Different Channels in 5G NR — Part 2
Channels are communication pathways between multiple layers in a 5G NR stack. 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 discuss?Logical?and?Transport?channels in 5G NR. So, let's begin.
Logical Channels
The logical channel is the link between RLC-layer and MAC-layer. So what happens is that RLC-layer receives generic data from the upper layer (say application-layer), segmentize 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, scheduling, initiating HARQ process etc.
Now we can appreciate why the logical channel is further is categorized as 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:
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:
There are some sidelink channels as well, but I have very little knowledge about them.
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”.