Testing FTTA - Fiber To The Antenna
Note: the techniques here are applicable to most short fiber optic cables, including those used in premises cabling, DAS (distributed antenna systems), etc.
Successful installation of a fiber optic cable on a cellular tower requires understanding the installation as well as how to handle, inspect, clean and test the cables. That includes the cable up the tower and the patchcords used on the top and bottom of the tower to connect active equipment.
Connector Handling, Inspection And Cleaning
You must never assume that factory-installed connectors are perfect or stay clean. Certainly they should have been perfect when made and tested at the factory, but the factory puts protective caps on the connectors to ship them. We call those caps “dust caps” and we say they are called “dust caps” because they are usually full of dust.
So after you receive the cables, you should first remove the dust caps and inspect the connector ferrule end face for dust and scratches with a special fiber optic inspection microscope. Then you clean them, inspect to assure yourself the cleaning was done properly, then test them. Likewise before you insert them into the receptacles to mate with another connector, give them a quick dry cleaning before insertion.
Never touch the end of the connector because the oils on your finger will
Dirt is the #1 enemy of fiber optic connectors because it can cause loss and reflectance, even damage connectors. Inspect every connector before you make a connection with it. Check the connector and the receptacle it will be plugged into as either or both may be dirty. To get low loss and reflectance, one needs a “perfectly clean” connector ferrule end face.
There are both simple optical microscopes and video microscopes available to check fiber optic connectors. The best magnification is 100-400X, with 200 perhaps the best, as 400X tends to have a smaller field of view – you would like to see some of the connector ferrule to be able to judge it’s condition as well as the fiber.
There are three areas of inspection on a connector endface – the fiber core zone where you must be most discriminating, the fiber cladding zone and its epoxy interface to the ferrule shown by dual zone lines, and the connector ferrule. Since the ferrule is slightly convex, it is the center of the ferrule that is most important.
Video microscopes provide better views of the connector endface plus they allow automatic analysis of the image. Then the results can be stored for submission to the user to verify the condition of the connector and kept for future reference.
More on connector inspection.
More on cleaning fiber optic connectors.
Testing FTTA Cables
When dealing with either prefab or onsite terminated FTTA cables, testing involves careful cleaning and inspection with a microscope, insertion loss testing and in some cases, OTDR testing.
Like any fiber optic cable and especially any prefab cable, the tower cable should not be installed until it has been tested to confirm that the cable is OK and has not been damaged in shipment or handling. This also includes the patchcords used on the tower. Even short cables can cause major problems if they have been damaged or are not clean.
Testing includes cleaning and inspecting the connectors, checking continuity with a visual fault locator (VFL), then do an insertion loss test with an optical loss test set to determine if all fibers are OK. Recording this data will help in the final test, after the cable has been installed, by comparing losses before and after installation to see if any damage was done during installation.
Remember to always keep protective caps on all the connectors except when cleaning, inspecting or testing.
After installation, the cable needs to be tested again to ensure no damage was done to the cable during installation. Insertion loss testing and perhaps OTDR testing will be required.
Insertion Loss Testing FTTA Cables
Insertion loss testing uses a test source like the transmitter and an opitcal power meter like the receiver to test a fiber the way it will be used in an operational link. This is considered the most valid test for optical fiber. The test diagram looks like this.
The power meter is used to calibrate the output of the test source and launch cable, then measure the loss. The launch and receive cables mate to the cable under test to check the connections on both ends of that cable. This test works well for the prefab cables that are waiting for installation, but once the cable is installed up the tower, this method is inconvenient so a different method has been devised.
More on insertion loss testing.
The classical method of insertion loss testing of an installed cable plant is not ideal for FTTA. This test would require the tech on the top of the tower to have a source and launch cable with him to connect to the cables. That is very inconvenient and dangerous.
There is an alternative method you can use that is safer and cuts test time almost in half. You have two fibers for each RRU, each transmitting in opposite directions. By putting a loopback on the two fibers at the top of the tower, you can leave both testers on the ground and test the two fibers at once.
This method uses a loopback, basically a short cable that plugs into the LC duplex connector in the upper distribution box to “loop back” to the bottom of the tower where the tech with the test instruments makes the measurements. You can purchase loopbacks made for this purpose. Then the tech at the top only has to be concerned with cleaning the loopback and moving it to another pair of fibers.
For practical reasons, having the loopback fairly long is a good idea. If it is ~20m long, it can also be used with a high-resolution OTDR to test the cables and record traces for future reference.
The problem with using an OTDR on FTTA cables is most cables are short for a typical OTDR’s limited resolution. The best OTDRs for FTTA are those intended for premises cabling or fiber to the home because they have higher resolution. Set the OTDR to a short range and set the shortest possible test pulse for highest resolution (5ns will give a dead zone of ~4meters, among the shortest resolutions available from today’s OTDRs.).
