Semi-active wavelength division multiplexing

A semi-active WDM system combines passive optical components at the remote site with active WDM equipment at the central site to optimize fiber usage, power consumption, and network management for 5G front-haul networks.

System Architecture

A semi-active WDM system typically consists of:

• Remote Passive Equipment: Located at the Active Antenna Unit (AAU) or Remote Radio Unit (RRU), this includes passive multiplexers/demultiplexers and optical couplers that combine or split multiple wavelength channels without requiring electrical power at the remote site .
• Local Active Equipment: Located at the Distributed Unit (DU) or Baseband Unit (BBU), this includes active WDM modules, optical switches, and multiplexers/demultiplexers that manage signal routing, amplification, and monitoring .
• Optical Channels: The system uses main and standby optical channels to provide redundancy and protection, ensuring reliable transmission even if one channel fails .

Operation and Signal Management

Semi-active WDM systems leverage pilot-tone relay detection and data preprocessing to manage optical carrier signals efficiently:

• Pilot-Tone Modulation: A low-amplitude pilot tone is superimposed on the optical signal to enable relay detection and monitoring of signal integrity .
• Data Preprocessing and Storage: Optical signals are filtered and stored based on preset waveform, buffering, or retention conditions, allowing selective output and improved transmission rates .
• OAM Functions: Operation, administration, and maintenance tasks, such as monitoring optical power, module temperature, and signal quality, can be performed using only downstream OAM demodulation units, reducing system complexity and cost .

Advantages

• Fiber Resource Optimization: By combining passive and active components, the system reduces the number of fibers required for multi-wavelength transmission .
• Lower Power Consumption: Passive components at the remote site eliminate the need for electrical power, reducing energy usage .
• Flexible Deployment: The semi-active architecture allows for scalable and adaptable network deployment with simplified OAM operations .
• High Performance: Experimental results demonstrate low sensitivity penalties (<0.3 dB), high extinction ratios (>4.2 dB), and error-free transmission of multiple 25-Gbps eCPRI channels over 10 km of single-mode fiber .

Applications

Semi-active WDM systems are particularly suited for:

• 5G Centralized Front-Haul Networks: Connecting AAUs/RRUs to DUs/BBUs efficiently while supporting high data rates and low latency .
• Metro and Access Networks: Where fiber resources are limited and cost-effective deployment is critical .
• Future 4G/5G Evolution: Supporting flexible, high-capacity optical transport with improved signal processing and network management capabilities .

Summary

A semi-active WDM system represents a hybrid approach between fully passive and fully active WDM architectures. By placing passive components at the remote site and active components at the central site, it achieves efficient fiber utilization, reduced power consumption, and enhanced network management, making it ideal for modern high-speed mobile networks such as 5G .