Channel Mapping and Multiplexing

Besides physical and transport channels, LTE also defines logical channels. Logical channels are multiplexed to transport channels within the MAC layer. Logical channels map different content connections to transport channels, like CCCHs to multiple UEs or DCCHs to a specific UE or dedicated transport channels carrying higher layer application data.

Logical channels are addressed with a logical channel ID. The logical ID is a field within the MAC header PDU. Logical channels are multiplexed by using logical channel IDs to transport channels specifying where the information should be transmitted. Finally, transport channels are transferred with physical channels as a service provided by the physical channel.

Figures 1 and 2 show the above-described channel architecture from the basic physical channels via transport channels to logical channels bearing higher layer messages for DL and UL respectively.

 
Figure 1: Downlink channel mapping and multiplexing from logical channels via transport channels to physical channels

 
Figure 2: Uplink channel mapping and multiplexing from logical channels via transport channels to physical channels

Two basic sets of logical channels are defined:

• Control channels: CCCH and DCCHs.
• Traffic channels: CCCH and DTCHs.

The nature of common channels is such that no specific UE is addressed, but the information is either general for all cell-wide subscribers or a message from a UE which has not yet established a dedicated control/traffic channel. A typical example of a CCCH is the broadcast of SIBs.

Traffic channels carry user plane protocols like the Packet Data Convergence Protocol (PDCP) and application IP packets, while control channels carry control plane protocols as RRC and NAS.

• DL logical channels:
  • – Broadcast Control Channel (BCCH).
  • – Paging Control Channel (PCCH).
  • – Common Control Channel (CCCH).
  • – Dedicated Control Channel (DCCH).
  • – Dedicated Traffic Channel (DTCH).
• UL logical channels:
  • – Common Control Channel (CCCH).
  • – Dedicated Control Channel (DCCH).
  • – Dedicated Traffic Channel (DTCH).