5G Data Channels: Physical; Transport; & Logical

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In order to be able to carry the data across the 5G radio access network, the data and information is organised into a number of data channels.

By organising the data into various channels the 5G communications system is able to manage the data transfers in an orderly fashion and the system is able to understand what data is arriving and hence it is able to process it in the required fashion.

As there are many different types of data that need to be transferred - user data obviously needs to be transferred, but so does control information to manage the radio communications link, as well as data to provide synchronisation, access and the like. All of these functions are essential and require the transfer of data over the radio access network.

The 5G mobile or wireless communications system uses a similar access stratum to that used by 4G LTE.

Although there are two protocol stacks: user plane and control plane, they still adopt the familiar OSI reference model.

As a result there are various protocol layers and accordingly there are several data channel layers that are defined for the radio communications.

5G channel hierarchy

In order to group the data to be sent over the 5G NR radio access network, the data is organised in a very logical way. As there are many different functions for the date being sent over the radio communications link, they need to be clearly marked and have defined positions and formats.

To ensure this happens, there are several different forms of data "channel" that are used. The higher level ones are "mapped" or contained within others until finally at the physical level, the channel contains data from higher level channels.

In this way there is a logical and manageable flow of data from the higher levels of the protocol stack down to the physical layer.

There are three main types of data channels that are used within mobile communications systems. This is true for 5G systems, and accordingly the hierarchy is given below.

• Logical channel:   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.
• Transport channel:   Is the multiplexing of the logical data to be transported by the physical layer and its channels over the radio interface.
• 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 of 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.

5G wireless communications system channel mapping

The channels names, for both the uplink and downlink are virtually the same as those used for 4G LTE. However the channel names and descriptions used for the 5G communications system are given below, in this instance for the downlink.

The uplink from the user equipment to the base station also has a variety of channels which are mapped in a similar way to those for the downlink of the 5G mobile communications system.

5G NR logical channels

There are several different logical channels that are used within the 5G NR radio access network. Some of thee will be familiar names from the 4G LTE system as the names have been carried over.

• Broadcast Control Channel, BCCH:   The BCCH is used within the downlink, and it is used for sending broadcast style information to the user equipments 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, SIBs. 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 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 equipments 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:   The 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.

5G NR transport channels

There are five different transport channels. Some are used on the uplink, others on the downlink, and some can be used on both.

• Broadcast Channel, BCH:   The BCH 5G channel is used in the downlink only for transmitting the BCCH system information and specifically the Master Information Block, MIB, information. In order that the data can be utilised, it has a specific format.
• Paging Channel, PCH:   The PCH is used for carrying paging information from the PCCH logical channel. The PCH supports discontinuous reception, DRX, to enable the UE to save battery power by waking up at a specific time to receive the PCH.
  
  In order that the PCH is received by all mobiles / UEs in the cell, the PCH must be broadcast over the entire cell as a single message, or where beam forming is used, this can be done using several different PCH instances.
• Downlink Shared Channel, DL-SCH:   As the name indicates, this is a downlink only channel. It is the main transport channel used for transmitting downlink data and it supports all the key 5G NR features. These include: dynamic rate adaptation; HARQ, channel aware scheduling, and spatial multiplexing.
  
  The DL-SCH is also used for transmitting some parts of the BCCH system information, specifically the SIB. Each UE has a DL-SCH for each cell it is connected to.
• 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:   The RACH is a transport channel, which carries the random access preamble which is used to overcome the message collisions that can occur when UEs access the system simultaneously.

5G NR physical layer data channels

The 5G physical channels are used to transport information over the actual radio interface. They have the transport channels mapped into them as seen in the diagrams, but they also include various physical layer data required for the maintenance and optimisation of the radio communications link between the UE and the base station.

The 5G mobile communications physical layer channels resemble those of 4G LTE, but PHICH and PCIICH have been removed. The HARQ operation has also been updated to be more flexible. Also the downlink control channel PDCCH is now administered by layer 3 procedures.

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 the capacity on a time and frequency basis. The PDSCH 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); & paging.
  
  The PDSCH uses an adaptive modulation format dependent upon 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 physical downlink control channel 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 synchronisation 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 synchronisation 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 aid with beam-forming.

5G NR Uplink Physical Channels

• Physical random access channel, PRACH:   This 5G channel - the physical random access 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 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 that dependent 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.

The use of these 5G channels provide a method for organising the data flow over the radio interface of the 5G communications network. Using channels enables the communications system to recognise the type of data that is being sent, and to deal with it accordingly. The format used is very similar to that employed on 4G LTE and it built on the technology of previous mobile communications or mobile phone generations.

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