4Physical and Logical Channels in LTE

As for most radio communication systems, the radio interface of E-UTRAN faces many challenges. In terms of requirements, the E-UTRAN shall be able to transmit high-rate and low-latency information in the most efficient way. However, not all the information flows require the same protection against transmission errors or Quality of Service handling.

In general, it is critical, especially in the case of radio mobility, that E-UTRAN signalling messages are transmitted as fast as possible, usingthe best error-protectionscheme. On the other hand, voice or data streaming applications can accept a reasonable frame loss due to radio transmission. Interactive connection-oriented applications (such as Web browsing) are also different, as the end-to-end retransmission can help to recover from radio propagation issues

In order to be flexible and allow different schemes for data transmission, the E-UTRAN specifications introduce several types of channels:

The logical channels correspond to data-transfer services offered by the radio interface protocols to upper layers. Basically, there are only two types of logical channels: the control channels (for the transfer of Control plane information) and the traffic channels (for the transfer of User plane information). Each of the channels of these two categories corresponds to a certain type of information flow.

The transport channels describe how and with what characteristics data are transferred over the radio interface. For example, the transport channels describe how the data are protected against transmission errors, the type of channel coding, CRC protection or interleaving which is being used, the size of data packets sent over the radio interface, etc.

As in the specification, the transport channels are classified into two categories:

The physical channels are the actual implementation of the transport channel over the radio interface. They are only known to the physical layer of E-UTRAN and their structure is tightly dependent on physical interface OFDM characteristics.