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eCPRI in O-RAN Fronthaul Deployment (O-DU <-> O-RU Communication)

  • Writer: Venkateshu
    Venkateshu
  • Mar 23
  • 3 min read

In O-RAN (Open RAN) architecture, eCPRI plays a crucial role in the fronthaul interface between the O-DU (O-RAN Distributed Unit) and O-RU (O-RAN Radio Unit). It enables efficient transport of radio signal data (IQ samples) over packet-based networks such as Ethernet, IP, and UDP.


In O-RAN Working Group 4 (WG4), which focuses on the Fronthaul Interface, the CUS-Plane (Control, User, and Synchronization Plane) is responsible for carrying data between O-DU (O-RAN Distributed Unit) and O-RU (O-RAN Radio Unit). The eCPRI protocol is used to efficiently transport IQ data (User Plane), Control information (Control Plane), and Timing Data (Synchronization Plane).


1. Role of eCPRI in O-RAN Fronthaul

  • eCPRI enables low-latency, high-bandwidth communication between O-DU and O-RU.

  • It carries IQ samples, control and management data, synchronization, and diagnostics.

  • Reduces the bandwidth requirement compared to legacy CPRI by using packet-based transport (Ethernet, UDP/IP).

  • Supports scalability and flexibility for cloud-based and virtualized RAN deployments.


2. eCPRI Transport Header

The definition of the eCPRI transport header is shown below( ORAN WG4 CUS spec)


eCPRI Message Types Used in O-RAN

The following eCPRI message types are commonly used in the O-RAN Fronthaul (O-DU ↔ O-RU communication):

Message Type (Hex)

Description

Usage in O-RAN

0x00 (IQ Data)

Carries digitized radio IQ samples

Transfers user data (e.g., LTE/5G signals) between O-DU and O-RU

0x02 (Real-time Control Data)

Timing and scheduling control

Synchronization, Beamforming, and Frame Alignment

0x05 (One-Way Delay Measurement)

Measures propagation delay in the fronthaul link

Essential for timing synchronization

 

ecpriConcatenation (eCPRI concatenation indicator)

 

This parameter indicates when eCPRI concatenation is in use (allowing multiple eCPRI messages in a single Ethernet payload). NOTE: This parameter is part of the eCPRI common header.

Value range: {0b=No concatenation, 1b=Concatenation}.

 

ecpriRtcid / ecpriPcid (real time control data / IQ data transfer message series identifier)  

 

This parameter is an eAxC identifier (eAxC ID) and identifies the specific data flow associated with each  C-Plane (ecpriRtcid) or U-Plane (ecpriPcid) message.

 

The eAxC ID value assigned to an O-RU’s endpoint is used to identify the endpoint within O-RU and must be unique among all endpoints of O-RU of the same direction (Tx or Rx).

 

The O-DU may decide to use same eAxC ID value to address endpoints on different O-RUs, but eAxC ID values assigned to one O-RU must be unique within the O-RU’s endpoints of same direction.


The assignment of the DU_port_ID, BandSector_ID, CC_ID, and RU_Port_ID as part of the eAxC ID is done solely by  the O-DU via the M-plane.

 

ecpriSeqid (message identifier)

 

This parameter provides unique message identification and ordering on two different levels.

 

  • The first octet of this parameter is the Sequence ID, used to verify that all messages are received and also to reorder messages that are received out of order.

  • The second octet of this parameter is the Subsequence ID. The Subsequence ID is used to verify ordering and implement reordering when radio-transport-level (eCPRI or IEEE-1914.3) fragmentation occurs.

 

Example eCPRI packet header:



3. eCPRI Packet Flow in O-RAN Fronthaul


a) Downlink (O-DU → O-RU)

  1. O-DU prepares IQ data for transmission.

  2. eCPRI encapsulates the IQ data into packets.

  3. The packets are transported over Ethernet or UDP/IP.

  4. O-RU receives the packets, extracts IQ data, and converts them to RF signals.


b) Uplink (O-RU → O-DU)

  1. O-RU digitizes RF signals into IQ samples.

  2. The samples are encapsulated in eCPRI packets.

  3. The packets travel over UDP/IP or Ethernet to O-DU.

  4. O-DU processes the IQ data for further baseband processing.

 

4. Timing and Synchronization in O-RAN Fronthaul

eCPRI enables precise timing and synchronization between O-DU and O-RU, which is critical for:


  • 5G NR Time-Sensitive Networking (TSN)

  • Beamforming & MIMO synchronization

  • TDD (Time-Division Duplex) frame alignment

  • Latency-sensitive applications (e.g., URLLC in 5G)


Synchronization can be achieved using:

  • IEEE 1588 Precision Time Protocol (PTP)

  • Synchronous Ethernet (SyncE)

  • GPS-based synchronization


5. Key Advantages of eCPRI in O-RAN


  • Reduced Bandwidth Requirements – Uses packet-based transport, reducing overhead compared to traditional CPRI.

  • Flexible Transport – Supports Ethernet, IP, and UDP, allowing use of existing networks.

  • Lower Latency – Essential for 5G fronthaul, ensuring low-latency data transport.

  • Scalability – Works well with virtualized RAN (vRAN) and cloud-based deployments.

  • Better Network Management – Supports remote monitoring, fault detection, and software updates.

 


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