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.

References

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

Related posts:

1. UMTS Logical Channels and Transport Channels
2. MAC (Medium Access Control) Architecture (25.321)

Logical channels define ‘WHAT’ type of information is being transferred and transport channel defines ‘HOW’ the information is being transferred. So one of the primary objective of MAC is to map appropriate logical channels on transport channel.

NOTE: Before proceeding check out the different types of logical channels and transport channel here.

UMTS Logical Channels and Transport Channels

MAC Entities

MAC-b is the MAC entity that handles the following transport channels:

1. broadcast channel (BCH)

MAC-c/sh/m, is the MAC entity that handles the following transport channels:

1. paging channel (PCH)
2. forward access channel (FACH)
3. random access channel (RACH)
4. downlink shared channel (DSCH). The DSCH exists only in TDD mode.
5. uplink shared channel (USCH). The USCH exists only in TDD mode.

MAC-d is the MAC entity that handles the following transport channels:

1. dedicated transport channel (DCH)

MAC-hs is the MAC entity that handles the following transport channels:

1. high speed downlink shared channel (HS-DSCH)

MAC-m is the MAC entity that handles the following transport channels:

1. forward access channel (FACH).

MAC-e/es are the MAC entities that handle the following transport channels:

1. enhanced dedicated transport channel (E-DCH).

MAC Architecture – UE Side

MAC-c/sh/m entity – UE Side

There is one MAC-c/sh/m entity present in the UE side.

The following functionality is covered by this MAC entity.

TCTF MUX:

The TCTF field indicates the common logical channel type, or if a dedicated logical channel is used;

add/read UE Id:

1. the UE Id is added for RACH transmissions;

2. the UE Id, when present, identifies data to this UE

read MBMS Id:

1. the MBMS Id is read in case of MTCH reception;

2. the MBMS Id identifies received data to an MBMS service.

UL: TF selection:

in the uplink, the possibility of transport format selection exists.

ASC selection:

For RACH, MAC indicates the ASC associated with the PDU to the physical layer. This is to ensure that RACH messages associated with a given Access Service Class (ASC) are sent on the appropriate signature(s) and time slot(s). MAC also applies the appropriate back-off parameter(s) associated with the given ASC. When sending an RRC CONNECTION REQUEST message, RRC will determine the ASC; in all other cases MAC selects the ASC;

scheduling /priority handling

this functionality is used to transmit the information received from MAC-d on RACH based on logical channel priorities. This function is related to TF selection.

TFC selection

transport format and transport format combination selection according to the transport format combination set (or transport format combination subset) configured by RRC is performed.

MAC-m entity – UE Side

The following functionality is covered:

TCTF DEMUX:

this function represents the handling (detection and deletion for downlink channels) of the TCTF field in the MAC header, and the respective mapping between logical and transport channels. The TCTF field indicates the common logical channel type;

read MBMS Id

1. the MBMS Id is read in case of MTCH reception;

2. the MBMS Id identifies received data to an MBMS service.

MAC-d entity – UE Side

NOTE: There is one MAC-d entity in the UE side.

The following functionality is covered:

Transport Channel type switching

Transport Channel type switching is performed by this entity, based on decision taken by RRC. This is related to a change of radio resources. If requested by RRC, MAC shall switch the mapping of one designated logical channel between common and dedicated transport channels.

C/T MUX:

The C/T MUX is used when multiplexing of several dedicated logical channels onto one transport channel (other than HS-DSCH) or one MAC-d flow (HS-DSCH) is used. An unambiguous identification of the logical channel is included. If MAC-ehs is configured, C/T MUX toward MAC-ehs is not used.

Ciphering:

Ciphering for transparent mode data to be ciphered is performed in MAC-d.

Deciphering:

Deciphering for ciphered transparent mode data is performed in MAC-d.

UL TFC selection:

Transport format and transport format combination selection according to the transport format combination set (or transport format combination subset) configured by RRC is performed.

