100G is the mainstream technology of current optical transmission networks. China has built the world's largest 100G WDM/OTN commercial transmission network. Under the continuous promotion of the "Broadband China" strategy and the "Internet +" action plan, the mobile Internet, cloud computing, and Internet of Things businesses are rapidly developing, resulting in a surge in traffic, which makes the optical transmission network as the basis of the network face greater bandwidth. pressure. 400G is an important evolution direction of ultra-high-speed and large-capacity optical transmission networks, which can further increase network bandwidth and reduce transmission cost per bit.
Different technical routes, products gradually mature
The 400G inherits key technologies such as polarization multiplexing, coherent reception, and digital signal processing used in 100G. However, 400G has multiple technical routes compared to the industry's unified 100G transmission solution. Generally speaking, using more subcarriers, higher baud rate, and higher order modulation formats can increase the transmission rate. At present, the 400G implementation is mainly based on two-carrier and single-carrier schemes, and each scheme has multiple coding and modulation modes of competition, as shown in the table. Among them, the transmission distance is n&TImes; 22 dB multi-span G.652 fiber and meets the non-electrical relay distance in the OSNR margin scenario. For different technical solutions, optical signal performance indicators such as optical signal-to-noise ratio and non-electrical relay transmission distance, as well as system capacity and cost, are significantly different.
The dual-carrier 400G system based on the 2&TImes;200G super-path technology solution adopts PM-QPSK, PM-8QAM or PM-16QAM coding. In 2014, China Mobile completed the first dual-carrier 400G (2&TImes; 200G) live network pilot test in China to verify its transmission performance. After the development of core devices and algorithms such as optical modules and chips in recent years, domestic mainstream transmission manufacturers have already supported the dual-carrier 400G solution, and the products have gradually matured and the performance has been greatly improved. China Mobile is guided by standards and accelerates the maturity of new technologies. In 2017, it took the lead in realizing dual-carrier 400G commercial use in China, which is suitable for metro transmission scenarios.
The research and standardization of single-carrier 400G technology was also launched at the same time. Compared to the dual-carrier scheme, single-carrier 400G uses higher-order modulation formats, such as PM-16QAM, PM-32QAM, and PM-64QAM, which require higher optical signal-to-noise ratio and are more susceptible to laser phase noise and fiber nonlinear effects. Impact. China Mobile completed the first phase of the first single-carrier 400G laboratory test in China in early 2018, which verified the performance and maturity of Huawei, ZTE and Shanghai Nokia Bell single-carrier 400G commercial products, involving stand-alone indicators, single-span/multi-span system indicators, Test items such as network management functions. The test results show that the performance of the test equipment during the test is stable, and the single-carrier 400G can further increase the system capacity, but the transmission distance of the non-electrical relay is significantly reduced.
Promote key technologies and enhance transmission capabilities in multiple dimensions
The 400G technology continues to evolve with a large space, which can improve system performance from chip modules and basic resources. At the 2018 OFC conference, probabilistic constellation shaping technology has become a hot spot of concern and research in the industry. This technique converts the equal probability source into a non-equal probability source and improves the transmission performance by compressing the entropy coding. Domestic mainstream transmission equipment manufacturers already have products that support this technology, and subsequent research and verification are required to improve the performance of different coding schemes. In addition to the new modulation technology, the increase in transmission media specifications also helps to improve the transmission performance of the system.
G.654.E fiber has the characteristics of ultra-low loss and large effective area, which reduces fiber loss and nonlinear effects and improves the system's non-electrical relay transmission distance. China Mobile verified the transmission performance of multiple types of optical fibers under different types of optical fibers in the 2014 dual-carrier 400G live network pilot test and the 2018 single-carrier 400G laboratory test. The data shows that the transmission performance of each scheme in G.654.E fiber is improved to some extent compared to G.652 fiber. For the differentiation of fiber optic manufacturer G.654.E product specifications, it is recommended that the typical loss value is less than 0.17dB/km and the effective area is 130μm2. The industry should promote the convergence of key indicators of G.654.E fiber through standard formulation; in addition, it should pay attention to the problem of G.654.E and G.652 fiber splice loss, and comprehensively consider the deployment strategy of G.654.E fiber.
Based on the performance of dual-carrier and single-carrier 400G systems, China Mobile is studying the applicable scenarios, promoting standard formulation and convergence technology routes, and further promoting industrial development, and strives to improve the system's key performance indicators.
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