Application Introduction of Optical Modules in 5G Architecture
5G has greatly improved the requirements of key performance indicators, which provides a good market opportunity for the large-scale application of new types of optical devices. But on the other hand, operators have faster and higher requirements for the return cycle of network construction investment. The market environment requires "just right" optical devices, that is, an end-to-end design with moderate redundancy and relatively low cost. Telecom experts said that "the growth rate of traffic exceeds Moore's Law (40% for backbone network, 60% for metropolitan area network, and 80% for mobile network), while the reduction rate of transmission cost is only half of Moore's Law. Optical devices will be the bottleneck of fronthaul cost."
5G transport network architecture
In order to support the diversity of services, in the 5G transmission network architecture, fronthaul, midhaul, and backhaul networks may be equally important. Among them, the prequel is from RRU to DU (the distance is generally within 10km, a few scene is within 20km; among them, to deal with the scene with high delay requirements, the transmission delay within 1-2km can be about 100μs), the medium transmission is from DU to CU (the distance is generally within 40km, and the maximum is no more than 80km), and the backhaul is CU to 5G core network (city area network, within 200km). Selecting different transmission technologies and corresponding optical devices according to business types and attributes is the key to commercial optical network construction core competitiveness.
25G/50/100/400Gbit/s optical transceiver in 5G
Table 2 lists the mainstream specification requirements for high-speed optical transceiver modules in the 5G transport network. Large bandwidth, small size, low power consumption and low cost have become the basic characteristics of the development of optical module technology. 5G base station interconnection optical modules are mainly upgraded from 6Gbit/s/10Gbit/s to 25Gbit/s/100Gbit/s, and backhaul optical modules are gradually based on 100Gbit/s, 200Gbit/s, 400Gbit/s rates. 25GBaud optoelectronic devices, PAM4 modulation and demodulation technology and metro coherent optical module technology will be widely used. In recent years, the construction of large-scale data centers has promoted and accelerated the application process of 25Gbit/s commercial-grade optical modules. In comparison, 5G fronthaul requires 25/100Gbit/s industrial-grade (- 40°C to 85°C) optical modules because it is an outdoor application.
At present, the technical solutions for realizing wide temperature operation mainly include:
(1) Commercial-grade 25Gbit/s directly modulated laser (DML) chip + cooling packaging method, the advantage is that the requirements for the laser chip are low, and the disadvantage is that the power consumption and cost are increased.
(2) Direct use of industrial-grade 25Gbit/s DML chips has the advantages of simple packaging and low power consumption costs, but the disadvantage is that it is difficult to realize industrial-grade laser chip technology.
Table 1 lists：Parameters of optical transceiver modules in 5G transmission network
In addition, the 5G fronthaul network has a tight demand for optical fiber resources, and the optical fiber transmission method usually adopts 25Gbit/s single-fiber bidirectional (Bidi) and 100Gbit/s 4WDM optical modules. At present, there are still two wavelength schemes under discussion in Bidi mode: scheme one is 1270nm and 1330nm; scheme two is 1270nm and 1310nm. The advantage of Solution 1 is that the 1270nm/1330nm wavelength configuration can directly follow the optical device packaging process of the 10Gbit/s Bidi module, inheriting the existing supply chain resources, which is conducive to the rapid increase of products in a short period of time and is compatible with early 4G base stations. And the 1310nm/1330nm wavelength dispersion penalty (within 10km or 20km transmission distance) is within the acceptable range of the system. Because the demand for fronthaul optical modules accounts for the vast majority of the entire 5G application optical module market, mainstream device and equipment manufacturers are also trying some new low-cost optical module implementation paths. For example, Japan’s Fujitsu company uses DMT technology to realize single-wavelength 100Gbit/s QSFP28 modules, American Molex manufacturers use PAM4 technology to realize single-wavelength 100Gbit/s QSFP28 modules, and Accelink and Hisense are trying to achieve overfrequency optical modules based on 10G DML operating temperature chips . The basic idea is to use more complex electrical modulation and demodulation technology to reduce the module's requirements on the physical bandwidth of the laser or reduce the number of lasers used.
Conclusion
5G medium transmission is applied in the computer room environment, the transmission distance is 10-40km, and commercial-grade optical modules are usually used. Regarding optical chips, the industry is more optimistic that the 50Gbit/s PAM4 module will become the mainstream application module for mid-haul and future PON network upgrades. At present, 50Gbit/s PAM4 electrical chips with high linearity are available in the market, and lasers and detectors with 25GBaud high linearity need further process exploration and improvement. 5G medium transmission may also adopt DWDM ring network structure, which requires 25Gbit/s NRZ (or 50Gbit/s PAM4) IPL optical transceiver module products. 5G backhaul belongs to the category of metropolitan area network, the distance is generally within 200km, and coherent optical transceiver modules are usually used. 200Gbit/s/400Gbit/s coherent optical modules will occupy a major share, and single-carrier 200Gbit/s DP-16QAM technology may become the mainstream. At present, silicon photonics integration has been applied on a large scale in coherent optical modules.
