MAC PDU Structure for non-HSPA Transport Channels

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This tutorial describes about the MAC PDU structure when HS-DSCH and E-DCH transport channel are not used. MAC PDUs are also called the MAC Transport Blocks (TB). MAC PDU is a bit string. The MAC PDU is consists of the MAC header and the MAC SDU (RLC PDU) The MAC header size and content depends on the type of logical channel used. The structure of the MAC PDU is as follows. clip image002 thumb1 MAC PDU Structure for non HSPA Transport Channels

 

 

 

 

 

 

NOTE: The MAC header may have none of the parameters in some cases.

MAC PDU Header Parameters

TCTF: Target Channel Type Field

TCTF field tells the type of logical channel mapped on a Transport Channel. The following combinations can be possible:

Transport Channel: FACH
For FDD Mode:

TCTF Designation
00 BCCH
01000000 CCCH
01000001-01001111 Reserved
01010000 MCCH
01010001-01011110 Reserved
01011111 MSCH
0110 MTCH
0111 Reserved
10000000 CTCH
10000001-10111111 Reserved
11 DCCH or DTCH over FACH
For TDD Mode

TCTF

Designation

000

BCCH

001

CCCH

010

CTCH

01100

DCCH or DTCH over FACH

01101

MCCH

01110

MTCH

01111

MSCH

100

SHCCH

101-111

Reserved

Transport Channel: RACH
Fore FDD Mode

TCTF

Designation

00

CCCH

01

DCCH or DTCH over RACH

10-11

Reserved

For TDD Mode

TCTF

Designation

00

CCCH

0100

DCCH or DTCH Over RACH

0101 – 0111

Reserved

10

SHCCH

11

Reserved

Transport Channel: USCH or DSCH (Only in TDD Mode)

TCTF

Designation

0

SHCCH

1

DCCH or DTCH over USCH or DSCH

C/T Field

The C/T field is used to identify the logical channel when multiple logical channels are mapped on the same transport channel. Example: In case of SRB (Signalling Radio Bearer), four SRBs are mapped on the same transport channel. C/T is 4 bit long.

C/T field

Designation

0000

Logical channel 1

0001

Logical channel 2

1110

Logical channel 15

1111

Reserved

UE-Id

UE-Id is used to identify a UE on common transport channel. There are mainly two types of identifiers are used:

U-RNTI: UTRAN Radio Network Temporary Identity

U-RNTI is always used in the downlink. The length of the U-RNTI is 32 bits. The U-RNTI is normally associated with connection to a Serving RNC (SRNC).

C-RNTI: Cell Radio Network Temporary Identity

The C-RNTI is used on DCCH or DTCH on uplink and may be used on DCCH on downlink. The length of the C-RNTI is 16 bits. The C-RNTI is normally associated with connection to a controlling RNC (CRNC).

UE-Id Type

The UE-Id type is used to ensure the correct decoding of the UE-Id field in the MAC header.

UE-Id Type field 2 bits

UE-Id Type

00

U-RNTI

01

C-RNTI

10

Reserved

11

Reserved

MBMS-Id

MBMS-Id is used to identify the MTCH for an MBMS service on FACH. MBMS-Id is always used on downlink.

MBMS-Id field

MBMS logical channel identity [7]

0000

1

0001

2

1110

15

1111

Reserved

Example

In case of Signalling Radio Bearer (SRB), four DCCH logical channels are multiplexed on one DCH transport channel. For SRBs the RLC PDU size is 144 bits.

MAC PDU

C/T

RLC PDU

C/T field can have the value from 0000 to 0011. clip image001 thumb MAC PDU Structure for non HSPA Transport Channels

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

Medium Access Control (MAC) protocol specification: 3GPP TS 25.321

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