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. Such special MAC structures carrying the control information's that 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 scheduled 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:

 

 

 

MAC-CE and reference:

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