Optical Modules: The "Invisible Guardians" of 5G Networks - An Analysis of Fronthaul, Midhaul, and Backhaul Applications

一、 Brief Overview of 5G Network Architecture

To better understand the application of optical modules, we first need to understand the architecture of the 5G Radio Access Network (RAN), which is mainly divided into three parts:

• The network connecting the Baseband Unit (DU) and the Active Antenna Unit (AAU) or Remote Radio Unit (RRU).
  
  

  • The network connecting the DU and the Centralized Unit (CU). (This is a new layer introduced in 5G).
    
    

    • The network connecting the CU and the 5G Core (5GC) network.

These three parts together form the complete data path from the user to the internet.

二、 Application of Optical Modules in Fronthaul

Challenge: Fronthaul requires extremely high bandwidth and ultra-low latency. Due to the large number and dense deployment of AAUs, fronthaul is also very cost-sensitive regarding optical modules.

Application Characteristics:

• • The mainstream rate is 25Gbps. For more advanced Massive MIMO configurations, 100Gbps (4x25G) or 50Gbps (e.g., based on 25G/50G PAM4 modulation) may be required.

    • Primarily follows the eCPRI standard, which is more efficient than the previous CPRI standard, effectively reducing bandwidth requirements.
      
      

      • Fronthaul distances are typically short (<10 km), so Grey (Standard) Optics and Dual-Fiber Bi-Directional modules are mainstream. To save fiber resources, Bi-Directional (BiDi) Optical Modules are also widely used.

Common Optical Modules:

• 25G SFP28 LR/ER/BiDi: 

  • The most mainstream fronthaul optical module.

• 100G QSFP28 (e.g., CWDM4/PSM4): 

  • Used for high-capacity fronthaul links.

三、Application of Optical Modules in Midhaul

Challenge: Midhaul is responsible for aggregating traffic from multiple DUs, requiring relatively high bandwidth and certain latency guarantees. Its requirements for bandwidth and cost fall between those of fronthaul and backhaul.

Application Characteristics:

• • 50Gbps and 100Gbps are the mainstream rates. With increasing network traffic, evolution towards 200Gbps and 400Gbps is underway.

    • Typically uses Ethernet protocols.
      
      

      • Midhaul distances are generally around 10-40 km. Besides grey optics, Coarse Wavelength Division Multiplexing (CWDM) technology is also beginning to be used to transmit multiple signals over a single fiber to save resources.

Common Optical Modules:

• 50G SFP56 / QSFP28: 

  • Meets medium-capacity midhaul requirements.

• 100G QSFP28 (e.g., CWDM4/LR4/ER4): 

  • The mainstream midhaul optical module.

• 200G/400G QSFP-DD/OSFP: 

  • Used for next-generation high-capacity midhaul nodes.

四、Application of Optical Modules in Backhaul

Challenge: Backhaul is the backbone network connecting the access network to the core network, requiring extremely high bandwidth, ultra-long transmission distances, and strong reliability.

Application Characteristics:

• • 100Gbps and 400Gbps are mainstream, rapidly evolving towards 800Gbps and even 1.6Tbps.

• Protocol: 

  • Standard Ethernet protocols.

    • Backhaul distances are long (can exceed 80 km), placing the highest performance demands on optical modules. Dense Wavelength Division Multiplexing (DWDM) technology is the absolute mainstay of backhaul, providing tens of Tbps capacity on a single fiber.

Common Optical Modules:

• 100G/400G/800G CFP2/QSFP-DD/OSFP:

  • High-speed, high-performance pluggable optical modules.
    
    

    • Including tunable DWDM optical modules, which operate on specific wavelengths and are the core of building DWDM systems.
      
      

Conclusion
In summary, optical modules are the cornerstone of the 5G bearer network. From fronthaul to backhaul, the requirements for optical modules show a distinct gradient:

• Fronthaul: 

  • Low cost and low latency are key.

• Midhaul:

  • Balancing cost and performance, evolving towards higher speeds.

• Backhaul:

  • Ultra-high bandwidth, ultra-long distance, and high reliability are the core demands.

With the development of 5G-Advanced and future 6G technologies, optical modules will continue to evolve towards higher speeds, lower power consumption, smaller form factors, and greater intelligence, continuously injecting powerful momentum into our digital lives.