Loopback testing works for OTDRs also as long as the loopback has a fiber length longer than the resolution of the OTDR. Here we show using an OTDR with a launch cable connected to one pair of a duplex link, a loopback inserted at the top of the tower (a loopback is a ~20m cable with connectors on each end, coiled up so the tech at the top can plug it into the fiber pair under test), then there is a receive cable at the other end of the looped-back fibers. If testing bi-directionally, move OTDR to end of the receive cable and take new trace.
When analyzing OTDR traces with such short cables, the short patchcords may not be resolved, but instead the reflectance of the connectors will be overlapping and cause the traces to be harder to analyze.
Schematic of OTDR trace
Actual OTDR trace of tower cable in loopback without receive cable so the end connector cannot be measured.
While this makes it harder to find which connectors are bad, it does not prevent the OTDR from making measurements. One test is to measure the end-to end loss of the cable using the OTDR by placing the OTDR markers as shown.
This measures the loss of all connectors and fiber, although the loss of the fiber is probably too small on such a short cable to make a significant contribution to the loss.
There is another way to make this measurement that can be more accurate. Use the "least squares" or "LSA" function of the OTDR for making the test. LSA is not intended to be used for a cable plant test but it works well for this test. Here is the diagram for a cable test using LSA:
Place the marker at the beginning of the cable. Use the LSA markers on the OTDR to set the limits for LSA at either end of the cable (red dotted lines on the diagram) and read the loss on the OTDR. The LSA option will calculate the loss of the complete cable plant from end to end with less uncertainty than the two point method, just like when measuring the loss of a single connector or splice.
One can also test just one of the cables, going up or down. This is how to test the fiber going down. And you can use the LSA function of the OTDR here in the same way.
If you need to test the cables in both directions to remove directional effects, simply disconnect the OTDR from the launch cable and attach to the far end of the receive cable and test in the reverse direction. (That's a good reason to use launch and receive cables of the same length. Also a long launch cable - longer than the cables under test - will ensure that one does not get "ghosts in the traces.)
More on OTDR testing.
What Loss To Expect? The Loss Budget
Calculating the loss budget is the best way to estimate what loss we should be measuring. To calculate the loss budget, we figure what is the maximum loss with a normal installation. To begin, we need to know the approximate length of the link and the number of connectors and splices. For connectors, count the connectors on each end as one each (we’ll mate them to reference connectors when we test them) and each mated pair used as a connection in the cable plant as one also.
Consider a simple FTTA cable shown as an example:
Link length: <0.1 km (~330 feet).
If it is SM fiber, it will have a minimal loss – the fiber attenuation at 1310nm is only ~0.4dB/km so our <0.1km link would have <0.04dB, basically ignorable. For MM fiber at 3.5dB/km, it would be <0.3dB which is not ignorable.
Connectors: 8, as noted in the diagram, including the ones on the ends. Good connectors should be under 0.5dB, typically 0.3dB, but we will use 0.5 for our loss budget, so 8X0.5 = 4.0dB.
Add the connector losses to the fiber losses and we should have:
SM: 0.04dB (fiber) + 4.0dB (connectors) = 4.04dB
MM: 0.3dB (fiber) + 4.0dB (connectors) = 4.3dB
These numbers become “pass/fail” numbers for testing. When you test the link, you should have less than those calculated losses. If the loss is higher than that, you may have problems with the cable installation or the terminations, and should troubleshoot the installation – beginning with cleaning. Remember these numbers are estimates, so some judgement is needed.
Testing Equipment Optical Power
Once the cables and equipment are installed, it may be necessary to test the optical power of the system. To measure the power output of a transmitter or the input power of a receiver, use an optical power meter. Set it to the wavelength being tested and the “dBm” or absolute power range. Note the system must be turned on and set to allow transmitter output. Also note the difference between the source output and the receiver input on the same fiber link is the loss in the cable plant.
The amount of light coupled into a fiber by a source is measured by attaching a patchcord to the source, either a known good system patchcord or a reference test cable. The cable used must have a connector that mates with the transmitter and a fiber size the same as the system cabling (50/125, 62.5/125 or SM) since the coupled power is highly dependent on the core size of the fiber. The meter connector adapter must be the same type as the connector on the cable to allow connection.
Connect the meter, set the range on dBm to measure power (dB is used for loss) and be sure to set the wavelength to the wavelength of the source, as the meter’s calibration will be different due to the wavelength sensitivity of its detector! Measure the power and record the results.
Receiver power is measured by removing the cable connected to the receiver input and connecting it to the power meter.
Set the meter range on dBm or watts as appropriate and be sure to set the wavelength to the wavelength of the source, as the meter’s calibration will be different due to the wavelength sensitivity of its detector!
Measure the power and record the results.
A Final Reminder (Warning?)
FTTA installation involves work that is dangerous and for which crews need specialized training and certification plus specialized equipment made for this job. The danger includes climbing towers, raising large cables up the towers, working on the top of the towers to install and test the cable, and something many forget, working around sources of RF energy from the antennas that may be subject to FCC regulation.
Follow all applicable rules and use the proper personal safety equipment!