All About MAC Control Elements

MAC Control Elements:

- Way of FAST Signaling Communication Exchange Between UE and eNodeB.

- Send as a part of MAC PDU.

- MAC control elements are always placed before any MAC SDU.

MAC Control Element Types:

• Buffer Status Report MAC Control Elements
• C-RNTI MAC Control Element
• DRX Command MAC Control Element
• UE Contention Resolution Identity MAC Control Element
• Timing Advance Command MAC Control Element
• Power Headroom MAC Control Element
• Extended Power Headroom MAC Control Element
• MCH Scheduling Information MAC Control Element
• Activation/Deactivation MAC Control Element

MAC CE Header:

- LCID field in MAC Subheader denotes MAC CE Type.

Values of LCID for DL-SCH:

Index	LCID values
00000	CCCH
00001-01010	Identity of the logical channel
01011-11010	Reserved
11011	Activation/Deactivation
11100	UE Contention Resolution Identity
11101	Timing Advance Command
11110	DRX Command
11111	Padding

Values of LCID for UL-SCH:

Index	LCID values
00000	CCCH
00001-01010	Identity of the logical channel
01011-11000	Reserved
11001	Extended Power Headroom Report
11010	Power Headroom Report
11011	C-RNTI
11100	Truncated BSR
11101	Short BSR
11110	Long BSR
11111	Padding

Buffer Status Report MAC Control Elements:

- Short BSR and Truncated BSR format :

- Long BSR format: 

- If extendedBSR-Sizes is not configured, the values taken by the Buffer Size field are in Table 6.1.3.1-1(3GPP TS 36.321). If extendedBSR-Sizes is configured, the values taken by the Buffer Size field are in Table 6.1.3.1-2(3GPP TS 36.321).

- Short BSR Header:

3D : MAC sub-header - Short BSR

R = 0

R = 0

E = 1

LCID = 11101 = Short BSR

1D : MAC sub-header - Short BSR

R = 0

R = 0

E = 0

LCID = 11101 = Short BSR

- Long BSR Header:

3E : MAC sub-header - Long BSR

R = 0

R = 0

E = 1

LCID = 11110 = Long BSR

1E : MAC sub-header - Long BSR

R = 0

R = 0

E = 0

LCID = 11110 = Long BSR

- Truncated BSR Header:

3C : MAC sub-header - Truncated BSR

R = 0

R = 0

E = 1

LCID = 11100= Truncated BSR

1C : MAC sub-header - Truncated BSR

R = 0

R = 0

E = 0

LCID = 11100= Truncated BSR

C-RNTI MAC Control Element Format : 

C-RNTI MAC control element

UE Contention Resolution Identity MAC Control Element :

UE Contention Resolution Identity MAC control element

- Has a fixed 48-bit size

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

3C : MAC subheader - Contention Resolution

R = 0

R = 0

E = 1

LCID = 11100 = Contention Resolution

Timing Advance Command MAC Control Element :

Timing Advance Command MAC control element

- 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/Deactivation MAC Control Element : 

- The C_i field is set to "0" to indicate that the SCell with SCellIndex i shall be deactivated.

Activation/Deactivation MAC control element

Padding MAC Sub-Header:

1F : MAC subheader - Padding

R = 0

R = 0

E = 0

LCID = 11111 = Padding

All About MAC PDU

MAC PDU: 
- A MAC PDU consists of'

• MAC Header
• Zero or More MAC SDU, 
• Zero or More MAC Control Elements and
• Optionally Padding.

- MAC header and MAC SDU are of variable SIZE.
- The MAC header and subheaders are octet aligned.

- A MAC PDU sub-header consists of the six header fields R/R/E/LCID/F/L.

- For the last sub-header in the MAC PDU, for fixed sized MAC control elements and Padding consist of the four header fields R/R/E/LCID.

- MAC Padding always placed at the end of the MAC PDU, with only exception when single-byte or two-byte padding is required.PDU subheaders have the same order as the corresponding MAC SDUs, MAC CE and Padding.
- A maximum of one MAC PDU can be transmitted per TB per UE.

MAC PDU consisting of MAC header, MAC control elements, MAC SDUs and padding