Transport Channels in LTE

The physical layer provides a transport service for MAC PDUs. This service is accessed by transport channels. Most transport channels are directly mapped to physical channels. Thus, in other words, transport channels are the gateway to physical channels and a selection for MAC PDUs where they are to be transmitted.

The following DL transport channel types are defined:

• Broadcast Channel (BCH):
  • – Uses a static transport format and has the requirement that all UEs within the cell have to receive its information error-free. The reception of the BCH is mandatory for accessing any service of a cell.
• DL-SCH:
  • – Carries all semi-static broadcast information (SIB) and all UE-specific traffic channels.
  • – DL-SCH is secured with HARQ algorithms.
  • – Efficiency is realized with AMC link adaptation.
  • – Various TMs are defined to meet different environment scenarios to increase efficiency in respect of current conditions.
  • – DRX is available in order to increase handset operating time.
  • – Makes use of spatial algorithms like beamforming or MIMO.
• Paging Channel (PCH):
  • – Needs to be received in complete cell coverage area.
  • – Supports DRX in order to increase battery operating cycle.
  • – Dynamically allocated via own physical identifier (P-RNTI).
• Multicast Channel (MCH):
  • – Broadcast to entire cell coverage area.
  • – MBMS transmission with use of multiple cells.

A designated DL control channel is not defined as the PDCCH is used for physical channel control only. All the higher layer control plane is transmitted via the DL-SCH.

Defined UL transport channel types are shown in the following items:

• UL-SCH:
  • – UL-SCH is secured with HARQ algorithms.
  • – Fully dynamic and semi-static resource allocation schemes.
  • – Can make use of multi-user MIMO (UL "virtual" MIMO).
  • – Uses dynamic link adaptation like AMC.
• RACH:
  • – Accessible without UL synchronization.
  • – Collision-based and collision-free operating modes.
  • – Various modes depending on cell size and interference.

As in the DL direction, no UL control transport channel is defined as all the higher layer control plane is transmitted on the UL-SCH. The PUCCH is a control channel used by the physical layer only.