MBMS Channel Structure

There exists two transmission modes to provide the MBMS service:

  • Point-to-point transmission (p-t-p/PTP)
  • Point-to-multipoint transmission (p-t-m/PTM)

Figure above [17] shows an example scenario where one cell uses p-t-m while an other cell has only one joines UE which is kept in p-t-p state. From the MBMS operation point of view, procedures are obviously simpler if the content is always provided in a point-to-multipoint manner without shifting users back and forth between different states.

Point-to-Point Transmission

Point-to-point transmission is used to transfer MBMS specific control/user plane information as well as dedicated control/user plane information between the network and one UE in RRC Connected Mode. It is used only for the multicast mode of MBMS.

For a UE in CELL_FACH and Cell_DCH, DCCH or DTCH is used, allowing all existing mappings to transport channels.

Point-to-multipoint Transmission

Point-to-multipoint transmission is used to transfer MBMS specific control/user plane information between the network and several UEs in RRC Connected or Idle Mode. It is used for broadcast or multicast mode of MBMS.

Logical Channels

MBMS point-to-multipoint Control Channel (MCCH):

This logical channel is used for a p-t-m downlink transmission of control plane information between network and UEs in RRC Connected or Idle Mode. The control plane information on MCCH is MBMS specific and is sent to UEs in a cell with an activated (joined) MBMS service. MCCH can be sent in S-CCPCH carrying the DCCH of the UEs in CELL_FACH state, or in standalone S-CCPCH, or in same S-CCPCH with MTCH. The MCCH is always mapped to one specific FACH in the S-CCPCH as indicated on the BCCH. If MCCH is the only logical channel mapped in to the FACH, the absence of the TCTF field is explicitly signalled otherwise the TCTF field is used in MAC header to identify MCCH logical channel type. In case of soft combining, the MCCH is mapped to a different S-CCPCH (CCTrCH in TDD) than MTCH. Reception of paging has priority over reception of MCCH for Idle mode and URA/CELL_PCH UEs.

MBMS point-to-multipoint Traffic Channel (MTCH):

This logical channel is used for a p-t-m downlink transmission of user plane information between network and UEs in RRC Connected or Idle Mode. The user plane information on MTCH is MBMS Service specific and is sent to UEs in a cell with an activated MBMS service. The MTCH is always mapped to one specific FACH in the S-CCPCH as indicated on the MCCH. The TCTF field is always used in MAC header to identify MTCH logical channel type.

MBMS point-to-multipoint Scheduling Channel (MSCH):

This logical channel is used for a p-t-m downlink transmission of MBMS service transmission schedule between network and UEs in RRC Connected or Idle Mode. The control plane information on MSCH is MBMS service and S-CCPCH specific and is sent to UEs in a cell receiving MTCH. One MSCH is sent in each S-CCPCH carrying the MTCH. The MSCH is always mapped to one specific FACH in the S-CCPCH as indicated on the MCCH. Due to different error requirements the MSCH is mapped to a different FACH than MTCH. If MSCH is the only logical channel mapped in to the FACH, the absence of the TCTF field is explicitly signalled otherwise the TCTF field is used in MAC header to identify MSCH logical channel type.

Transport Channel

FACH is used as a transport channel for MTCH, MSCH and MCCH.

Physical Channel

SCCPCH is used as a physical channel for FACH carrying MTCH or MCCH or MSCH.

Mapping between channels

Only in downlink, the following connections between logical channels and transport channels exist:

  • MCCH can be mapped to FACH
  • MTCH can be mapped to FACH
  • MSCH can be mapped to FACH

The mappings as seen from the UE and UTRAN sides are shown in Figure a and Figure b below respectively [6]:


Figure a: Logical channels mapped onto transport channel, seen from the UE side


Figure b: Logical channels mapped onto transport channel, seen from the UTRAN side

Data Flows through Layer 2

Data flow for MCCH mapped to FACH:

For MCCH, the RLC mode to be employed is UM-RLC, with required enhancements to support out of sequence SDU delivery. A MAC header is used for logical channel type identification.

Data flow for MTCH mapped to FACH

For MTCH, the RLC mode to be employed is UM-RLC, with required enhancements to support selective combining. Quick repeat may be used in RLC-UM. A MAC header is used for logical channel type identification and MBMS service identification.

Data flow for MSCH mapped to FACH:

For MSCH, the RLC mode to be employed is UM-RLC. A MAC header is used for logical channel type identification.

MBMS Notification Indicator Channel

MBMS notification utilizes a new MBMS specific PICH called the MBMS Notification Indicator Channel (MICH) in each cell.









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