WDM Technology in the 5G Era: A Cost-Effective Solution for 5G Era Fiber Demands

The Basic of 5G Wireless Communication

The fifth generation of mobile wireless communication, also known as 5G wireless communication, is a new generation of broadband mobile communication technology characterized by high speed, low latency, and large connectivity. The communication infrastructure of 5G is the foundation for achieving the interconnection of people, machines, and things.

5G networks are divided into three parts: access network, transport network, and core network. The access network generally refers to the wireless access network, also known as RAN (Radio Access Network), with the well-known base stations belonging to the access network. Currently, base station networking is divided into two modes: DRAN and CRAN. DRAN refers to the dispersed layout of AAU and BBU, with AAU located on rooftop antennas and BBU located in building equipment rooms. CRAN involves stacking BBUs in specific equipment rooms while dispersing AAUs on rooftops several kilometers away. Essentially, CRAN utilizes more optical fiber resources in exchange for fewer leased equipment rooms, thus reducing costs. Cost-driven, the application of CRAN is becoming increasingly widespread in the 5G era.

The Architecture of the 5G Bearer Network

The access network and the bearer network in 5G are closely related, and the architecture of the access network will directly influence the bearer network. Currently, the 5G access network consists of Central Unit (CU), Distributed Unit (DU), and Active Antenna Unit (AAU). Therefore, the architecture of the 5G bearer network has also evolved into fronthaul, midhaul, and backhaul. The connection between the base station and the DU is defined as fronthaul, the connection between the DU and the CU is defined as midhaul, and the connection between the CU and the urban core network is defined as backhaul.

Although compared to backhaul, the bandwidth requirement of fronthaul is not as high, because fronthaul is the closest link to AAU, coupled with the increase in the density and number of base stations, 5G fronthaul will be huge in scale and need to strictly control the cost, and fiber resources will also face great challenges.

WDM Technology in 5G Fronthaul

In traditional transmission modes, a single optical fiber can only transmit an optical carrier signal carrying one type of information. To transmit different services, numerous separate optical fibers are needed. In the era of 5G, with the explosion of service information, the traditional transmission mode requires a large number of optical fibers for transmission, posing significant challenges in terms of cabling space and costs. In contrast, the application of a WDM system can quickly address the above issues.

WDM, short for Wavelength Division Multiplexing, is a technology that allows two or more optical wavelength signals to be transmitted on the same optical fiber. The most common WDM technologies are Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM). For the differences between the two, you can refer to this article: CWDM vs DWDM: What’s the Difference?

A WDM system can carry multiple formats of "service" signals, such as ATM, IP, etc., and transmit multiple service signals on a single optical fiber through the techniques of multiplexing and demultiplexing, greatly reducing the number of optical fibers. It is an ideal means of capacity expansion for network expansion and development.

Passive WDM Solution

In the passive WDM scheme, colored transceivers are directly employed at the AAU end and connected to the mux demux via optical fibers. Multiplexers and demultiplexers enable bidirectional transmission through a pair of optical fibers or a single optical fiber. They multiplex and demultiplex wavelengths at both ends of the link to, thereby achieving a one-to-one correspondence of colored light wavelengths from the AAU end to the DU end. The passive WDM scheme is simple to deploy, but due to the passive nature of the equipment, it poses challenges such as difficulty in management, maintenance, and fault detection.

Active WDM Solution

The active WDM solution uses gray transceivers, AAU stations and DU sides are equipped with corresponding OTN and other active devices, while active mux demux is used in the network nodes for multiplexing and demultiplexing of the electrical or optical layer. Additionally, active mux-demux units are employed at network nodes for the multiplexing and demultiplexing of signals at either the electrical or optical layer. This approach, compared to passive solution, offers greater network flexibility and ease of management but also raises concerns about high costs. As 5G deployment progresses, an increasing number of AAUs need to connect to DUs, indicating significant pressure on 5G fronthaul. Clearly, the high-cost passive solution is not the optimal choice at this juncture.

Semi-Active WDM Solution

As the scale of 5G network deployments continues to expand, the fronthaul network will have tens of thousands or even millions of nodes. Network maintenance, management and cost control capabilities have become an urgent need. In order to solve the disadvantages of passive WDM and active WDM solutions, semi-active WDM solutions supporting maintenance and management capabilities emerge as the times require. Semi-active, as the name suggests, means half active and half passive. As shown in the diagram below, in general, the DU side is usually changed to active, while the AAU side remains passive. The semi-active WDM solution is both a simplification of the active WDM solution and an enhancement of the passive solution. The remote AAU side is passive WDM, not subject to power supply limitations, while the DU side deploys active WDM equipment, which can effectively realize maintenance, management, fault detection, and other capabilities.

Conclusion

As an important optical fiber transmission technology, Wavelength Division Multiplexing (WDM) technology offers significant advantages in achieving multi-channel transmission, enhancing optical fiber communication capacity, and efficiency. With the advent of the 5G era, greater challenges are posed for WDM technology. In subsequent development and research, active efforts can be made to optimize and improve 5G fronthaul WDM solutions by considering deployment requirements, transmission performance, industrial chain development, and other factors.