Optical fiber will be the foundation of 5G network
April 01 , 2021
With the continuous changes in 5G wireless standards, what measures can network operators take to lay the foundation for 5G transmission networks? The good news is that at least the path of 5G at the physical layer is clear: optical fiber will be the foundation of 5G networks, and centralized RAN (C-RAN) will become the 5G network architecture.
C-RAN was introduced through 4G (commercial deployment is now expanding) and adds a new transmission network segment to the mobile network: outbound. After using C-RAN, the radio units remain in the base station tower, but the baseband processing units (BBUs) are moved from the cell tower to the central office in order to achieve communication with each other and with other components. Using the standard CPRI protocol, the distance between base station towers and BBUs can reach up to 20 kilometers.
C-RAN has two main points: 1) C-RAN is the required transmission network architecture for 5G, because the virtualization of BBUs (Cloud RAN) will become a key component to realize 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 fronthaul network will be mainly based on optical fiber.
The testing requirements for the physical layer are also very simple. The focus is on the testing of optical fiber characteristics that are critical to any optical network. In other words, there are some differences when preparing 5G data rates and architectures.
attenuation Attenuation is the reduction in power of an optical signal as it propagates in an optical fiber. Common causes of attenuation include poor connector quality, dense fiber bending, faulty fiber connectors, and fiber defects due to increased transmission distance. Compared with distributed RAN, C-RAN introduces two important factors that may increase loss: 1) Larger optical fiber transmission distance-the physical isolation distance between the remote head end and BBUs is tens of meters from the distributed RAN Increase to 10 kilometers to 20 kilometers; 2) More connectors in the transmission route.
The Optical Time Domain Reflectometer (OTDR) is the correct test tool for accurately measuring attenuation and should be performed on any new C-RAN fiber installation. If the OTDR point connector has abnormally high losses, inspecting the probe can help determine whether the fiber end face should be cleaned.
Chromatic Dispersion & Polarization Mode Dispersion Dispersion is the spread of optical pulses and may lead to an increase in bit error rate in optical transmission. The two most important forms at present are chromatic dispersion (CD) and polarization mode dispersion (PMD). CD is caused by different wavelengths (colors) in light pulses running at different speeds, and PMD is caused by differences in the propagation speeds of different polarization states.
At sub-10G rates, CD and PMD tolerance rates are very high, but at 10G and above, dispersion becomes a problem. This is an important consideration, because mobile backhaul networks can reach data rates of 10Gbps (which will eventually be higher).
In addition, distance is also a factor. Test and measurement provider EXFO recommends dispersion testing for any spans over 15 kilometers to 20 kilometers away; these tests should be performed before commissioning to avoid CD/PMD related failures.
The migration of coherent 100G transmission in long-distance networks and in metropolitan area networks, due to the function of digital signal processing, has reduced many problems related to dispersion reduction.
However, coherent detection brings some limitations that do not exist in 10G direct detection systems, such as the sensitivity to rapid changes in the state of polarization (SOP) and PMD. Since SOP and PMD can change within a few microseconds, the coherent receiver must compensate PMD and SOP in real time. If they change too fast, sometimes they can't be achieved, which will result in signal loss.
The best way to prevent SOP and PMD compensation failures in coherent receivers is to avoid using fibers with higher PMD, because in higher PMD fibers, rapid changes in SOP and PMD are more frequent.
All in all, operators planning for the 5G future can now take measures at the physical layer to extend optical fibers to their cell sites to meet the higher-level requirements of the centralized RAN architecture. From the point of view of physical layer testing, the method is very simple, and will focus on fiber characteristics.
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