First, we know that 5G,like LTE, has physical ,transport and logical channels. Also the MAC layer is the responsible for handling logical channels multiplexing.
The physical layer is responsible for coding, physical-layer hybrid-ARQ processing, modulation, multi-antenna processing, and mapping of the signal to the appropriate physical time–frequency resources. It also handles mapping of transport channels to physical channels.
A physical channel corresponds to the set of time–frequency resources used for transmission of a particular transport channel and each transport channel is mapped to a corresponding physical channel.
In addition to the physical channels with a corresponding transport channel, there are also physical channels without a corresponding transport channel. These channels, known as L1/L2 control channels, are used for downlink control information (DCI), providing the device with the necessary information for proper reception and decoding of the downlink data transmission, and uplink control information (UCI) used for providing the scheduler and the hybrid-ARQ protocol with information about the situation at the device.
NR Physical Channels
- The Physical Downlink Shared Channel (PDSCH) is the main physical channel used for unicast data transmission, but also for transmission of, for example, paging information, random-access response messages, and delivery of parts of the system information.
- The Physical Broadcast Channel (PBCH) carries part of the system information, required by the device to access the network.
- The Physical Downlink Control Channel (PDCCH) is used for downlink control information, mainly scheduling decisions, required for reception of PDSCH, and for scheduling grants enabling transmission on the PUSCH.
- The Physical Uplink Shared Channel (PUSCH) is the uplink counterpart to the PDSCH. There is at most one PUSCH per uplink component carrier per device.
- The Physical Uplink Control Channel (PUCCH) is used by the device to send hybrid-ARQ acknowledgments, indicating to the gNB whether the downlink transport block(s) was successfully received or not, to send channel-state reports aiding downlink channel-dependent scheduling, and for requesting resources to transmit uplink data upon.
- The Physical Random-Access Channel (PRACH) is used for random access.
Note that some of the physical channels, more specifically the channels used for downlink and uplink control information (PDCCH and PUCCH) do not have a corresponding transport channel mapped to them.
Logical Channels and Transport Channels
The MAC provides services to the RLC in the form of logical channels. A logical channel is defined by the type of information it carries and is generally classified as a control channel, used for transmission of control and configuration information necessary for operating an NR system, or as a traffic channel, used for the user data.
NR Logical Channels
- The Broadcast Control Channel (BCCH), used for transmission of system information from the network to all devices in a cell. Prior to accessing the system, a device needs to acquire the system information to find out how the system is configured and, in general, how to behave properly within a cell. Note that, in the case of non-standalone operation, system information is provided by the LTE system and there is no BCCH.
- The Paging Control Channel (PCCH), used for paging of devices whose location on a cell level is not known to the network. The paging message therefore needs to be transmitted in multiple cells. Note that, in the case of non-standalone operation, paging is provided by the LTE system and there is no PCCH.
- The Common Control Channel (CCCH), used for transmission of control information in conjunction with random access.
- The Dedicated Control Channel (DCCH), used for transmission of control information to/from a device. This channel is used for individual configuration of devices such as setting various parameters in devices.
- The Dedicated Traffic Channel (DTCH), used for transmission of user data to/from a device. This is the logical channel type used for transmission of all unicast uplink and downlink user data.
The above logical channels are in general present also in an LTE system and used for similar functionality. However, LTE provides additional logical channels for features not yet supported by NR.
From the physical layer, the MAC layer uses services in the form of transport channels . A transport channel is defined by how and with what characteristics the information is transmitted over the radio interface.
Data on a transport channel are organized into transport blocks. In each Transmission Time Interval (TTI), at most one transport block of dynamic size is transmitted over the radio interface to/from a device.
Associated with each transport block is a Transport Format (TF), specifying how the transport block is to be transmitted over the radio interface. The transport format includes information about the transport-block size, the modulation-and-coding scheme, and the antenna mapping. By varying the transport format, the MAC layer can thus realize different data rates, a process known as transport-format selection.
NR Transport Channels
- The Broadcast Channel (BCH) has a fixed transport format, provided by the specifications. It is used for transmission of parts of the BCCH system information, more specifically the so-called Master Information Block (MIB).
- The Paging Channel (PCH) is used for transmission of paging information from the PCCH logical channel. The PCH supports discontinuous reception (DRX) to allow the device to save battery power by waking up to receive the PCH only at predefined time instants.
- The Downlink Shared Channel (DL-SCH) is the main transport channel used for transmission of downlink data in NR. It supports key NR features such as dynamic rate adaptation and channel-dependent scheduling in the time and frequency domains, hybrid ARQ with soft combining, and spatial multiplexing. It also supports DRX to reduce device power consumption while still providing an always-on experience. The DL-SCH is also used for transmission of the parts of the BCCH system information not mapped to the BCH. Each device has a DL-SCH per cell it is connected to. In slots where system information is received there is one additional DL-SCH from the device perspective.
- The Uplink Shared Channel (UL-SCH) is the uplink counterpart to the DL-SCH—that is, the uplink transport channel used for transmission of uplink data.
The Random-Access Channel (RACH) is also defined as a transport channel, although it does not carry transport blocks.
Mapping between logical, transport, and physical channels
Part of the MAC functionality is multiplexing of different logical channels and mapping of the logical channels to the appropriate transport channels.
- 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation.
- 3GPP TS 38.331, NR; Radio Resource Control (RRC) Protocol Specification (Release 15).
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