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5G for the transmission network: fiber is 5G future

Time: 2017-08-20
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5G for the transmission network: fiber is 5G future

It is reported that this year's MWC and OFC congress of the same theme is: for the upcoming 5G ready to transfer network. The current industry consensus is to start deploying 5G in 2020. But because the 5G NR is still in the early stages of standardization, so 5G preparation is also a difficult problem.

With the 5G wireless standard changes, the current network operators can take measures to lay the foundation for the 5G transmission network? The good news is that at least 5G in the physical layer of the road is clear: the fiber will be 5G network foundation, centralized RAN (C-RAN) will become a 5G network architecture.

C-RAN is introduced through 4G (commercial deployment is now expanding) and adds a new transport network segment to the mobile network: to the way. After using the C-RAN, the radio units remain in the base station tower, but the baseband processing units (BBUs) move from the cell towers to the central office in order to communicate with each other and between other components. Using the standard CPRI protocol, the distance between the base station and BBUs is 20 km.

C-RAN has two main points: 1) C-RAN is 5G required transmission network architecture, because BBUs (Cloud RAN) virtualization will be a key component to achieve 5G. In order to extend and implement virtualization, we need to implement the C-RAN architecture immediately. 2) Due to the combination of capacity and distance requirements, the outbound network will be mainly fiber-based.

Physical layer testing requirements are also very simple, focusing on any fiber optic network is essential to the test of fiber characteristics. In other words, there are some differences in preparing the 5G data rate and architecture.

attenuation

The attenuation is the power of the optical signal propagating in the fiber. Common causes of attenuation include poor connector quality, dense fiber bending, faulty fiber optic connectors, and defects in the fiber itself due to increased transmission distances. Compared with the distributed RAN, the C-RAN introduces two important factors that may increase the loss: 1) Larger fiber transmission distance - The physical isolation distance between the remote head end and the BBUs increases from tens of meters of the distributed RAN To 10 km to 20 km; 2) a greater number of connectors in the transmission route.

Optical Time Domain Reflectometer (OTDR) is the correct test tool for accurate measurement of attenuation and should be performed on any new C-RAN fiber installation. If the OTDR point connector has unusually high losses, the inspection probe helps determine whether the fiber end face should be cleaned.

Chromatic dispersion & polarization mode dispersion

Dispersion is an extension of the optical pulse and may result in an increase in the bit error rate in the optical transmission. The two most important forms are chromatic dispersion (CD) and polarization mode dispersion (PMD). The CD is caused by the different wavelengths (colors) in the light pulses running at different speeds, and the PMD is caused by the difference in the propagation speed of the different polarization states.

At sub-10G rates, CD and PMD tolerances are very high; but at 10G and above, dispersion becomes a problem. This is an important consideration because the mobile backhaul network can reach 10 Gbps of data rate (which will eventually be higher).

In addition, distance is also a factor. Testing and Measurement Providers EXFO recommends dispersing any span of more than 15 km to 20 km; perform these tests before commissioning to avoid CD / PMD-related failures.

The migration of coherent 100G transmissions in remote networks as well as in metropolitan area networks reduces the number of issues related to dispersion impairments due to the function of digital signal processing.

However, coherent detection leads to limitations that do not exist in some 10G direct detection systems, such as sensitivity to rapid changes in polarization (SOP) and PMD. Since SOP and PMD can vary within a few microseconds, the coherent receiver must compensate PMD and SOP in real time; however, if they change too quickly and sometimes can not be achieved, the signal is lost.

The best way to prevent SOP and PMD compensation failures in coherent receivers is to avoid the use of fibers with higher PMDs because the rapid changes in SOP and PMD are more frequent in higher PMD fibers.

All in all, for operators planning for future 5G, it is now possible to take steps at the physical level to extend the fiber to its cell site to anticipate the need for a centralized RAN architecture at a higher level. From the physical layer test point of view, the method is very simple, that is, focus on the fiber characteristics.


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