Fiber optics will be the foundation of 5G networks.
Time:
Jun 02,2025
As the 5G wireless standard continues to evolve, what measures can network operators currently take to lay the foundation for 5G transmission networks? The good news is that, at least on the physical layer, the path for 5G is clear: fiber optics will serve as the backbone of the network, and centralized RAN (C-RAN) will become the architectural cornerstone of 5G networks.
As the 5G wireless standard continues to evolve, what measures can network operators currently take to lay the foundation for 5G transmission networks? The good news is that, at least on the physical layer, the path forward for 5G is clear: fiber optics will serve as the backbone of the network, and centralized RAN (C-RAN) will become the architectural cornerstone of 5G networks.
C-RAN was introduced via 4G (commercial deployment is now expanding) and has added a new transmission segment to mobile networks: the uplink. With C-RAN, radio units remain located in the base station towers, while the baseband processing units (BBUs) have been moved from the tower sites to central offices, enabling communication among themselves as well as with other network elements. Using the standard CPRI protocol, the distance between base station towers and BBUs can reach up to 20 kilometers.
2. Due to the combined requirements of capacity and distance, the outbound network will primarily be fiber-optic based.
The testing requirements at the physical layer are also quite straightforward, focusing on the measurement of fiber-optic characteristics that are critical to any fiber-optic network. In other words, there are certain differences when preparing for 5G data rates and architectures.
Attenuation
Attenuation refers to the reduction in power of an optical signal as it propagates through an optical fiber. Common causes of attenuation include poor-quality connectors, tight bends in the fiber, faulty fiber splices, and inherent defects in the fiber itself that become more pronounced with increasing transmission distances. Compared to Distributed RAN, C-RAN introduces two key factors that can significantly increase loss: 1) longer fiber transmission distances—where the physical separation between remote radio heads and BBUs has increased from tens of meters in Distributed RAN to between 10 and 20 kilometers; and 2) a greater number of connectors along the transmission path.
The Optical Time-Domain Reflectometer (OTDR) is the right testing tool for precisely measuring attenuation and should be used in any new C-RAN fiber optic installation. If the OTDR shows abnormally high loss at a connector, inspecting the probe can help determine whether the fiber end face needs to be cleaned.
Chromatic Dispersion & Polarization Mode Dispersion
Dispersion refers to the broadening of optical pulses and can lead to an increase in the bit-error rate during optical transmission. Currently, the two most significant forms of dispersion are chromatic dispersion (CD) and polarization-mode dispersion (PMD). CD is caused by the different wavelengths (colors) within an optical pulse traveling at different speeds, while PMD arises from differences in the propagation velocities of light waves with different polarization states.
At sub-10G rates, the tolerance for CD and PMD is very high; however, once the rate reaches 10G or higher, dispersion becomes a significant issue. This is an important consideration, given that mobile backhaul networks can achieve data rates of 10 Gbps (and ultimately even higher).
In addition, distance is also a factor. Testing and measurement supplier EXFO recommends performing dispersion tests on any span exceeding 15 to 20 kilometers in length. These tests should be conducted prior to commissioning to avoid CD/PMD-related failures.
The migration to coherent 100G transmission in both remote networks and metropolitan area networks has significantly reduced many issues related to dispersion-induced impairments, thanks to the capabilities of digital signal processing.
However, coherent detection introduces certain limitations that are absent in 10G direct-detection systems—for example, sensitivity to rapidly varying state of polarization (SOP) and polarization mode dispersion (PMD). Since both SOP and PMD can change within just a few microseconds, coherent receivers must compensate for PMD and SOP in real time. If these changes occur too rapidly, such compensation may sometimes become impossible, leading to signal loss.
The best way to prevent SOP and PMD compensation failures in coherent receivers is to avoid using optical fibers with high PMD, since in fibers with higher PMD, rapid variations in both SOP and PMD occur more frequently.
In short, for operators planning for the future of 5G, it is now possible to take measures at the physical layer to extend fiber optics to their cell site locations, thereby meeting the requirements of centralized RAN architectures at higher layers. From a physical-layer testing perspective, this approach is straightforward—simply focus on the characteristics of the fiber optic cables.
Reposted from Fiber Online
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