The 5G fronthaul passive WDM system is mainly used to solve the problem of long-distance optical fiber between DU-AAU and lack of transmission optical fiber resources in the C-RAN architecture. The optical module replaces the original gray optical module on the wireless device, thereby providing business optical signals of different wavelengths. The passive wavelength division multiplexers on both sides then multiplex the business optical signals of different wavelengths on one optical fiber for bidirectional transmission. Instead of laying optical cables, it provides operators with a low-cost, high-performance optical fiber expansion solution.

The 5G fronthaul passive solution adopts WDM technology, and the main configuration products are passive wavelength division devices and 10G/25G color light modules. For different needs, 5G passive WDM has two connection solutions: point-to-point and WDM-PON. According to different wavelengths, there are four wavelength configuration schemes: CWDM, DWDM, LWDM and MWDM.

5G fronthaul connection solution
Passive point-to-point WDM solution
The passive WDM solution adopts passive wavelength combiner and demultiplexer with color light direct drive. Color light modules are used on DU and AAU. Optical combiner and demultiplexer and optical add-drop multiplexer are respectively configured at the front end of DU and AAU node. Reduce fiber resource consumption. WDM equipment transparently transmits fronthaul services without introducing delay. However, the wavelengths of AAU and DU are connected point-to-point on the physical layer, which requires complex wavelength planning. The direct use of the color light module in the AAU base station will require the management of the base station, and the power budget is also a key indicator that needs to be considered.

WDM-PON solution
WDM-PON is a broadband access network evolution technology. In this system, the optical line terminal OLT is deployed on the DU side, which can provide multiple light sources with different wavelengths to realize control, switching and management functions. Optical network unit ONU is deployed on the AAU side to provide specific wavelengths. OWDN (Optical Wavelength Distribution Network) is deployed between OLT and ONU to realize wavelength distribution. The pre-designed wavelengths are used between the OLT and the ONU. However, in the case of multiple wavelengths, there are too many types of ONUs with a specific wavelength light source, which is easy to cause storage problems. Therefore, the current solution based on colorless ONUs is the mainstream of the WDM-PON system. Maturity is still at a low level.

The 5G fronthaul solution involves huge investment in network construction and occupies a large amount of optical fiber resources. The choice of the solution has a significant impact on the resource occupation and construction period of each deployment point. Both of the above two passive solutions for 5G fronthaul can meet the performance requirements of 5G fronthaul bandwidth, delay, and transmission distance.