NR Protocol Stack: MAC
Functions of the MAC:
Logical channel multiplexing
Hybrid ARQ retransmission
Scheduling
Multiplexing/demultiplexing for CA(Carrier Aggregation)
MAC layer provides services to the RLC layer in the form of logical channels and takes the services from the PHY layer in the form of transport channels.
Logical Channels: ‘what type of information’ it carries
PCCH — This channel is used for paging whose location is not known to the network. Therefore , paging needs to be transmitted in multiple cells.
BCCH — This channel is used for broadcasting system information from the network to all the devices in the cell.
CCCH — This is used for transmission of controlled information in conjunction with Random Access Procedure.
DTCH — This is used for the transmission of user data from and to the devices. This logical channel is used for all unicast UL and DL user data.
DCCH — This is used for transmission of control information to and from the device. This channel is used for individual configuration of devices such as setting various parameters to the devices to support the communication.
Transport Channels: ‘how and with what characteristics information’ is transmitted over the radio interface.
Data in transport channel are organized into transport blocks. In each transmission time interval, utmost 1 block of dynamic size is transmitted over the radio interface to and from a device.
The transport block is transmitted using a variety of transport formats in terms of what is modulation scheme, what is the coding parameters, what is the antenna configuration, etc.
The MAC layer controls the transport format and the process is called transport format selection.
PCH — This is used for paging information from the PCCH.
BCH — This is used for the transmission of the parts of the BCCH system information especifically the so called master information block.
DLSCH — This is used for the transmission of DL data in NR
ULSCH — This is used for the transmission of UL data
In addition to the multiplexing of the different logical channels, the MAC layer can also insert MAC control elements into the transport blocks that are to be transmitted over transport channels.
The MAC control element is used for in-bank control signals. MAC puts together both the data from the above layers and some of the control elements together to construct the transport block.