Introduction to O-RAN Architecture

Introduction to O-RAN Architecture

O-RAN (Open Radio Access Network) is an industry-standard framework that aims to virtualize and disaggregate the RAN (Radio Access Network) by adopting open interfaces and interoperable components. The goal is to create a more flexible, vendor-neutral, and cost-effective RAN ecosystem.

The O-RAN Alliance has defined a standardized architecture consisting of several logical components connected by well-defined interfaces, which enable multivendor interoperability and more agile network deployment.

Key Components in O-RAN Architecture

Fig: Logical O-RAN architecture (ref- ORAN WG1 spec)

The O-RAN architecture is divided into the following functional elements:

1. Non-Real-Time RIC (Non-RT RIC)

• Located within the Service Management and Orchestration Framework.

• Handles non-real-time functions such as:

  • Policy management

  • Network optimization

  • Configuration management

• Operates on time scales greater than 1 second.

• Communicates with the Near-RT RIC over the A1 interface.

2. Near-Real-Time RIC (Near-RT RIC)

• Manages near-real-time RAN optimization functions.

• Handles time-sensitive RAN control with response times of 10 milliseconds to 1 second.

• Controls:

  • Load balancing

  • Mobility management

  • Interference mitigation

• Communicates with other RAN components via:

  • E2 interface – For direct control over O-CU and O-DU.

  • Y1 interface – For data exchange with Y1 consumers.

3. O-CU (Centralized Unit)

The O-CU is split into two functional parts:

• O-CU-CP (Control Plane):

  • Handles RRC (Radio Resource Control) signaling.

  • Manages mobility, handovers, and session management.

  • Communicates with:

    • O-DU via F1-c interface

    • O-CU-UP via E1 interface

    • Core network via NG-c interface

• O-CU-UP (User Plane):

  • Handles user data transmission.

  • Communicates with:

    • Core network via NG-u interface

    • O-DU via F1-u interface

4. O-DU (Distributed Unit)

• Handles real-time Layer 1 and Layer 2 processing (PHY, MAC, RLC).

• Controls scheduling, HARQ (Hybrid Automatic Repeat Request), and retransmissions.

• Communicates with:

  • O-RU via Open FH CUS-Plane and Open FH M-Plane interfaces.

  • O-CU via F1-c and F1-u interfaces.

5. O-RU (Radio Unit)

• Handles RF (Radio Frequency) functions and signal transmission.

• Processes:

  • Digital beamforming

  • Antenna control

• Communicates with:

  • O-DU via Open FH CUS-Plane and Open FH M-Plane interfaces.

6. O-eNB

• Represents a legacy 4G eNodeB integrated into the O-RAN framework.

• Communicates with:

  • O-CU and O-DU over standard X2 and E2 interfaces.

7. O-Cloud

• Provides the infrastructure to host O-RAN components.

• Supports virtualization, containerization, and cloud-native functions.

• Provides:

  • Compute resources

  • Storage

  • Networking

Key Interfaces in O-RAN Architecture                 

The O-RAN architecture defines several standardized interfaces for interoperability between components:

1. A1 Interface – Between Non-RT RIC and Near-RT RIC

• Used for:

  • Policy-based control

  • AI/ML model updates

  • Performance monitoring

2. E1 Interface – Between O-CU-CP and O-CU-UP

• Enables separation of control plane and user plane.

• Supports low-latency data exchange for fast handovers and session handling.

3. E2 Interface – Between Near-RT RIC and O-CU/O-DU

• Used for:

  • Real-time data collection and analysis

  • Load balancing and interference management

4. F1-c and F1-u Interfaces – Between O-CU and O-DU

• F1-c – Control plane signaling

• F1-u – User data transmission

• Supports:

  • Split of processing between O-CU and O-DU

  • Flexibility for resource allocation

5. NG-c and NG-u Interfaces – Between O-CU and Core Network

• NG-c – Control signaling between O-CU-CP and AMF (Access and Mobility Management Function).

• NG-u – User data path between O-CU-UP and UPF (User Plane Function).

6. X2-c and X2-u Interfaces – Between O-CU and O-eNB

• X2-c – Control plane interface for handovers and mobility management.

• X2-u – User plane interface for data forwarding.

7. Xn-c and Xn-u Interfaces – Between O-CU and other gNBs

• Xn-c – Control plane interface for handovers and mobility across gNBs.

• Xn-u – User plane interface for data forwarding.

8. Open FH CUS-Plane and M-Plane – Between O-DU and O-RU

• CUS-Plane – Used for user and control data transmission.

• M-Plane – Used for management and configuration of O-RU.

9. Y1 Interface – Between Near-RT RIC and Y1 Consumers

• Y1 consumers could include:

  • Network analytics platforms

  • AI/ML training models

• Provides monitoring and insights for network optimization.

Advantages of O-RAN Architecture

• Vendor Independence: Open interfaces enable multi-vendor interoperability.

• Cost Efficiency: Lower CAPEX and OPEX due to disaggregation and use of COTS (Commercial off-the-shelf) hardware.

• Agility and Flexibility: Cloud-native and virtualized functions enable quick deployment and scaling.

• Enhanced Performance: Real-time and AI-based optimization through the RIC.

• Improved Network Slicing: Efficient use of resources for different use cases (eMBB, URLLC, mMTC).

O-RAN Challenges

While O-RAN presents a promising path toward a more open and cost-efficient RAN ecosystem, the challenges related to performance, interoperability, security, and operational complexity remain significant.

Conclusion

O-RAN's open and disaggregated architecture represents a significant shift from traditional, vendor-specific RAN implementations. The use of standardized interfaces allows for interoperability, cost reduction, and enhanced network performance. The integration of AI and ML through the RIC further enhances the intelligence and adaptability of the network. O-RAN is expected to drive the future evolution of 5G and pave the way for the next generation of wireless communication networks.