5G Channels: Explained

If you've worked in physical layer or are new to physical layer you may have come across confusing channel acronyms. In this article we'll get the overview of these channels.

Remember the time we used to flip channels on the TV with the remote buttons Ch+ and Ch- ? Yeah gone are those days with streaming apps like Netflix, YouTube, etc.

What channel meant there on those old remotes is the same thing in 5G. At physical level channels are simply a set of frequencies where our smartphones “tune” or look for a specific information. As you go higher up in the layers channels are a way to categorize and mix certain type of data that’s used in transmission between a smartphone and the base station. From a programmer’s perspective these higher level channels can be just an array or a data structure.

In telecommunications usually any equipment with a cellular capability is called an user equipment (UE). In the following sections you may see references to UE.

Figure 1: Hierarchy of downlink channels. Source: electronics-note.

Figure 2: Hierarchy of uplink channels. Source: electronic-note.

In 5G channels follow a hierarchical structure according to their purpose. There are three categories of channels:

1. Logical - These are higher level channels. Here the segregation of data is purely on logical basis i.e what type of data and piece of information can be carried together. There are two types of logical channels:

   1. Control - These channels carry control (signaling) information:

      1. Common Control Channel (CCCH): Transmits control information during random access to/from UEs.

      2. Dedicated Control Channel (DCCH): Carries control information to/from a specific UE.

   2. Traffic - These channels carry user data (traffic):

      1. Dedicated Traffic Channel (DTCH): Transmits user data to/from a specific UE.

2. Transport - These channel map and multiplex the data from logical channels to physical channels. From a programmer’s perspective you can simply think of mixing various logical channel arrays into a single array called a transport block which is ready to be sent over a physical channel. They are a link between the MAC layer and the physical layer. They carry data organized into transport blocks by the MAC layer.

   1. Broadcast Channel (BCH): Transmits the Master Information Block (MIB) containing essential system information.

   2. Paging Channel (PCH): Carries paging information from the PCCH logical channel.

   3. Downlink Shared Channel (DL-SCH): Main channel for downlink data transmission, including system information not mapped to BCH. You can see in the figure 1 how the data from CCCH, DCCH, DTCH and BCCH are multiplexed into DL-SCH.

   4. Uplink Shared Channel (UL-SCH): Main channel for uplink data transmission.

3. Physical - These channels carry the actual data over physical resource i.e an actual radio frequencies. Depending on their use case, different characterizations apply to each of these physical channels like: modulation, timing, power control, error correction etc. The layering of channels as transport and logical is necessary to map them to correct type physical channel.

   1. Downlink physical channels:

      1. Physical Downlink Shared Channel (PDSCH): Carries data from the DL-SCH transport channel, including user data, control messages, and system information.

      2. Physical Downlink Control Channel (PDCCH): Carries control information for scheduling downlink and uplink transmissions.

      3. Physical Broadcast Channel (PBCH): Transmits the master information block(MIB) from the BCH transport channel.

   2. Uplink physical channel:

      1. Physical Random Access Channel (PRACH): Used for initial access and transmission of random access preambles by UE to access network.

The key points are that logical channels organize data based on its type (control or traffic), transport channels multiplex logical channels for physical layer transmission, and physical channels carry the actual data over the radio interface.