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5G NR Logical Channels,Transport Channels and Physical Channels Mapping

5G NR Logical Channels, Transport Channels, and Physical Channels Mapping

Logical Channel Definition: Medium Access Control (MAC) Layer of NR provides services to the Radio Link Control (RLC) Layer in the form of logical channels. A logical channel is defined by the type of information it carries and is generally differentiated as a control channel, used for transmission of control and configuration information or as a traffic channelused for the user data.

List of Logical Channels for NR:


Transport Channel Definition: A transport channel is defined by how and with what characteristics the information is transmitted over the radio interface. From the physical layer, the MAC layer uses services in the form of transport channels. . Data on a transport channel are organized into transport blocks.

List of Transport Channels for NR:


Logical, Transport and Physical Channel Mapping:

As a part of MAC layer’s multiplexing/de-multiplexing function mapping of the logical channels to the appropriate transport channels is done. The mapping between logical channel  and transport channel is shown in Figure below. This figure clearly indicates how DL-SCH and UL-SCH are the main downlink and uplink transport channels, respectively.

In the figures, the mapping between logical, transport channels and physical channels is illustrated.


To support priority handling, multiple logical channels at MAC layer, each logical channel given its own RLC entity, which can be multiplexed into one transport channel by the MAC layer. At the receiver, the MAC layer handles the corresponding de-multiplexing and forwards the RLC PDUs to their respective RLC entity. To support the de-multiplexing at the receiver, a MAC header is used.

The placement of the MAC headers has been improved in NR compared to LTE, considering the low latency applications in mind. Instead of placing all the MAC header information at the beginning of a MAC PDU (which implies that assembly of the MAC PDU can not start until the scheduling decision is available)  the sub-header corresponding to a certain MAC SDU is placed immediately before the SDU.

This helps the PDUs to be pre-processed before scheduling decision is received . If necessary, padding can be appended to align the transport block size supported in NR. The sub-header contains the identity of the logical channel (LCID) from which the RLC PDU originated and the length of the PDU in bytes. There is also a flag indicating the size of the length indicator, as well as a reserved bit for future use as shown in above figure.

In addition to multiplexing of different logical channels, the MAC layer can also insert MAC control elements into the transport blocks to be transmitted over the transport channels. A MAC control element is used for inband control signaling and identified with reserved values in the LCID field, where the LCID value indicates the type of control information.


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