LTE channel mapping : logical, transport, physical channels in LTE

LTE (Long-Term Evolution) uses a hierarchical structure of channels to facilitate efficient data transmission between the user equipment (UE) and the network. The data flows through logical channels, transport channels, and physical channels to ensure that information is carried correctly at different layers of the LTE protocol stack. In this page, we will explore channel mapping in LTE including logical, transport, and physical channels and learn the key functions of each channel type in LTE networks.

LTE logical transport physical channels mapping

The diagram provides mapping between various three channel types namely logical, transport and physical channels.

Logical Channels

Logical channels define what type of data is being transmitted. They exist at the MAC (Medium Access Control) layer and are categorized based on the nature of the information they carry. Logical Channels are of two types namely control and traffic channels.
➨Control Channels:
• BCCH (Broadcast Control Channel): Broadcasts system information to all UEs.
• PCCH (Paging Control Channel): Used for paging UEs when network wants to initiate communication.
• DCCH (Dedicated Control Channel): Carries control information between UE and network.
• CCCH (Common Control Channel): Used for control signaling when a UE does not have an RRC connection.
➨Traffic Channels:
• DTCH (Dedicated Traffic Channel): Carries user data between the network and UE.
• MTCH (Multicast Traffic Channel): Used for multimedia broadcasting services.

Transport Channels

Transport channels represent how the data is transmitted over the air interface. They define the format of data transport blocks and are responsible for transferring data from the MAC layer to the physical layer. The transport channels are of two types namely contrl and traffic.
➨Control Transport Channels:
• BCH (Broadcast Channel): Carries broadcast system information (like cell-wide parameters).
• PCH (Paging Channel): Carries paging messages to alert the UE about incoming data.
• RACH (Random Access Channel): Used by UEs to initiate communication with the network.
➨Traffic Transport Channels:
• DL-SCH (Downlink Shared Channel): The main transport channel for user data and control information on the downlink.
• UL-SCH (Uplink Shared Channel): Used for carrying uplink data from UE to the network.
• MCH (Multicast Channel): Supports Multimedia Broadcast Multicast Service (MBMS).

Transport Channels

Physical channels define where and when data is transmitted over the radio interface. They map the transport channels onto the actual resource elements (subcarriers and OFDM symbols) in the LTE air interface. Physical Channels are categorized as per uplink and downlink directions.
➨Physical Downlink Channels:
• PDSCH (Physical Downlink Shared Channel): Carries user data and system information.
• PDCCH (Physical Downlink Control Channel): Carries control information for scheduling and resource allocation.
• PCFICH (Physical Control Format Indicator Channel): Informs UEs about the format of the downlink control channel.
• PHICH (Physical Hybrid ARQ Indicator Channel): Carries HARQ feedback for uplink transmission.
➨Physical Uplink Channels:
• PUSCH (Physical Uplink Shared Channel): Carries user data in the uplink.
• PUCCH (Physical Uplink Control Channel): Carries control information such as acknowledgment (ACK) or scheduling requests.
• PRACH (Physical Random Access Channel): Used by the UE for initial access to the network.

Process of Channel Mapping in LTE

In LTE, data flows through these layers:
• Logical Channels - define the type of data.
• Transport Channels - define how the data will be carried.
• Physical Channels - define the actual transmission over the radio.
For example:
• A UE requests system access using the RACH (Transport Channel) mapped to the PRACH (Physical Channel).
• The network responds with a paging message sent over the PCCH (Logical Channel), mapped to the PCH (Transport Channel), and transmitted over the PDSCH (Physical Channel).

Conclusion

In LTE, data transmission is organized through logical, transport, and physical channels, each serving a distinct purpose. Logical channels define the type of data, transport channels handle how data is transferred, and physical channels manage its transmission over the air interface. This structured channel mapping in LTE ensures efficient and reliable communication across networks.

LTE channels

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