is
4 bits
- RAPID: This
field identifies the transmitted Random Access Preamble. The size of the
RAPID field is 6 bits (value range: 0 to 63)
Padding is
placed at the end of the MAC PDU if present. Presence and length of padding is
implicit based on TB size, size of MAC subPDU(s)
MAC
subPDU for Backoff Indicator only:
A MAC subPDU
with ‘Backoff Indicator only’ is placed at the beginning of the MAC PDU, if
included. It consists of a MAC subheader which contains five header fields E/T/R/R/BI as shown in figure
below.
MAC
subPDU for RAPID only:
The network
includes ‘MAC subPDU with RAPID only’ in the MAC PDU if the RA procedure was
triggered for SI request (On-demand SI) using CFRA procedure i.e., it is used as
an acknowledgment for SI request from the network.
'MAC subPDU(s)
with RAPID only' can be placed anywhere between MAC subPDU with Backoff
Indicator only (if any) and padding (if any). It consists of a MAC subheader which
contains three header fields E/T/RAPID
as shown in figure below.
MAC
subPDU for ‘MAC subheader with RAPID and MAC RAR’:
'MAC subPDU(s)
with RAPID and MAC RAR' consists of a RAPID subheader with fields E/T/RAPID and a MAC RAR.
A MAC subPDU with RAPID and MAC RAR payload is shown in the figure below.
As shown in the
figure above, the MAC RAR is of fixed size (7 octets) and consists of the
following fields.
-
Timing Advance Command (12-bits):
The Timing Advance Command field indicates the index value TA used
to control the amount of uplink transmission timing. The index values of TA
range: 0, 1, 2, ..., 3846.
-
UL Grant (27-bits): The Uplink Grant
field indicates the resources to be used on the subsequent uplink UL-SCH
transmission. UL Grant contents are presented in the figure below:
-
Temporary C-RNTI (16-bits):
The Temporary C-RNTI field indicates the temporary identity that is used by the
MAC entity during Random Access procedure. It can take values between 0001 and
FFEF. It is used for Msg3 transmission in the uplink and for the purpose of
Contention Resolution in the downlink (when no valid C-RNTI is available with
the UE)
'MAC subPDU(s)
with RAPID and MAC RAR' can be placed anywhere between MAC subPDU with Backoff
Indicator only (if any) and padding (if any).
If the RAPID in
the MAC subheader of a MAC subPDU corresponds to one of the RA Preambles
configured for SI request, MAC RAR is not included in the MAC subPDU.
MAC Control Elements (CE) used in Random Access Procedure
C-RNTI
MAC CE:
The C-RNTI MAC
CE is identified by MAC subheader with LCID: 58. UE includes this field in Msg3
if the UE already has a C-RNTI.
It is a fixed
size MAC CE which contains 16-bit C-RNTI as shown below.
UE
Contention Resolution Identity MAC CE:
The UE
Contention Resolution Identity MAC CE is identified by MAC subheader with LCID:
62.
It is a fixed
size MAC CE which 48-bit sized UE Contention Resolution Identity as shown
below.
UE Contention
Resolution Identity: This field contains the UL CCCH SDU. If the UL CCCH SDU is
longer than 48 bits, the network includes the first 48 bits of the UL CCCH SDU
for contention resolution purpose.
Reference:
3GPP 38.321 and 38.213
