Different Channels in 5G NR — Part 2

Fig.1: Mapping between logical, transport, and physical channels. Image Ref: Fig 6..11 from E. Dahlman et al. “5G NR the Next Generation Wireless Access Technology”, Academic Press 2020

Channels are communication pathways between multiple layers in a stack NR. For example, RLC and MAC layers are connected via the Logical channel, whereas MAC and PHY layers are connected via the Transport channel. The radio transmission (between gNB and user device) takes place over the Physical channel.

The above highlighted are the three main channels in 5G NR. In part 1 of this article, we discussed the Physical channel. In this part, we will discuss Logical and Transport channels in 5G NR. So, let's begin.

Fig. 2: Roles of layers and channels in 5G NR

Logical Channels

The logical channel is the link between RLC-layer and MAC-layer and provides data from RLC to MAC layer. So what happens is that RLC-layer receives generic data from the upper layer (say application-layer), segmentizes the generic data into different categories (e.g. data or control type), and then map that data onto logical channels. From logical channels, MAC-layer takes over the data and perform operations such as multiplexing data from logical channels to respective transport channels, initiating HARQ process etc.

Now we can understand why the logical channel is further is categorized as per the type of information it carries, i.e. Control and User (or Traffic) Channel.

The control channel, as the name suggests, transmit control and configuration information necessary for operating an NR system. The traffic channel transmits user data.

The 5G NR has five logical channels are:

1. Broadcast Control Channel (BCCH): This channel is used for the transmission of System Information (SI) from the network to all devices in a cell. Prior to accessing the system, a device needs to acquire the SI to find out how the system is configured and, in general, how to behave properly within a cell. In case of a non-standalone (NSA) operation, SI is provided by the LTE BCCH is not used. BCCH carries MIB information to BCH-to-PBCH. For other SI (i.e. SIBs), BCCH carries SIBs to DLSCH-to-PDSCH. By decoding MIB, the UE is able to decode SIB and understand the dynamic information that SIB carries.
2. Paging Control Channel (PCCH): This channel is used for paging devices whose cell-level locations are unknown to the network. Therefore, the paging message must be transmitted in multiple cells. In the case of NSA operation, paging is provided by the LTE system and there is no PCCH.
3. Common Control Channel (CCCH): This channel is used for the transmission of control information in conjunction with random access. RRC messages is also sent over CCCH during RRC connection setup phase.
4. Dedicated Control Channel (DCCH): This channel is used for the individual device configuration, such as, setting various parameters in devices; and transmitting control information to/from a device. RRC messages are also sent over DCCH, but this is different than RRC on CCCH as this is done once RRC connection is established.
5. Dedicated Traffic Channel (DTCH): This is the logical-channel type used for transmission of all unicast uplink and downlink user data.

Transport Channels

The Transport channel is the link between MAC-layer and PHY-layer. In fact, part of the MAC functionality is the multiplexing and mapping of different logical channels to the appropriate transport channels. (See Fig. 1 for mapping example).

Data on the transport channel is organized into Transport Blocks (TB) by MAC-layer, which is then carried to PHY-layer. At PHY-layer data is mapped over physical channels (e.g. PDCCH, PDSCH etc.). (This should easily clarify the difference between layers and channels. Actual data manipulation takes place at layers, channels are only carriers of data).

In each Transmission Time Interval (TTI), at most one dynamic-sized TB is transmitted over the radio interface to/from a device (in case of spatial multiplexing with more than four layers, there are two TBs per TTI)[1].

Associated with each TB is a Transport Format (TF), specifying how the TB is to be transmitted over the radio interface. The TF includes information about the transport-block size, the modulation-and-coding scheme, and the antenna mapping [1]. By varying the TF, the MAC layer can thus play with different data rates. This process is known as transport-format selection. (this is useful in the link adaptation error-free transmission is ensured even in the worst radio conditions).

Anyways, 5G NR has five transport channels as well. They are:

1. Broadcast Channel (BCH): It is used for transmission of parts of the BCCH SI, more specifically known as Master Information Block (MIB).
2. Paging Channel (PCH): This channel is used for the transmission of paging information from the PCCH logical channel. The PCH supports discontinuous reception (DRX) to allow devices to save battery power by waking up to receive the PCH only at predefined time instants.
3. Downlink Shared Channel (DL-SCH): This is the main transport channel used for the transmission of DL data in NR. 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 SI is received there is one additional DL-SCH from the device perspective.
4. Uplink Shared Channel (UL-SCH): This channel is the uplink counterpart to the DL-SCH.
5. Random Access Shared Channel (RACH): It is used for random access procedures but it does not carry any TB and hence it is different from other transport channels. RACH has no higher layer. It starts and ends at the MAC layer.

There are some sidelink channels as well, but I have very little knowledge about them.

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Further Reading

For a detailed discussion, see 6.4.4.1 of an excellent book titled “5G NR, The Next Generation Wireless Access Technology” by Erik Dahlman et. al.

[1] Erik Dahlman et. al.“5G NR, The Next Generation Wireless Access Technology”.