5G(NR): MAC Control Elements

 MAC Control Elements:

- As we know that RRC and NAS messages functions are used to exchange the signalings between UE and gNodeB, But there are several communication path at MAC layer. so there are special MAC structures that carries special control information. These special MAC structure carrying the control information is called 'MAC CE', which means 'MAC Control Element'.

-

 -MAC CE works between UE(MAC) and gNodeB(MAC) for FAST Signaling Communication Exchange without  involving upper layers. 

- It is sent as a part of MAC PDU. It  always placed before any MAC SDUs.  LCID field denotes MAC CE Types.

There are several Type of MAC-CE, MAC CE List specified in 38.321 v15.3:

 

 

LCID VALUES FOR DL-SCH 

 LCID values for UL-SCH channel.

 

 

MAC-CE and reference:

 

MAC CE	Reference
Buffer Status Report	38.321 - 6.1.3.1
C-RNTI	38.321 - 6.1.3.2
UE Contention Resolution Identity	38.321 - 6.1.3.3
Timing Advance Command	38.321 - 6.1.3.4
DRX Command	38.321 - 6.1.3.5
Long DRX Command	38.321 - 6.1.3.6
Configured Grant Confirmation	38.321 - 6.1.3.7
Single Entry PHR	38.321 - 6.1.3.8
Multiple Entry PHR	38.321 - 6.1.3.9
SCell Activation/Deactivation	38.321 - 6.1.3.10
Duplication Activation/Deactivation	38.321 - 6.1.3.11
SP CSI-RS / CSI-IM Resource Set Activation/Deactivation MAC CE	38.321 - 6.1.3.12
Aperiodic CSI Trigger State Subselection MAC CE	38.321 - 6.1.3.13
TCI States Activation/Deactivation for UE-specific PDSCH MAC CE	38.321 - 6.1.3.14
TCI State Indication for UE-specific PDCCH MAC CE	38.321 - 6.1.3.15
SP CSI reporting on PUCCH Activation/Deactivation MAC CE	38.321 - 6.1.3.16
SP SRS Activation/Deactivation MAC CE	38.321 - 6.1.3.17
PUCCH spatial relation Activation/Deactivation MAC CE	38.321 - 6.1.3.18
SP ZP CSI-RS Resource Set Activation/Deactivation MAC CE	38.321 - 6.1.3.19
Recommended bit rate MAC CE	38.321 - 6.1.3.20

 

 

 

MAC CE Header:

- below are the MAC Control Elements of Buffer Status Report:

1- Short BSR and Truncated BSR format :

2- Long BSR format: 

Format of C-RNTI MAC Control Element: 

C-RNTI MAC control element fields

UE Contention Resolution Identity MAC Control Element :

UE Contention Resolution Identity MAC control element IEs

- it has fixed 48-bit size

- UE Contentions Resolution Identity: This field contains uplink CCCH SDU.

3C : MAC subheader - Contention Resolution:

R = 0

R = 0

E = 1

LCID = 11100 = Contention Resolution.

Timing Advanse Command MAC Control Element :

Timing Advance Command MAC control element

1- Timing Advance Command is of 6 bits in length. T_A (0, 1, 2… 63). 
Power Headroom MAC Control Element : 

Power Headroom MAC control element

Activation/De-activation MAC Control Element : 

- The C_i field is set to '0' to indicate that the SCell with SCellIndex i shall be de-activated.

Activation/De-activation MAC control elements

Padding MAC Sub-Header:

1F : MAC subheader - Padding

R = 0

R = 0

E = 0

LCID = 11111 = Padding