How to identify RRU optical modules

The optical module on an RRU is a compact, rectangular device that provides optical-electrical conversion, allowing fiber optic cables to transmit signals between the RRU and other network equipment.

Appearance and Placement

The optical module is typically a small, rectangular unit inserted into a dedicated port on the RRU panel. It is designed to accommodate fiber optic connectors and is often located near the RRU's cabling cavity or interface panel. The module may have a metal or plastic casing with a slot for the optical fiber and sometimes includes indicators for operational status.

Before replacing the module, optical cables must be removed to avoid signal interruption.

Function

The primary function of the optical module is optical-electrical conversion, enabling high-speed data transmission between the RRU and the Baseband Unit (BBU). It converts electrical signals from the RRU into optical signals for fiber transmission and vice versa, supporting CPRI or eCPRI interfaces. This allows the RRU, which is mounted on towers, to communicate efficiently with the BBU located in the server room.

Types and Specifications

In 4G networks, common optical modules include 1. 25G, 2. 5G, 6G, and 10G modules, often using single-mode or multi-mode fiber.

For 5G networks, higher-speed modules such as 25G SFP or 10G SFP+ are used. These modules typically operate at wavelengths like 850nm, with working temperatures ranging from 0–75°C and power consumption under 1W. Transmission distances vary depending on the module and fiber type, e.g., 550m for 1.

25G SFP over multi-mode fiber.

Connectivity

The optical module connects to the RRU via a dedicated optical port. Fiber optic patch cords are inserted into the module, linking the RRU to the BBU or to a CWDM multiplexer in serial networking setups. Passive wavelength division multiplexing is often used to optimize fiber usage when multiple RRUs are connected over long distances.

Maintenance Notes

Replacing an optical module interrupts signal transmission, so care must be taken during maintenance. Modules are designed for easy insertion and removal, and proper handling of fiber connectors is essential to prevent damage. In summary, the optical module is a small, rectangular interface on the RRU that enables optical communication with the BBU, comes in various speeds and types depending on network generation, and is critical for high-speed, long-distance signal transmission in distributed base stations.