Data channels are channels which are used to carry data across the 5G radio access network, data and information is organized into a number of data channels.
Types of data channels–
- Logical channel: 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 channel used for the user data. Medium Access Control (MAC) Layer of NR provides services to the Radio Link Control (RLC) Layer in the form of logical channels.
Logical channels can be one of two groups: control channels and traffic channels:
- Control channels: The control channels are used for the transfer of data from the control plane
- Traffic channels: The traffic logical channels are used for the transfer of user plane data.
Types of logical channels–
- Broadcast Control Channel, BCCH: BCCH is used within the downlink, and it is used for sending broadcast style information to the user equipment within that cell. The system information transmitted by the 5G NR BCCH is divided into different blocks:
- Master Information Block, MIB: There is one MIB and this is mapped onto the BCH transport channel and then to the PBCH physical channel.
- System Information Block, SIB: There are several system information blocks (SIB). These are mapped onto the DL-SCH transport channel and then onto the PDSCH physical channel.
- Paging Control Channel, PCCH: This is a Downlink channel. It is used to page the User Equipment (UEs) whose location at cell level is not known to the network. As a result the paging message needs to be transmitted in multiple cells. The PCCH is mapped to the PCH transport channel and then to the PDSCH physical channel.
- Common Control Channel, CCCH:This 5G channel is used on both the downlink and uplink for transmitting control information to and from the user equipment or mobiles. The channel is used for initial access, i.e., those mobiles that do not have a radio resource control (RRC) connection.
- Dedicated Control Channel, DCCH: DCCH is used within the uplink and downlink to carry dedicated control information between the UE or mobile and the network. It is used by the UE and the network after a radio resource control, RRC connection has been established.
- Dedicated Traffic Channel, DTCH: This 5G channel is present in both the uplink and downlink. It is dedicated to one UE and is used for carrying user information to and from a specific UE and the network.
- Transport channel: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. The multiplexing of the logical data to be transported by the physical layer and its channels over the radio interface.
Types of transport channels –
- Broadcast Channel (BCH): It is used for transmitting the BCCH system information, more specifically the Master Information Block (MIB). It has a fixed transport format, provided by the specifications.
- Paging Channel (PCH): This channel 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.
- Downlink Shared Channel (DL-SCH): This is the main transport channel used for transmitting downlink data in NR. It supports all key NR features such as dynamic rate adaptation and channel aware scheduling, HARQ and spatial multiplexing. DL-SCH is also used for transmitting some parts of the BCCH system info which is 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.
- Uplink Shared Channel (UL-SCH): This is the uplink counterpart to the DLSCH that is, the uplink transport channel used for transmission of uplink data.
- Random-Access Channel (RACH): RACH is also a transport channel, although it does not carry transport blocks.
- Physical channel: The physical channels are those which are closest to the actual transmission of the data over the radio access network / 5G RF signal. They are used to carry the data over the radio interface. The physical channels often have higher level channels mapped onto them to provide a specific service. Additionally, the physical channels carry payload data or details of specific data transmission characteristics like modulation, reference signal multiplexing, transmit power, RF resources, etc.
Types of Physical channels–
There are three physical channels for each of the uplink and downlink:
5G NR Downlink Physical Channels
- Physical downlink shared channel, PDSCH: The 5G NR physical downlink shared channel, PDSCH carries data sharing capacity on a time and frequency basis. The PDSCH’s physical channel carries a variety of items of data – user data, UE-specific higher layer control messages mapped down from higher channels, system information blocks (SIBs), and paging.
The PDSCH uses an adaptive modulation format dependent on the link conditions, i.e. signal to noise ratio. It also uses a flexible coding scheme. The combination of these means that there is a flexible coding and data rate.
- Physical downlink control channel, PDCCH: As the name implies, the 5G PDCCH carries downlink control data. Its primary function is scheduling the downlink transmissions on the PDSCH and also the uplink data transmissions on the PUSCH.
The PDCCH uses QPSK as its modulation format and polar coding as the coding scheme, except for small packets of data.
- Physical broadcast channel, PBCH: This 5G channel forms part of the synchronization signal block. Its function is to provide UEs with the Master Information Block, MIB. A further function of the PBCH in conjunction with the control channel is to support the synchronization of time and frequency. This aids with cell acquisition, selection and re-selection. The PBCH uses a fixed data format and there is one block that extends over a TTI of 80 ms.
The PBCH uses QPSK modulation and it transmits a cell specific demodulation reference signal, DMRS pattern that can be used with beam-forming.
5G NR Uplink Physical Channels
- Physical random-access channel, PRACH: This 5G channel PRACH is used for channel access. It transmits an initial random-access pre-amble consisting of sequences which may be of two different lengths:
- A long sequence length is 839 which is applied to the subcarrier spacings of 1.25kHz and 5 kHz
- Short sequence lengths of 139 are applied to subcarrier spacings of 15 kHz and 30 kHz (FR1 bands) and 60 kHz and 120 kHz (FR2 bands).
- Physical uplink shared channel, PUSCH: The 5G physical uplink shared channel, PUSCH, is the counterpart of the PDSCH. It is used to carry data from the UL-SCH and its higher mapped channels on a frequency and time-shared basis.
Like the PDSCH, the PUSCH also has a very flexible format. The allocation of frequency resources is undertaken using resource blocks along with a flexible modulation and coding scheme dependent upon the link signal to noise ratio.
To support the channel link estimation and demodulation, the PUSCH contains DMRS signals.
- Physical uplink control channel, PUCCH: The 5G physical uplink control channel, PUCCH, carries the uplink control data. It is also possible, depending upon the resource allocation the uplink control information or data may also be sent on the PUSCH, even though in the downlink direction, control information is always sent on the PDCCH.
In the figures below, the mapping between logical, transport channels and physical channels is illustrated.
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Written by- Ankit Puri