Logical, Transport & Physical Channels in LTE

All these channels help UE to establish the connection with the eNodeB, maintain the connection and terminate the same.  

Logical channels

• Characterized by the information that is transferred (what kind information).
• Provide services for the Medium Access Control (MAC) layer within the LTE protocol structure.

Transport channels 

• Characterized by how the data are transferred over the radio interface (way/ method data is being transmitted).
• The physical layer transport channels offer information transfer to Medium Access Control (MAC) and higher layers. 

Physical channel

• These are transmission channels that carry user data and control messages.
• Corresponds to a set of resource elements used by the physical layer. Channels are further divided into control channel (signalling) and traffic channel (user/data) at logical channel stage.

Physical Channels

PDSCH - Physical Downlink Shared Channel, mainly used to carry high speed data/multimedia information. Can be either QPSK/16QAM/64QAM.

PDCCH - Physical Downlink Control Channel, mainly used to carry UE specific control information. It will have QPSK modulation used. It is mapped on resource elements (REs) in first 3 OFDM symbols in the first slot of sub-frame.

CCPCH - Common Control Physical Channel, carries cell-wide control information. QPSK is used. CCPCH is transmitted exclusively on 72 sub-carriers centered around DC carrier.
 

Physical Signals

Physical signals do not convey L2/L3 layer information, but mainly used for synchronization and channel estimation purpose. RS is used for estimating channel response. P-SS and S-SS synchronization signals used for determining network frame timing information i.e. start of the information.
 

Transport Channels

• Downlink and Uplink transport channels carry L2/L3 information. 
• It also configures LTE PHY layer. 
• It sends status information such as packet error and CQI to upper layers. 
• Also supports peer-peer signaling between higher layers. 

 

Based on broadcast, unicast or multicast concept different transport channels exist. 

 

Downlink channels include BCH (broadcast channel), DL-SCH( downlink shared channel, to multiple mobile subscribers or UEs), PCH (paging channel, used for UE DRX and broadcasted over entire cell ), MCH (multicast channel, transmitted over entire cell).  

 

Uplink channels include RACH (Random Access Channel), UL-SCH(Uplink Shared Channel).

 

Uplink PRBs (Physical Resource Blocks) are assigned to UE by eNodeB scheduler. 

 

PUSCH is used and shared by multiple UEs to carry upper layer information towards eNodeB. It will employ QPSK/16QAM/64QAM modulation types.