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 PDUIt  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 Table for DL-SCH

 LCID values for UL-SCH channel.

LCID Table for UL-SCH



MAC-CE and reference:




Buffer Status Report

38.321 -


38.321 -

UE Contention Resolution Identity

38.321 -

Timing Advance Command

38.321 -

DRX Command

38.321 -

Long DRX Command

38.321 -

Configured Grant Confirmation

38.321 -

Single Entry PHR

38.321 -

Multiple Entry PHR

38.321 -

SCell Activation/Deactivation

38.321 -

Duplication Activation/Deactivation

38.321 -

SP CSI-RS / CSI-IM Resource Set Activation/Deactivation MAC CE

38.321 -

Aperiodic CSI Trigger State Subselection MAC CE

38.321 -

TCI States Activation/Deactivation for UE-specific PDSCH MAC CE

38.321 -

TCI State Indication for UE-specific PDCCH MAC CE

38.321 -

SP CSI reporting on PUCCH Activation/Deactivation MAC CE

38.321 -

SP SRS Activation/Deactivation MAC CE

38.321 -

PUCCH spatial relation Activation/Deactivation MAC CE

38.321 -

SP ZP CSI-RS Resource Set Activation/Deactivation MAC CE

38.321 -

Recommended bit rate MAC CE

38.321 -




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

Power Headroom MAC control element
Activation/De-activation MAC Control Element : 
- The Ci 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

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