Passive wavelength division multiplexer (WDM) designed to address fiber resources for long-haul transmission between distributed units (DUs) and active antenna units (AAUs) in Centralized Radio Access Network (C-RAN) 5G fronthaul architectures In addition, passive WDM can save fiber resources. In passive WDM, color optical modules are directly deployed on AAUs and DUs, and multiple AAUs can share one optical fiber for transmission through passive WDM without power supply at the far end. Passive WDM is the most suitable solution for 5G fronthaul. According to different wavelengths, 5G passive wavelength division multiplexers can be divided into CWDM (Coarse Wavelength Division Multiplexing), DWDM (Dense Wavelength Division Multiplexing), MWDM (Medium Wavelength Division Multiplexing) division multiplexing) and LWDN (fine wavelength division multiplexing).

Application of WDM Network Topology in 5G Transmission
The passive WDM network topology in 5G transmission consists of fronthaul and backhaul. The 5G fronthaul interconnects the AAU/RRH (active antenna unit processing unit/remote radio head unit) to the CU/BBU (central unit/baseband unit), and the 5G backhaul interconnects the CU/BBU to the core network.

Advantage
The solution to carry passive WDM in 5G fronthaul has the following advantages: high bandwidth, high reliability, cost-effective CPRI rate, low latency, low insertion loss, low cost, 4/6/8/12/18/24 /48 optional channels, significant fiber saving, plug and play, easy installation and deployment, easy maintenance.

Application scenarios
Passive WDM is suitable for end-to-end C-RAN network scenarios, areas where optical fibers are scarce, and areas where pipeline resources are lacking. Potential application scenarios for passive wavelength division multiplexing include: 5G fronthaul, oil and gas, industrial, power, mining, cable TV, FTTx, passive optical fiber networks, and others

Different passive WDM solutions:
Passive WDM solutions include CWDM, DWDM, MWDM and LWDM.

(1) CWDM
CWDM adopts wavelength multiplexing technology, which has the advantages of high bandwidth, high channel isolation, low temperature sensitivity, low cost, etc. It enables operators to transmit 18 bands simultaneously in a pair of fibers.

(2) DWDM
DWDM MUX is a cost-effective solution with a point absorption modulated laser (EM). It features high reliability and stability, high channel isolation, high bandwidth, low insertion loss and lower complexity.

(3) MWDM
MWDM pays attention to the first 6 wavelengths of CWDM, compresses the 20nm wavelength interval of CWDM to 7nm, and uses the thermal electron cooler (TEC) temperature control technology to expand 1 wave to 2 waves, which can further save fiber resources and achieve capacity improvement. Increase

(4) LWDM
LWDM is based on Ethernet communication wavelength division multiplexing (LAN WDM). Its channel spacing is 200~800GHz, and this range is between DWDM (100GHz, 50GHz) and CWDM (about 3THz). LWDM provides high reliability and stability, high channel isolation and low insertion loss. In addition, LWDM can support 12 waves of 25G to increase capacity and save fiber.

Another passive WDM solution for 5G fronthaul is WDM-PON.

Wavelength Division Multiplexed Passive Optical Networks (WDM-PONs) use multiple different wavelengths over a physical point-to-multipoint fiber infrastructure. WDM-PON realizes 1310/1490/1550nm bidirectional communication through single fiber, which can expand single fiber capacity and save fiber resources. It provides high reliability, high bandwidth, high communication isolation, low latency, low insertion loss, plug-and-play ONU (Optical Network Unit) and simple operation and maintenance (O&M).

Passive WDM can help solve 5G front-end transmission challenges by saving fiber resources and reducing costs. Passive WDM bearer rates include 10G, 25G, 40G and 100G. In addition, passive wavelength division multiplexing has the advantages of high bandwidth, high channel isolation, low delay, low insertion loss, simple maintenance, and easy deployment.