5G NR Protocol Structure Changes - An Overview
2020-07-23 | 7 min read
In a previous blog, I provided an overview of the 5G New Radio (NR) standard and key differences compared to 4G LTE. I also covered major differences between the two frequency ranges of the standard, frequency range 1 (FR1), commonly referred to as sub-6 GHz even though it extends to 7.125 GHz, and frequency range 2 (FR2), known as millimeter-wave (mmWave). In this blog, we will cover the protocol structure of 5G NR.
Figure 1. 5G NR protocol layers
In the control plane, control-relevant information is exchanged between the network and the user equipment. The establishment and management of sessions occurs at the highest layer on the control plane called non-access stratum (NAS). The next layer, radio resource control (RRC), exchanges control information with the device to set important parameters for the session.
In the user plane, the network and the user equipment exchange user data. The highest layers are the application and IP layers and refer to the worldwide web and other applications running on it. Data then goes through the service data adaptation protocol (SDAP), a new protocol layer for quality of service (QoS) management.
The higher protocol layers are based on LTE. The IP header is replaced with a 5G equivalent at the packet data convergence protocol (PDCP) layer. The radio link control (RLC) layer organizes the data and retransmission, if necessary. Prioritization and hybrid automated retransmission requests take place at the media access control (MAC) layer.
The last layer in the protocol structure is the physical layer (PHY). This layer involves aspects relevant for the communication channel between the user equipment and the core network as well as other aspects like modulation and beamforming.
The greatest changes for the protocol structure in 5G are at the PHY layer. There are a few important changes on layer 2 as well including:
- A new SDAP layer for QoS management in the user plane that provides mapping between QoS flow and data radio bearers and marking for QoS flow IDs in downlink and uplink packets all the way to the 5G core.
- A new feature that enables PDCP duplication, mapping packet data units (PDUs) to more than one logical channel and sending them over different component carriers.
- RLC/MAC layer support for beam management procedures and transmission modes that use different numerologies and transmission time intervals. Processing latency reduces as well. In LTE, the MAC layer had a huge header that pointed to the different PDUs. In 5G, the header is placed in front of the packet greatly reducing latency.
Data flows between the RLC, MAC, and PHY layers of the stack through channels. The logical channels are between the RLC and the MAC layers. These channels define the type of data that can be transferred.
The transport channels carry information from the MAC layer to the PHY layer. These channels define how the information will be carried to the physical layer and the characteristics of the data.
Finally, the physical layer communicates directly with the user equipment through the physical channels. Physical channel characteristics include timing, access protocols, and data rates.
Figure 2. 5G NR channel types
Logical channels on the downlink include:
- Broadcast control channel (BCCH) to manage broadcasting for all user equipment
- Paging control channel (PCCH) to manage paging messages
- Common control channel (CCCH) and dedicated control channel (DCCH) for common messages for all user equipment in the same cell
- Dedicated traffic channel (DTCH) to send data to specific user equipment in a cell
Transport channels for the downlink include the broadcast channel (BCH), the paging channel (PCH), and the downlink shared channel (DL-SCH). The BCCH logical channel has a direct connection to the DL-SCH. This route carries the system information block. The master information block has its own transport channel (BCH). The paging control channel also has its own transport channel (PCH).
Physical channels include the physical broadcast channel (PBCH), physical downlink shared channel (PDSCH), and the physical downlink control channel (PDCCH) channels. PDSCH transports the paging channel and the downlink shared channels. Broadcast messages have their own channel (PBCH). PDCCH handles downlink control information (DCI) and slot-format information (SFI).
Figure 3. 5G NR downlink channel mapping for logical, transport, and physical channels
Uplink channel mapping is similar to the downlink. A self-contained process called random access channel (RACH) exists for the initial connection. The user equipment sends back uplink control information (UCI) such as channel reports, HARQ-ACK, and scheduling requests.
Figure 4. 5G NR uplink channel mapping for logical, transport, and physical channels
It is also important to note that, compared to LTE, 5G NR physical channels have a few additions. They include a phase tracking reference signal (PTRS) to track phases and time scheduling and a demodulation reference signal for the uplink control channel and the downlink broadcast channel.
The 5G NR standard is complex and introduces significant changes in comparison to the previous generation standard. For more information on 5G NR technologies, challenges, and solutions, download the Engineering the 5G World eBook or visit Keysight’s 5G Solutions webpage.