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Layer 2

This section introduces transport channels, Layer 2 structure, and logical channels in LTE.

Transport Channels

Downlink transport channels:

  • Broadcast Channel (BCH)characterized by:
    • Fixed, pre-defined transport format
    • Requirement to be broadcast in the entire coverage area of the cell.
  • Downlink Shared Channel (DL-SCH)characterized 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 beamforming
    • Support for both dynamic and semi-static resource allocation
    • Support for UE discontinuous reception (DRX) to enable UE power saving.
  • Paging Channel (PCH)characterized 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 or other control channels.
  • Multicast Channel (MCH) (from Release 9)characterized 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 channels:

  • Uplink Shared Channel (UL-SCH)characterized 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.
  • Random Access Channel(s) (RACH)characterized by:
    • Limited control information
    • Collision risk