MAC-hs entity – UE Side

HARQ:

The HARQ entity is responsible for handling the MAC functions relating to the HARQ protocol. The HARQ functional entity handles all the tasks that are required for hybrid ARQ. It is responsible for generating ACKs or NACKs. The detailed configuration of the hybrid ARQ protocol is provided by RRC over the MAC-Control SAP. The maximum number of HARQ process per HS-DSCH per TTI on which an HS-DSCH transmission can be received is one.

Reordering Queue distribution:

The reordering queue distribution function routes the MAC-hs PDUs to the correct reordering buffer based on the Queue ID.

Reordering:

The reordering entity reorders received MAC-hs PDUs according to the received TSN. MAC-hs PDUs with consecutive TSNs are delivered to the disassembly function upon reception. MAC-hs PDUs are not delivered to the disassembly function if MAC-hs PDUs with lower TSN are missing. There is one reordering entity for each Queue ID configured at the UE.

Disassembly:

The disassembly entity is responsible for the disassembly of MAC-hs PDUs. When a MAC-hs PDU is disassembled the MAC-hs header is removed, the MAC-d PDUs are extracted and any present padding bits are removed. Then the MAC-d PDUs are delivered to higher layer.

MAC-e/es entity – UE Side

The MAC-es/e handles the E-DCH specific functions.

MAC-e/es comprise the following entities:

HARQ:

The HARQ entity is responsible for handling the MAC functions relating to the HARQ protocol. It is responsible for storing MAC-e payloads and re-transmitting them.

For FDD: The HARQ entity provides the E-TFC, the retransmission sequence number (RSN), and the power offset to be used by L1. Redundancy version (RV) of the HARQ transmission is derived by L1 from RSN, CFN and in case of 2 ms TTI from the sub-frame number.

Multiplexing and TSN setting:

The multiplexing and TSN setting entity is responsible for concatenating multiple MAC-d PDUs into MAC-es PDUs, and to multiplex one or multiple MAC-es PDUs into a single MAC-e PDU, to be transmitted in the next TTI, as instructed by the E-TFC selection function.

It is also responsible for managing and setting the TSN per logical channel for each MAC-es PDU.

E-TFC selection:

This entity is responsible for E-TFC selection according to the scheduling information, Relative Grants (FDD only) and Absolute Grants, received from UTRAN via L1 and Serving Grant value signalled through RRC, and for arbitration among the different flows mapped on the E-DCH.

Scheduling Access Control (TDD only):

The Scheduling Access Control entity is responsible for routing associated uplink signalling via E-UCCH and MAC-e PDU (in the case that E-DCH resources are assigned) or via E-RUCCH (in the case that no E-DCH resources are assigned). It is also responsible for obtaining and formatting the appropriate information to be carried on E-UCCH/E-RUCCH.

NOTE: HARQ process ID and RSN are carried on E-UCCH.

MAC Architecture – UTRAN Side

MAC-c/sh/m entity – UTRAN Side

The following functionalities are covered:

Scheduling – Buffering – Priority Handling;

This function manages FACH and for TDD DSCH resources between the UEs and between data flows according to their priority and delay requirements set by higher layers.

TCTF MUX:

The TCTF field indicates the common logical channel type, or if a dedicated logical channel is used;

UE Id Mux

For dedicated type logical channels, the UE Id field in the MAC header is used to distinguish between UEs;

MBMS Id Mux

For MTCH channels, the MBMS Id field in the MAC header is used to distinguish between MBMS services.

TFC selection

In the downlink, transport format combination selection is done for FACH and PCH and for TDD DSCHs.

Demultiplex

For TDD operation the demultiplex function is used to separate USCH data from different UEs, i.e. to be transferred to different MAC-d entities.

DL code allocation

For TDD this function is used to indicate the code used on the DSCH.

Flow control

A flow control function exists toward MAC-d to limit buffering between MAC-d and MAC-c/sh/m entities.

A flow control function also exists towards MAC-hs in case of configuration with MAC-c/sh/m.

MAC-d entity – UTRAN Side

The following functionalities are covered:

Transport Channel type switching

If requested by RRC, MAC shall switch the mapping of one designated logical channel between common and dedicated transport channels.

This is the case when UE moves from CELL_DCH state to CELL_FACH state.

