O-RAN – Fronthaul Delay and Its Management

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O-RAN Split Option 7-2x, introduced a functional split between O-DU and O-RU within the PHY Layer of the radio interface. This split cause some delay in transmission and reception of data exchanged between O-DU and O-RU, this delay is know as Fronthaul Delay. The amount of fronthaul depend up on the topology, how O-DU and O-RU are connected and underlaying transport (router and Switches) network.

Fronthaul Delay  Management

Fronthaul Delay Management is required to transmit C/U-Plane messages on the fronthaul in accordance with transmit/receive timing of the radio interface (Uu) and retransmission timing of the HARQ procedure. This delay management enables to adopt the concept of a receive window and transmit window based on the eCPRI framework.

Delay management for transmission from the O- DU to O-RU is shown in below figure. Upon receiving the IQ sample sequence of the OFDM signal in the frequency domain from the fronthaul, the O-RU does certain IFFT, analog conversion, BF, etc. in time for transmitting the signal on the radio interface given specific time resources (radio frame, subframe, slot, OFDM symbol). For this reason, the receive window position of the O-RU  is set before transmission timing on the radio interface at an offset corresponding to this O-RU processing delay.

The O-DU, meanwhile, transmit a C/U-Plane message to the fronthaul so that it is delivered within the O-RU receive window. Accordingly, the transmit window position of the O-DU is set before transmission timing to the radio interface at an offset corresponding to O-RU processing delay and fronthaul delay. Here, fronthaul delay includes variable elements such as fronthaul distance and switch processing delay.

  • The size of the O-RU receive window is calibrate to a length that can cover this fluctuation in fronthaul delay and size of the O-DU transmit window.
  • The size of the O-DU transmit window is calculated after accounting the processing time required for the O-DU to transmit the C/U-Plane message to the fronthaul.

Delay management using the same type of windows is also applied for transmission in the direction from O-RU to O-DU. Additionally, the fronthaul specifications define separate windows for the C-Plane and U-Plane.

Reference:

  • Overview of O-RAN Fronthaul Specifications- White Paper From Docomo
  • O-RAN Specification

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