O-RAN Fronthaul Latency or Delay model

Latency is a critical factor in O-RAN fronthaul networks, as it impacts real-time signal processing and synchronization between the O-DU (O-RAN Distributed Unit) and O-RU (O-RAN Radio Unit). The O-RAN Alliance defines specific latency models and requirements to ensure low-latency performance in fronthaul communication.

1. O-RAN Fronthaul Latency Model

O-RAN fronthaul latency is influenced by:

1. Transport Network Delay (Propagation Delay) – Time taken for packets to travel between O-DU and O-RU.

2. Switching & Processing Delay – Includes queuing delays, serialization delays, and hardware processing at O-DU/O-RU.

3. Synchronization & Compensation Mechanisms – Using eCPRI timestamping, PTP (IEEE 1588v2), and SyncE for accurate timing. 

a) Latency Components in the O-RAN Fronthaul

The reference points defined for eCPRI are reflected below. The reference points are:

• O-DU: R1/ R4 – Transmit/ Receive interface at O-DU

• O-RU: R2/ R3 – Receive/ Transmit interface at O-RU

• Ra: Antenna interface at O-RU

• Transmission delay between O-DU and O-RU are specified as T12 (downlink) and T34 (uplink)

• Transmission and reception at the reference points are measured relative to Ra. Below are the resulting parameters.

Timing Parameter Relationships

To make sure data is received correctly over the network, certain timing rules must be followed. First, it's important to understand how data is transmitted. In both uplink (O-RU to O-DU) and downlink (O-DU to O-RU) directions, it takes some time for the sender to put data packets onto the network.

The maximum time allowed for sending all data within a given period is called the Transmission Window, which is defined as T1amax - T1amin. This window depends on the network and O-RU characteristics.

The receiver (O-DU or O-RU) uses a Reception Window to manage variations in transmission time.

The position (in time) of the reception/ transmission windows at the O-RU is fixed relative to the air interface. However, the position of the corresponding windows at the O-DU is a function of the O-RU and transport parameters.

For guaranteed reception of packets sent from O-DU to O-RU within the O-RU reception window, the following relationships must be met: 

For ex: Downlink timing using the below parameter values:

T2amin = 100µs, T2amax = 200µs, T12min = 0µs, T12max = 50µs

From the table, the downlink transmission timing constraints for O-DU are:

1. T1amax ≤ T2amax + T12min

2. T1amin ≥ T2amin + T12max

Final Downlink Timing Constraints for O-DU:

• T1amax must be ≤ 200µs (O-DU cannot transmit earlier than this).

• T1amin must be ≥ 150µs (O-DU must transmit no later than this).

O-DU must transmit data between 150µs and 200µs to ensure proper synchronization with O-RU.

If O-DU sends data too early (before 150µs), O-RU may not be ready to receive it.

If O-DU sends data too late (after 200µs), it may not align with the scheduled radio transmission at O-RU.

U-Plane/ C-Plane Timing

C-Plane messages must be received before the related U-Plane data packets at the O-RU. This ensures that the system has enough time to process control instructions before the actual data arrives.

To manage this, the O-RAN interface includes a timing rule:

• The C-Plane message must reach the O-RU at least a certain amount of time before the first related U-Plane message arrives. This advance time is called Tcp_adv_dl.

• In some cases, Tcp_adv_dl can be set to 0, meaning both control and data packets could arrive at the same time. However, the standard O-RAN delay model does not assume this as the default case.

Think of it like a traffic signal system:

• The C-Plane is like a green traffic signal that tells cars (U-Plane data) when to move.

• If the signal (C-Plane message) is delayed, cars (U-Plane data) might arrive at the intersection with no instructions, causing confusion.

• That’s why the system ensures that the green signal (C-Plane) arrives before the cars (U-Plane) to keep traffic (data flow) smooth.

How Data & Control Information Flow in DL:

Step 1: O-DU Transmits C-Plane First

The C-Plane message (control information) for a specific symbol (e.g., symbol #n) is sent first from the O-DU.

It is transmitted during a specific Transmission Window (green line, C-Plane DL).

Step 2: O-RU Receives C-Plane Before U-Plane

The O-RU must receive the C-Plane before the related U-Plane data arrives.

