5G Radio Protocols: A Quick Structural Guide

Protocol stack layers

• Layer 1 (Physical Layer - L1): Handles wireless transmission, modulation, coding, and signal processing.

• Layer 2 (Data Link Layer - L2): Ensures reliable data transfer, error correction, and efficient resource allocation.

• Layer 3 (Network Layer - L3): Manages connections, mobility, and security between devices and the core network.

Layered Protocol Organization

1️⃣ Layer 1 (Physical Layer - L1)

🔹 PHY (Physical Layer): handles the transmission and reception of raw data over the air, including power control, synchronization, random access, and HARQ (Hybrid Automatic Repeat Request).

2️⃣ Layer 2 (Data Link Layer - L2)

🔹 MAC (Medium Access Control): Manages radio resource allocation and scheduling.

🔹 RLC (Radio Link Control): Provides segmentation, reassembly, and retransmission of data packets.

🔹 PDCP (Packet Data Convergence Protocol): Handles data compression, encryption, and integrity protection.

🔹 SDAP (Service Data Adaptation Protocol): maps 5G core network QoS flows to data radio bearers for QoS (Quality of Service) purposes. Used only for 5G SA (Standalone).

3️⃣ Layer 3 (Network Layer - L3)

🔹 RRC (Radio Resource Control): Manages connection setup, handovers, mobility, and security.

🔹 NAS (Non-Access Stratum): Facilitates communication between the UE and the 5G Core, handling authentication, session management, and mobility across different network nodes.

User & Control Plane Protocol Stack

We can now examine the RAN protocol architecture for both the user and control planes. The following figure provides an overview of this architecture, including the AMF for completeness, even though it is not part of the RAN.

In the next section, we will first explore the user-plane protocols, followed by the control-plane protocols. Since many protocol layers, such as PDCP, RLC, MAC, and PHY, are shared between both planes, they will be covered only in the user-plane section.

User Plane Protocols

The NR user-plane protocol stack shares similarities with LTE but introduces key differences, particularly in Quality-of-Service (QoS) handling. When connected to a 5G core network, the SDAP (Service Data Adaptation Protocol) manages QoS flows, mapping them to radio bearers. However, SDAP is not used when the NR user plane connects to the EPC.

The uplink protocol structure closely resembles the downlink architecture, as shown in Figure 3, with a few key differences, such as in the selection of transport formats and the management of logical channel multiplexing.

Notes on Protocol Functionalities with Non-Descriptive Names:

• PDCP Header compression: Robust Header Compression (RoHC) is employed to optimize real-time data transmission by significantly reducing the size of headers across multiple layers, enabling more efficient use of bandwidth.

• RLC Automatic Repeat reQuest (ARQ): A mechanism used to identify and correct errors by retransmitting lost or corrupted data.

• MAC HARQ (Hybrid ARQ) combines Error Correction (FEC) and Automatic Repeat reQuest (ARQ). FEC adds redundant bits for error detection and correction, while ARQ requests retransmission of data if an error is detected.

• PHY CBG retransmissions: Code Block Groups (CBGs) enable selective retransmission of erroneous code block groups instead of the entire transport block, optimizing resource usage. These retransmissions occur at the physical layer as part of the hybrid-ARQ mechanism but remain invisible to the MAC layer.

Dual Connectivity

In dual connectivity, a device connects two cell groups: the Master Cell Group (MCG) and the Secondary Cell Group (SCG), which may be managed by different gNBs. PDCP handles data distribution between the MCG and SCG, including split bearers where one radio bearer is shared by both groups.

Carrier Aggregation

Carrier Aggregation (CA) enhances network capacity by combining multiple component carriers within the same gNB. This coordination allows for efficient scheduling, where all cells share a common scheduler. Unlike dual connectivity, which can connect to multiple cells, CA requires tight coordination between cells within the same gNB.

CA can also be used alongside dual connectivity, creating master and secondary cell groups, where carrier aggregation is applied within each group to optimize data throughput.

Control Plane Protocols

NAS (Non-Access Stratum): Operates between the device and the AMF in the core network. It handles authentication, security, idle-mode procedures (e.g., paging), and IP address assignment.

RRC (Radio Resource Control): Operates between the device and the gNB. It controls system information broadcast, paging, connection management, mobility, measurement reporting, and device capability handling. RRC messages use signaling radio bearers (SRBs) and can be multiplexed with user-plane data at the MAC layer.

RRC states

• RRC_IDLE: Supports DRX (Discontinuous Reception), system information broadcast, cell reselection, and paging.

• RRC_CONNECTED: The device has an active RRC connection and Access Stratum (AS) context stored in the gNB.

• RRC_INACTIVE: Uses DRX with mobility managed in a RAN-based notification area, like UMTS URA_PCH.

Logical, Transport & Physical Channels

As shown in the previous Figure 3, the placement of logical channels, transport channels, and the physical layer can be visualized, illustrating their interaction within the protocol stack.

The Medium Access Control (MAC) layer manages logical-channel multiplexing, scheduling, hybrid-ARQ retransmissions, and carrier aggregation. It provides services to the Radio Link Control (RLC) layer via logical channels, which are categorized as control or traffic channels. These include BCCH, PCCH, CCCH, DCCH, and DTCH.

The MAC layer also interacts with the physical layer through transport channels, which define how data is transmitted. Key transport channels include BCH, PCH, DL-SCH, UL-SCH, and RACH. The MAC layer prioritizes and multiplexes logical channels into transport channels, optimizing transmission efficiency.

Channel structure

List of channels and their acronyms:

Logical channels:

• BCCH – Broadcast Control Channel

• PCCH – Paging Control Channel

• CCCH – Common Control Channel

• DCCH – Dedicated Control Channel

• DTCH – Dedicated Traffic Channel

Transport channels:

• BCH – Broadcast Channel

• PCH – Paging Channel

• DL-SCH – Downlink Shared Channel

• UL-SCH – Uplink Shared Channel

• RACH – Random Access Channel

Physical channels:

• PBCH – Physical Broadcast Channel

• PDCCH – Physical Downlink Control Channel

• PDSCH – Physical Downlink Shared Channel

• PRACH – Physical Random-Access Channel

• PUCCH – Physical Uplink Control Channel

• PUSCH – Physical Uplink Shared Channel

This guide provides a quick reference to 5G radio protocols, highlighting key functionalities and channel structures. Understanding these concepts is essential for working with modern wireless networks.

References

My favorite book on 5G:

https://www.ericsson.com/en/reports-and-papers/books/5g-nr-the-next-generation-wireless-access-technology

5G Protocols Mpirical course:

https://www.mpirical.com/courses/5g-protocols