O-RAN Working Group 4 (WG4): Open Fronthaul Interfaces

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The O-RAN Alliance aims to drive open, intelligent, and virtualized RAN architectures by defining standard interfaces between different network components. O-RAN Working Group 4 (WG4) is responsible for defining the Open Fronthaul Interface, which serves as the communication link between the O-RAN Distributed Unit (O-DU) and the O-RAN Radio Unit (O-RU).

WG4 plays a critical role in enabling vendor-neutral, multi-supplier deployments in 4G LTE, 5G, and future 6G networks. By establishing protocol specifications, transport architecture, functional splits, and management mechanisms, WG4 ensures:

• Interoperability between O-RU and O-DU from different vendors.
• Optimized latency and performance through split-based architectures.
• Efficient network synchronization for Time Division Duplex (TDD) operations.
• Advanced management capabilities through NETCONF/YANG.

Functional Split Architecture in WG4

The fronthaul interface in O-RAN follows a Lower Layer Functional Split (LLS), specifically Split 7-2x. This split was chosen for its ability to reduce fronthaul bandwidth while maintaining centralized processing advantages for beamforming and MIMO.

The Open Fronthaul Architecture consists of the following core components:
O-DU (O-RAN Distributed Unit):

• Handles Layer 2 and higher-layer processing (PDCP, RLC, MAC).
• Manages scheduling, beamforming control, and hybrid ARQ (HARQ).
• Encodes and transmits IQ samples to O-RU.

O-RU (O-RAN Radio Unit):

• Processes Layer 1 (Physical Layer) and RF functions.
• Applies precoding, beamforming, and modulation.
• Converts digital signals into RF transmissions.

Fronthaul Transport Network:

• Connects O-DU and O-RU via Ethernet/IP or eCPRI.
• Implements latency control and time synchronization.

Understanding Functional Split 7-2x

WG4’s Open Fronthaul Interface is based on Split 7-2x, which separates PHY processing between O-DU and O-RU:
Downlink (DL) Processing

• O-DU generates IQ data for transmission.
• Beamforming weights are applied at the O-DU.
• O-DU transmits IQ samples over the fronthaul link.
• O-RU performs precoding and RF transmission.

Uplink (UL) Processing

• O-RU receives RF signals from mobile users.
• Extracts IQ samples and forwards them to O-DU.
• O-DU processes and demodulates the data.

This split reduces fronthaul bandwidth requirements while allowing O-DU to centrally control massive MIMO beamforming.

WG4 Fronthaul Protocols and Transport Mechanisms

WG4 specifies protocols for data transport, control signalling, and synchronization. These protocols operate over the following three planes:

Control Plane (C-Plane)

• Manages signalling between O-DU and O-RU.
• Controls beamforming commands, PRACH, and scheduling.
• Uses Ethernet-based encapsulation with eCPRI.

User Plane (U-Plane)

• Transfers IQ data for uplink and downlink.
• Supports compression for bandwidth efficiency.
• Implements fragmentation and reassembly of packets.

Synchronization Plane (S-Plane)

• Manages frequency and phase synchronization.
• Uses IEEE 1588v2 Precision Time Protocol (PTP).
• Ensures low-latency and jitter-free transmissions.

Transport & Protocol Stack

The transport layer is designed to minimize latency, jitter, and packet loss. WG4 supports three encapsulation methods:
Ethernet (Layer 2)

• Low-latency fronthaul with minimal overhead.
• Supports VLAN tagging and QoS.

IP/UDP (Layer 3)

• Enables routing over IP networks.
• Provides flexibility but increases overhead.

eCPRI (Ethernet-based Common Public Radio Interface)

• Optimized for radio-over-Ethernet transport.
• Reduces bandwidth requirements by 50% compared to CPRI.

Management Plane (M-Plane)

The Management Plane (M-Plane) in WG4 ensures configuration, monitoring, and fault management for O-RU.

Key Management Functions

• Device Discovery & IP Assignment: O-RU automatically obtains an IP address via DHCP.
• Software & Firmware Updates: Supports remote software upgrades for O-RUs.
• Performance Monitoring: Collects real-time KPIs, counters, and logs.
• Security Mechanisms: Implements TLS-based encryption and NETCONF/YANG-based authentication.

Security Considerations

WG4 enforces strong security measures to protect the fronthaul network:

• Encryption: Uses TLS and IPsec to secure control traffic.
• Authentication: Devices must use PKI certificates.
• Access Control: Only authorized O-DUs can control O-RUs.

WG4’s Open Fronthaul Interface is critical for the success of Open RAN by ensuring interoperability, performance optimization, and cost reduction. By defining control, user, synchronization, and management planes, WG4 facilitates vendor-neutral deployments and scalable network evolution.