The required advance time is called Tcp_adv_dl (C-Plane advance time).

The reception window (green) at the O-RU ensures the control information is processed before handling the actual user data.

Step 3: O-DU Sends U-Plane Data (IQ Samples)

After sending the C-Plane, the O-DU starts sending the actual data (U-Plane IQ samples) for symbol #n.

This happens during the Transmission Window for U-Plane DL (blue section).

This U-Plane data includes the actual information that will be transmitted to the user.

Step 4: O-RU Receives U-Plane Data

The U-Plane data must arrive at the O-RU within the Reception Window (blue section).

However, due to fronthaul delay (T12_max, red arrow), there is a slight delay in receiving the data at the O-RU.

The O-RU waits for the full reception window before forwarding data to the antenna port.

Step 5: Data is Sent to the User

Once both C-Plane and U-Plane information is correctly received, the O-RU processes the symbol and sends it to the user via the antenna.

This happens at the correct time so that the symbol aligns properly in the air interface.

How Data & Control Information Flow in UL:

Step 1: O-DU Sends C-Plane to O-RU

• The C-Plane message (control information) is always sent first from the O-DU to the O-RU.

• This message is sent within the C-Plane Transmission Window (green section on the left).

• The C-Plane contains instructions, such as scheduling information, that guide the O-RU on when and how to send uplink data.

Step 2: O-RU Receives and Processes C-Plane

• The O-RU must receive the C-Plane before it can send the U-Plane data.

• This reception happens within the C-Plane Reception Window (green section at O-RU).

Step 3: O-RU Transmits U-Plane Data (IQ Samples) to O-DU

• After processing the C-Plane instructions, the O-RU transmits the uplink user data (U-Plane IQ samples).

• This happens within the U-Plane Transmission Window (blue section on the right).

• The U-Plane contains the actual signal received from the user, which must reach the O-DU on time.

Step 4: O-DU Receives U-Plane Data from O-RU

• The O-DU must receive the U-Plane data within the Reception Window (blue section at O-DU).

• Due to fronthaul delay (T34_max, red arrow), the actual reception time can vary.

• The O-DU then processes this data for further transmission within the network.

The resulting O-RAN delay model parameters are summarized below.

In O-RAN, there are two ways to handle delay calculations:

• Defined Transport Method: the network delay is pre-defined (usually by the network operator). The definition generally considers the maximum network latency, with an assumption that a smaller delay can more easily be accommodated.  

Below is the example of defined transport method computed delay profile.

• Measured Transport method: the network delay is estimated based on actual measurements of packet latency on the downlink and uplink.

O-RAN specifies the use of the One-Way Delay Measurement messages as defined in the eCPRI specification.

Here’s a step-by-step explanation of the process for measuring T12 in O-RAN:

1. O-DU Sends Request:

• The O-DU starts the measurement by sending a One-Way Delay Measurement Request to the O-RU.

• This request includes:

  • A timestamp (t1), which marks when the request was transmitted from the O-DU.

  • A compensation value, which adjusts for the expected processing delay before the packet is actually sent.

2. O-RU Receives and Timestamps the Request:

• The O-RU receives the request and records a timestamp (t2) when it arrives.

3. O-RU Sends a Response:

• The O-RU creates a One-Way Delay Measurement Response message.

• This response includes:

  • The timestamp (t2) recorded when the request was received.

  • A compensation value, which accounts for the expected delay from when the packet was received until it was timestamped. 

4.O-DU Calculates T12:

• After receiving the response from the O-RU, the O-DU computes T12 using a formula that considers:

  • The timestamps t1 and t2

  • The compensation values (tcv1 and tcv2), which adjust for known processing delays in the O-DU and O-RU.

4. Conclusion & Key Takeaways

1. O-RAN latency model defines precise timing references (T12, T34, T1a, T2a, Ta3, Ta4) for fronthaul transport.

2. The relative time error of the S-plane measurement signals between the O-DU and O-RU should be within a limit of 3μs (±1.5 μsec).

3. PTP (IEEE 1588v2) and SyncE are essential for maintaining time & frequency synchronization.

Reference:

O-RAN Control, User and Synchronization Plane Specification, https://specifications.o-ran.org/specifications