C/T MUX

The function includes the C/T field when multiplexing of several dedicated logical channels onto one transport channel (other than HS-DSCH) or one MAC-d flow (HS-DSCH) is used.

Priority setting

This function is responsible for priority setting on data received from DCCH / DTCH.

Ciphering

Ciphering of transparent mode data is performed in MAC-d.

Deciphering

Deciphering of transparent mode data is performed in MAC-d.

DL Scheduling/Priority handling

In the downlink, scheduling and priority handling of transport channels is performed within the allowed transport format combinations of the TFCS assigned by the RRC.

Flow Control

A flow control function exists toward MAC-c/sh/m to limit buffering between MAC-d and MAC-c/sh/m entities. This function is intended to limit layer 2 signalling latency and reduce discarded and retransmitted data as a result of FACH or for TDD DSCH congestion. For the Iur interface, a flow control function also exists towards MAC-hs/ehs in case of configuration without MAC-c/sh/m.

MAC-hs entity – UTRAN Side

The MAC-hs is comprised of four different functional entities.

Flow Control

This is the companion flow control function to the flow control function in the MAC-c/sh/m in case of configuration with MAC-c/sh/m and MAC-d in case of configuration without MAC-c/sh/m. Both entities together provide a controlled data flow between the MAC-c/sh/m and the MAC-hs (Configuration with MACc/sh/m) or the MAC-d and MAC-hs (Configuration without MAC-c/sh/m) taking the transmission capabilities of the air interface into account in a dynamic manner. This function is intended to limit layer 2 signalling latency and reduce discarded and retransmitted data as a result of HS-DSCH congestion.

Scheduling/Priority Handling

This function manages HS-DSCH resources between HARQ entities and data flows according to their priority. Based on status reports from associated uplink signalling either new transmission or retransmission is determined. Further it determines the Queue ID and TSN for each new MAC-hs PDU being serviced, and in the case of TDD the HCSN is determined.

HARQ

One HARQ entity handles the hybrid ARQ functionality for one user. One HARQ entity is capable of supporting multiple instances (HARQ process) of stop and wait HARQ protocols. There shall be one HARQ process per HS-DSCH per TTI

TFRC selection

Selection of an appropriate transport format and resource for the data to be transmitted on HS-DSCH.

MAC-es entity – UTRAN Side

For each UE, there is one MAC-es entity in the SRNC.

MAC-es comprises the following entities:

Reordering Queue Distribution

The reordering queue distribution function routes the MAC-es PDUs to the correct reordering buffer based on the SRNC configuration.

Reordering

This function reorders received MAC-es PDUs according to the received TSN and Node-B tagging i.e. (CFN, subframe number). MAC-es PDUs with consecutive TSNs are delivered to the disassembly function upon reception.

There is one Re-ordering Process per logical channel.

Macro diversity selection (FDD only)

The function is performed in the MAC-es, in case of soft handover with multiple Node-Bs. This means that the reordering function receives MAC-es PDUs from each Node-B in the E-DCH active set.

Disassembly

The disassembly function is responsible for disassembly of MAC-es PDUs. When a MAC-es PDU is disassembled the MAC-es header is removed, the MAC-d PDU’s are extracted and delivered to MAC-d.

MAC-e entity – UTRAN Side

There is one MAC-e entity in the Node B for each UE and one E-DCH scheduler function in the Node-B.

E-DCH Scheduling

This function manages E-DCH cell resources between UEs. Based on scheduling requests, Scheduling Grants are determined and transmitted.

E-DCH Control

The E-DCH control entity is responsible for reception of scheduling requests and transmission of Scheduling Grants.

De-multiplexing

This function provides de-multiplexing of MAC-e PDUs. MAC-es PDUs are forwarded to the associated MAC-d flow.

HARQ

One HARQ entity is capable of supporting multiple instances (HARQ processes) of stop and wait HARQ protocols. Each process is responsible for generating ACKs or NACKs indicating delivery status of E-DCH transmissions.

