What are the different Channels in 5G NR, and what are their functions?
In order to be able to carry the data across the 5G radio access network, the data and information is organized into a number of data channels. By organizing 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.
There are three types of channels in 5G NR:
1️⃣ 𝐋𝐨𝐠𝐢𝐜𝐚𝐥 𝐂𝐡𝐚𝐧𝐧𝐞𝐥𝐬:
Offered by MAC to RLC. Control channels carry CP packets. Traffic channels carry UP packets. Each logical channel maps to an RLC channel coming from RLC layer. 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.
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 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: BCCH-PCCH-CCCH-DCCH-DTCH.
2️⃣ 𝐓𝐫𝐚𝐧𝐬𝐩𝐨𝐫𝐭 𝐂𝐡𝐚𝐧𝐧𝐞𝐥𝐬:
Offered by PHY to MAC. MAC layer multiplexes one or more logical channels to a transport channel. Whereas logical channels describe what is carried, transport channels describe how they’re carried. 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. 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). 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.
5G NR has five transport channels as well: BCH-PCH-(DL-SCH)-(UL-SCH)-RACH.
3️⃣ 𝐏𝐡𝐲𝐬𝐢𝐜𝐚𝐥 𝐂𝐡𝐚𝐧𝐧𝐞𝐥𝐬:
Channels that carry information on the air interface. The physical channels are the closest to the actual over-the-air transmission. Transport channels map to physical channels.
In total, there are six Physical Channels : PBCH-PDCCH-PUCCH-PDSCH-PUSCH-PRACH.
