Logical Channels

• Control Channels

Control channels are used for transfer of control plane information only. The control channels offered by MAC are:

• Broadcast Control Channel (BCCH)
  A downlink channel for broadcasting system control information.
• Paging Control Channel (PCCH)
  A downlink channel that transfers paging information and system information change notifications. This channel is used for paging when the network does not know the location cell of the UE.
• Common Control Channel (CCCH)
  Channel for transmitting control information between UEs and network. This channel is used for UEs having no RRC connection with the network.
• Multicast Control Channel (MCCH)
  A point-to-multipoint downlink channel used for transmitting MBMS control information from the network to the UE, for one or several MTCHs. This channel is only used by UEs that receive MBMS.
• Dedicated Control Channel (DCCH)
  A point-to-point bi-directional channel that transmits dedicated control information between a UE and the network. Used by UEs having an RRC connection.

• Traffic Channels
  
  Traffic channels are used for the transfer of user plane information only. The traffic channels offered by MAC are:

• Dedicated Traffic Channel (DTCH)
  A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
• Multicast Traffic Channel (MTCH)
  A point-to-multipoint downlink channel for transmitting traffic data from the network to the UE. This channel is only used by UEs that receive MBMS.

Transport Channels in LTE

The physical layer offers information transfer services to MAC and higher layers. The physical layer transport services are described by how and with what characteristics data are transferred over the radio interface.

Downlink transport channel types are:

1. Broadcast Channel (BCH) characterised by:

• fixed, pre-defined transport format;
• requirement to be broadcast in the entire coverage area of the cell.

2. Downlink Shared Channel (DL-SCH) characterised by:

• support for HARQ;
• support for dynamic link adaptation by varying the modulation, coding and transmit power;
• possibility to be broadcast in the entire cell;
• possibility to use beam forming;
• support for both dynamic and semi-static resource allocation;
• support for UE discontinuous reception (DRX) to enable UE power saving;

3. Paging Channel (PCH) characterised by:

• support for UE discontinuous reception (DRX) to enable UE power saving (DRX cycle is indicated by the network to the UE);
• requirement to be broadcast in the entire coverage area of the cell;
• mapped to physical resources which can be used dynamically also for traffic/other control channels.

4. Multicast Channel (MCH) characterised by:

• requirement to be broadcast in the entire coverage area of the cell;
• support for MBSFN combining of MBMS transmission on multiple cells;
• support for semi-static resource allocation e.g. with a time frame of a long cyclic prefix.

Uplink transport channel types are:

1. Uplink Shared Channel (UL-SCH) characterised by:

• possibility to use beamforming; (likely no impact on specifications)
• support for dynamic link adaptation by varying the transmit power and potentially modulation and coding;
• support for HARQ;
• support for both dynamic and semi-static resource allocation.

2. Random Access Channel(s) (RACH) characterised by:

• limited control information;
• collision risk;

Paging in LTE

The paging procedure is used by the network to request the establishment of a NAS signalling connection to the UE. The NAS signalling connection thus established can also be used to transport cdma2000® signalling messages to the UE. Another purpose of the paging procedure is to prompt the UE to reattach if necessary as a result of a network failure. If the UE is not attached when it receives a paging for EPS services, the UE shall ignore the paging. Additionally, the network can use the paging procedure to initiate the mobile terminating CS fallback procedure.

- Paging for EPS services through E-UTRAN using S-TMSI

The network shall initiate the paging procedure for EPS services using S-TMSI with CN domain indicator set to "PS" when NAS signalling messages, cdma2000® signalling messages or user data is pending to be sent to the UE when no NAS signalling connection exists. Upon reception of a paging indication, the UE responds to the paging with a SERVICE REQUEST message.

- Paging for EPS services through E-UTRAN using IMSI

Paging for EPS services using IMSI is an abnormal procedure used for error recovery in the network. The network may initiate paging for EPS services using IMSI with CN domain indicator set to "PS" if the S-TMSI is not available due to a network failure.

Upon reception of a paging for EPS services using IMSI, the UE shall locally deactivate any EPS bearer context(s) and locally detach from EPS. Additionally the UE shall delete the following parameters: last visited registered TAI, TAI list, GUTI and KSIASME. The UE shall set the EPS update status to EU2 NOT UPDATED and change the state to EMM-DEREGISTERED.

After performing the local detach, the UE shall then perform an attach procedure. If the UE is operating in CS/PS mode 1 or CS/PS mode 2 of operation, then the UE shall perform a combined attach procedure.

-Paging for CS fallback to A/Gb or Iu mode

The network may initiate the paging procedure for non-EPS services when the UE is IMSI attached for non-EPS services. To initiate the procedure when no NAS signalling connection exists, the EMM entity in the network requests the lower layer to start paging. The paging message includes a CN domain indicator set to "CS" in order to indicate that this is paging for CS fallback.

To notify the UE about an incoming mobile terminating CS service when a NAS signalling connection exists, the EMM entity in the network shall send a CS SERVICE NOTIFICATION message. This message may also include CS service related parameters (e.g. Calling Line Identification, SS or LCS related parameters).

Upon reception of a paging indication, the UE shall respond with an EXTENDED SERVICE REQUEST. If the paging is received in EMM-IDLE mode, the UE shall respond immediately. If the paging is received as NAS CS NOTIFICATION message in EMM-CONNECTED mode, the UE may request upper layers input i.e. to accept or reject CS fallback before responding with an EXTENDED SERVICE REQUEST. The response is indicated in the CSFB response information element in the EXTENDED SERVICE REQUEST message in both EMM-IDLE and EMM-CONNECTED modes.