References

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

Related posts:

1. UMTS Logical Channels and Transport Channels
2. HSUPA: High-Speed Uplink Packet Access
3. MAC PDU Structure for non-HSPA Transport Channels

Logical Channels

Logical channels reside between Radio Link Control (RLC) and Medium Access Control (MAC) layer. There are broadly two categories of logical channels available.

1. Control Channels
2. Traffic Channels

Control Channels

Control channels are used to carry control plane information e.g. information from Radio Resource Control (RRC) Protocol.

Types of control channels

1. Broadcast Control Channel (BCCH)
2. Paging Control Channel
3. Dedicated Control Channel
4. Common Control Channel
5. Shared Channel Control Channel

Broadcast Control Channel

BCCH is a downlink channel and are used for broadcasting system information.

Paging Control Channel

PCCH is a downlink channel. This is used for carrying downlink paging information from UTRAN and Core Network.

Dedicated Control Channel

DCCH is a point to point bi-directional logical channel used to transmit dedicated control information between network and UE. This channel is commonly established through RRC connection establishment procedure.

Common Control Channel

CCCH is a bi-directional channel used to transfer control information between the UE and the network. CCCH is used when UEs have no RRC connection available.

Shared Control Channel

Bi-directional channel that transmits control information for uplink and downlink shared channels between network and UEs. This channel is for TDD only.

MBMS point-to-multipoint Control Channel (MCCH)

A point-to-multipoint downlink channel used for transmitting control information from the network to the UE. This channel is only used by UEs that receive MBMS.

MBMS point-to-multipoint Scheduling Channel (MSCH)

A point-to-multipoint downlink channel used for transmitting scheduling control information, from the network to the UE, for one or several MTCHs carried on a CCTrCH. This channel is only used by UEs that receive MBMS.

Traffic Channels

Dedicated Traffic Channel

DTCH is a point to pint bi-directional channel dedicated to one UE, for transfer of user information.

Common Traffic Channel

A point to multipoint unidirectional channel to carry user information for all or a group of specified UEs.

MBMS point-to-multipoint Traffic Channel (MTCH)

A point-to-multipoint downlink channel used for transmitting traffic data from the network to the UE. This channel is only used for MBMS.

Transport Channels

There are broadly two types of transport channels available.

1. Common Transport Channel
2. Dedicated Transport Channel

Common Transport Channel

Random Access Channel (RACH)

A contention based uplink channel used for transmission of relatively small amounts of data, e.g. for initial access or non-real-time dedicated control or traffic data.

Forward Access Channel (FACH)

Common downlink channel without closed-loop power control used for transmission of relatively small amount of data. In addition FACH is used to carry broadcast and multicast data.

Downlink Shared Channel (DSCH)

A downlink channel shared by several UEs carrying dedicated control or traffic data, used in TDD mode only.

Uplink Shared Channel (USCH)

An uplink channel shared by several UEs carrying dedicated control or traffic data, used in TDD mode only.

Broadcast Channel (BCH)

A downlink channel used for broadcast of system information into an entire cell.

Paging Channel (PCH)

A downlink channel used for broadcast of control information into an entire cell allowing efficient UE sleep mode procedures. Currently identified information types are paging and notification. Another use could be UTRAN notification of change of BCCH information.

High Speed Downlink Shared Channel (HS-DSCH)

A downlink channel shared between UEs by allocation of individual codes, from a common pool of codes assigned for the channel.

Dedicated Transport Channel

Dedicated Channel (DCH)

A channel dedicated to one UE used in uplink or downlink.

Enhanced Dedicated Channel (E-DCH)

A channel dedicated to one UE used in uplink only. The E-DCH is subject to Node-B controlled scheduling and HARQ.

Logical Channels and Transport Channels mapping

Mapping in Uplink

NOTE: The color green indicates a logical channel can be mapped on a transport channel.

Mapping in Downlink

NOTE: The color green indicates a logical channel can be mapped on a transport channel.

Related posts:

1. MAC PDU Structure for non-HSPA Transport Channels
2. MAC (Medium Access Control) Architecture (25.321)
3. Introduction to LTE
4. WCDMA Physical Layer: Principles and Features
5. HSUPA: High-Speed Uplink Packet Access