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O-RAN Fronthaul Split 7-2x Architecture

  • Writer: Venkateshu
    Venkateshu
  • Mar 19
  • 2 min read

Split 7-2x is a widely adopted split option in O-RAN that defines how PHY layer functions are distributed between the DU and RU.


Functional Division in Split 7-2x


Message Flows in Split 7-2x Between High-PHY (DU) and Low-PHY (RU)

The Split 7-2x architecture divides the PHY layer into two parts:

  • High-PHY (in the DU)

  • Low-PHY (in the RU)

These two parts communicate via a well-defined fronthaul interface. The eCPRI (enhanced Common Public Radio Interface) protocol is commonly used to manage this communication.

High-PHY (DU) Functions

  • Channel coding (e.g., LDPC, Polar coding)

  • Rate matching

  • Modulation (e.g., QPSK, 16QAM, 64QAM)

  • Resource mapping

  • Time-frequency resource grid management

Low-PHY (RU) Functions

  • FFT/iFFT (Fast Fourier Transform)

  • PRACH filtering (for uplink random access)

  • Beamforming

  • RF front-end control

Message Types in Split 7-2x

The Split 7-2x interface defines key message types to enable seamless coordination between the DU and RU. These messages are categorized as follows:


User Plane (U-Plane) Messages

Carry data traffic between DU and RU.

  • Contains IQ samples representing the actual data payload.

  • Supports both frequency-domain and time-domain data formats.

  • Efficient compression techniques (e.g., block floating point) are often applied to reduce bandwidth.


Control Plane (C-Plane) Messages

Carry control instructions from DU to RU.

  • Includes parameters for:

    • Beamforming weights

    • Timing advance

    • PRACH configuration

    • Synchronization control

    Synchronization Plane (S-Plane) Messages

Manages timing and synchronization between the DU and RU.

  • Critical to maintain precise frame timing, symbol boundaries, and subframe alignment for smooth data transmission.

 Message Flow Process in Split 7-2x

Below is a step-by-step breakdown of the typical message exchange between the DU (High-PHY) and RU (Low-PHY):



 

Why is Split 7-2x Popular in O-RAN?

  • The Split 7-2x option is widely preferred because it balances the trade-offs between:

  • Fronthaul Bandwidth Efficiency: Split 7-2x significantly reduces fronthaul data rate compared to Split 8 (full PHY in RU).

  • Low Latency: Enables real-time processing in the RU with minimal delay.

  • Flexible Deployment: Split 7-2x allows the DU to be located centrally, simplifying network scaling.

  • Cost-Effective Design: By moving complex PHY functions to the DU, the RU design becomes simpler and more power-efficient.

  • Multi-Vendor Compatibility: The Split 7-2x interface is standardized, ensuring interoperability between different vendors.

Conclusion

The Split 7-2x architecture is a powerful and flexible design that optimizes data flow between DU and RU. Its ability to reduce fronthaul bandwidth, improve latency, and enable interoperability makes it ideal for 5G O-RAN deployments.

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