Executing fault detection and isolation in analog RFoF links

By Meir Bartur / Optical Zonu

Analog radio frequency signal transport over fiber (RFoF) systems are becoming a crucial telecommunication technology for helping industries cope with the growing reliance on high-frequency communications. While high-frequency RF signals provide low latency and high-throughput data transmission, they are also easily obstructed by manmade and natural obstacles.

RFoF technology converts RF signals into light waves for transport, which are then reconverted back into RF at their destination to provide a resilient alternative to analog transmission methods. Unlike coaxial cables, attenuation limited range (e.g., a few hundred feet at most, depending on the frequency), which can only travel a few hundred feet at frequency without compromising signal performance, optical fiber can carry signals up to 100 kilometers.

Analog RFoF uses

RFoF has highly important use cases in modern society. Its secure and resilient nature in military communications is ideal for connecting antennas mounted on military vehicles, aircraft, or ground stations to centralized command centers or tactical communication networks. Since RFoF is impervious to electromagnetic interference, it can replace direct line-of-sight (LOS) microwave links.

In Global Navigation Satellite System (GNSS) timing distribution, critical for constantly synchronizing clocks to maintain data integrity, RFoF transports the GNSS RF signals (e.g., GPS L1, L2 frequencies) over optical fiber to remote sites. This is particularly useful for extending the reach of GNSS signals within facilities where direct RF distribution would suffer from losses and interference. RFoF is also ideal for long-haul connectivity between wireless base stations and distributed antenna systems (DAS), commonly found to provide mission-critical connectivity in hotels, campuses, stadiums, and airports.

Fault isolation and segmentation

Considering the importance of applications that typically leverage RFoF, operators cannot afford the downtime of these systems. Fiber fault isolation and segmentation are necessary to fix problems quickly. This is easier said than done, considering RFoF links can span many kilometers, and several components could be at fault, from the laser diode, electronics, or power supply to the fiber itself.

Beyond the obvious benefit of locating fiber faults faster, knowing precise information about the fault as soon as it occurs also helps improve triaging the situation, saving organizations both capital and operations expenditure based on the type of technical competency required. Unsurprisingly, the technical training necessary to identify and fix specific problems with RFoF components comes with a high price tag. For example, if an operator can confirm that the laser is dead, there is no need to send a specialized technician to diagnose the fiber. Conversely, if it’s the power supply, it might be needed. Significant time can be lost by sending multiple teams to figure out an issue, leading to lengthy downtime for a critical link.

Isolating RFoF link failures

Detecting and isolating failure in an RFoF link requires monitoring various functional and parametric indicators to identify and pinpoint potential issues early.

Functional indications

Most RFoF systems come equipped with LED indicators and basic alarms that provide immediate visual feedback about the system's status. For example, LEDs on the transmitter and receiver units may change color or flash to indicate issues such as power faults, signal loss, or degraded performance. Alarms often accompany these indicators, offering basic diagnostic information to operators at a glance. These allow technicians to detect problems quickly without complex diagnostic equipment, making them a valuable first step in troubleshooting.

Parametric monitoring

Parametric monitoring systems go beyond simple status indicators by tracking key operational parameters like voltage levels and signal ranges. Many RFoF systems are designed to alert users if voltages or signal levels drift out of predefined ranges, providing early warnings before these issues impact performance. For example, suppose the voltage powering the transmitter or receiver starts fluctuating or falls below safe limits. In that case, an alert is triggered, signaling a potential degradation in power integrity that could affect signal transmission. By notifying users of such changes early, parametric alarms enable preventive action to maintain system integrity.  Parametric monitoring is extended via SNMP to a centralized location that simultaneously monitors many locations.

Fiber integrity monitoring

Of course, fiber integrity is crucial in RFoF links since fiber issues directly impact signal transmission. Loss of signal (LOS) alarms are fundamental indicators that notify operators of a complete signal loss over the fiber. While a LOS alarm indicates an apparent problem, it does not specify the cause (e.g., physical fiber damage, connection issues, or component failure). Further diagnostic steps, such as using an optical time-domain reflectometer (OTDR), can help pinpoint the exact location or cause of the signal loss within the fiber link.

But fixing a broken fiber is a reactive measure. Ideally, network operators would prefer to intervene before a break occurs. Monitoring the gradual degradation of optical power is essential for predicting and preventing future LOS events. Advanced RFoF systems may include features for signal strength monitoring that alert operators if the optical power drops below a certain threshold—or additional spurious reflections are identified by the OTDR. This early-warning mechanism provides a proactive approach to detecting fiber aging, attenuation, or gradual connector damage, allowing operators to remediate issues before a complete loss of signal occurs. This level of monitoring is rare but valuable in high-reliability systems, as it provides both detection and mitigation options.

Recovering from link failures

Once a network operator has a link failure, they can repair it in multiple ways, including human intervention and automation.

Human intervention

Technicians can review different system data sets once an alarm is triggered. Operators can also develop customized thresholds for those alarms if they know what they specifically wish to monitor and automatically set up emails to be sent based on specific behaviors. The problem with this approach is that it’s based on whether an integrator or operator has the proper specialized training. Even if the customized alarms are set, unless the person overseeing the system knows what they are looking at, it may lead to more confusion and still require outsourcing to others to solve the problem.

Automation

The best way to avoid link failure is to build redundancy into the RFoF links that will automatically reroute in the event of a failure. The only downside to this is that, depending on the type of redundancy, it can be costly.

In a 1+1 redundancy setup, two identical RFoF links (primary and secondary) operate in parallel. The primary link handles the leading signal transmission, while the secondary link is continuously ready to take over if the primary link fails. In a 1+1 redundancy, every component has a backup. This essentially means deploying two systems with the purpose of only ever using one. 

Most of the time, operators find 1+1 redundancy too costly and opt for an N+1 setup. In an N+1 redundancy configuration for RFoF links, there is the primary system and one additional link as a shared backup for the whole system. By using only one backup for multiple links, it achieves a balance between reliability and cost. However, it also puts a lot of pressure on detecting and isolating the problem quickly to recreate the redundancy once a break or fault occurs.

The OTDR is excellent for helping pinpoint fiber faults within a few meters, but it can still be challenging to act upon that data alone. Operators should make sure to have network management systems capable of overlaying that OTDR data on a management panel and providing a visual representation of fiber continuity on terrain maps to view fiber faults and reflection events with accurate map locations.

Analog RFoF systems provide a robust, high-capacity solution for long-distance RF signal transmission, making them essential for applications requiring secure and uninterrupted communication. By bypassing typical RF signal barriers, RFoF systems enhance military, GNSS timing, and DAS applications significantly.