It is reported that the theme of this year’s MWC and OFC conference is to prepare the transmission network for the upcoming 5G. The current industry consensus is that 2020 will begin to deploy 5G extensively. But because 5G NR is still in the early stages of standardization, 5G preparation is also a tricky issue.
C-RAN was introduced through 4G (commercial deployment is now expanding) and adds a new transport segment to the mobile network: go. After the C-RAN is used, the radio units remain in the base station tower, but the baseband processing units (BBUs) move from the unit tower to the central office to enable communication with each other and with other components. Using the standard CPRI protocol, the distance between the base station tower and the BBUs can be as much as 20 kilometers.
C-RAN has two main points: 1) C-RAN is the transport network architecture required for 5G because the virtualization of BBUs (Cloud RAN) will become a key component for implementing 5G. In order to expand and implement virtualization, the C-RAN architecture needs to be implemented immediately; 2) Due to the combination of capacity and distance requirements, the outbound network will be primarily based on fiber.
The physical layer test requirements are also very simple, with a focus on testing the fiber characteristics that are critical to any fiber network. That is to say, there are some differences when preparing the 5G data rate and architecture.
attenuation
Attenuation is the reduction in power of an optical signal as it propagates through the fiber. Common causes of attenuation include poor connector quality, tight fiber bending, faulty fiber splices, and defects in the fiber itself due to increased transmission distance. Compared with distributed RAN, C-RAN introduces two important factors that may increase loss: 1) Larger fiber transmission distance – physical separation distance between the remote headend and BBUs increases from tens of meters of distributed RAN Up to 10 km to 20 km; 2) A larger number of connectors in the transmission route.
The Optical Time Domain Reflectometer (OTDR) is the correct test tool for accurate attenuation measurement and should be performed on any new C-RAN fiber installation. If the OTDR point connector has an abnormally high loss, the inspection probe helps determine if the fiber end face should be cleaned.
Chromatic dispersion & polarization mode dispersion
Dispersion is an extension of the light pulse and may result in an increase in the bit error rate in optical transmission. The two most important forms currently are chromatic dispersion (CD) and polarization mode dispersion (PMD). The CD is caused by different wavelengths (colors) in the light pulses operating at different speeds, and the PMD is caused by the difference in propagation speeds of different polarization states.
At the sub-10G rate, the CD and PMD tolerances are very high; however, at 10G and above, dispersion becomes a problem. This is an important consideration because mobile backhaul networks can achieve data rates of 10 Gbps (and eventually higher).
In addition, distance is also a factor. Test and measurement supplier EXFO recommends dispersion testing for any span greater than 15 km to 20 km; these tests are performed prior to commissioning to avoid CD/PMD related failures.
The migration of coherent 100G transmissions in remote networks and in metropolitan area networks has reduced many problems with dispersion impairment due to the function of digital signal processing.
However, coherent detection introduces some limitations that are not present in 10G direct detection systems, such as sensitivity to rapid changes in polarization state (SOP) and PMD. Since SOP and PMD can change in a few microseconds, coherent receivers must compensate PMD and SOP in real time; but if they change too fast, sometimes they are not implemented, resulting in signal loss.
The best way to prevent SOP and PMD compensation faults in coherent receivers is to avoid using fibers with higher PMD because SOPs and PMDs change more frequently in higher PMD fibers.
In summary, for operators planning the future of 5G, it is now possible to take measures at the physical layer to extend the fiber to its cell site in anticipation of the need for a centralized RAN architecture at a higher level. From a physical layer test point of view, the method is simple and will focus on fiber characteristics.
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