Todd Nesnick
In 2020, with the large-scale construction of 5G, China Mobile began to build a large-scale SPN network. By the end of 2020, the country will realize the large-scale commercial use of more than 150,000 SPN devices. Base station deployment requirements will further expand the deployment of SPN. At present, most of China Mobile’s 5G sites adopt the C-RAN method, and most of the access rings form a 50GE network. The technical solution of the 50Gb/s optical module is mainly based on the 25G baud rate optical chip and the fourth-order pulse amplitude modulation (PAM4) modulation format. Compared with the 10G optical module, the quality of the optical path is higher. Optical path reflection will cause multipath crosstalk, which will have a great impact on the 50GE SPN system. Therefore, this paper chooses to study the influence of light path reflection on SPN network and proposes corresponding solutions, which is of great significance for improving the performance of SPN network.
There is a problem with the SPN network
Compared with the PTN access system of 10GE and 1GE, the SPN system of 50GE has a greater impact and has higher requirements on the quality index of the optical path. When the reflection intensity of the optical path is high, the output of the light source will be unstable, the receiving device will generate noise, and crosstalk, line width, and frequency interference will occur in the transmission system, which will eventually cause the instability of the emitted light. In the SPN transmission system, the main performance is that the port is down, the port is frequently up-down, the port has a bit error rate alarm before correction, or the port is up but has CRC.
SPN 50Gb/s optical module working principle
The 50Gb/s dual-fiber bidirectional and single-fiber bidirectional optical modules mainly use 25GBaud optical chips and PAM4 modulation format, adding PAM4 chips on the basis of 25Gb/s optical modules to double the transmission capacity.
PAM4 operating principle: PAM4 is a modulation method that uses different pulse carrier amplitudes to carry information. PAM4 refers to the use of four different levels to transmit information, so at the same baud rate, the amount of information that can be transmitted by a PAM4 signal is twice that of a non-return-to-zero (NRZ) signal, as shown in Figure 3. The amplitude difference between every two adjacent levels of the PAM4 signal is only 1/3 of that of the NRZ code, the signal-to-noise ratio is low, and the signal is easily interfered by the reflection of the optical path to generate bit errors. PAM4 is a high-order modulation, and it needs to transmit four states at the same amplitude. Although it is compensated by the FEC algorithm, the link performance, especially the noise tolerance, is still significantly reduced, which shows that it has high requirements on the quality of the optical path.
Theoretical analysis of light path reflection
The reflection of the optical fiber optical path is mainly caused by fiber scattering and Fresnel reflection on the end face of the flange connector. Optical path reflection will cause multipath crosstalk in the transmission link.
When the light is transmitted in the optical fiber, due to the uneven distribution of the refractive index in the optical fiber, scattering will occur, mainly in three forms: Rayleigh scattering, Brillouin scattering and Raman scattering. In Rayleigh scattering, after the incident light is scattered, the wavelength and frequency do not change, which is a kind of elastic scattering; both Brillouin scattering and Raman scattering belong to inelastic scattering.
In the connection between the optical fiber and optical fiber, and in the connection between the optical fiber and optical device, due to the discontinuity of the refractive index at the connection interface, part of the light returns along the human injection path, which is called Fresnel reflection. In the reflected light of the optical path, the intensity of Fresnel reflected light is much greater than that of scattered light, and the multipath crosstalk is mainly caused by Fresnel reflection. The following mainly studies the Fresnel reflected light intensity of different fiber end faces. The end faces of optical fiber connectors are generally divided into PC, UPC, and APC. Among them, PC and UPC are flat end faces of optical fibers, while APC end faces of optical fibers are at an angle of 8 degrees.
The reflection intensity of Daoguang at the end face of the fiber decreases with the increase of the inclination angle of the end face. According to the analysis of the types of fiber end faces commonly used at present, the reflected light intensity of the APC (inclined at an angle of 8 degrees) is much smaller than that of the PC (or UPC) end face. It can effectively deal with the interference of optical path reflection on the transmission signal. It can be seen that if the end face of the optical fiber is APC, that is, the middle of the fiber optic connection is air, the light reflection intensity can also meet the requirement of less than -37dB. When the light wavelength is 1310nm, the reflected light intensity of the end face of the APC pigtail is about 87dB smaller than that of the PC (or UPC) end face under the same environment. At present, most of the pigtails purchased by China Mobile are PC (or UPC) end faces, which have a great impact on the reflection index of the optical path. The SPN systems of 50GE and 200GE have higher requirements on the reflection index of the optical path, and attention should be paid to the selection of the end face type of the pigtail.
Summary
At present, the 50Gb/s optical module adopts PAM4 technology, which has high requirements on the reflection intensity of the optical path. The reflection intensity of the joint is less than -37dB). If there is no OTDR instrument, it is recommended to clean each joint of the entire optical path before opening the station. In order to better solve the impact of optical path reflection on the 50GE SPN system, it is recommended to replace the PC (or UPC) pigtail with an APC pigtail. The APC end face can better meet the highest reflection of each reflection point in the IEEE802.3cn-2019 standard strength requirements. At present, the 200Gb/s optical module of SPN basically adopts PAM4 technology, and the problem of light path reflection should also be paid attention to in the application.