Testing Optical Feeds in Wireless Systems

Testing Optical feeds in Wireless Systems

Many wireless equipment vendors are transitioning from coax cables to lower-loss optical feeds in Remote Radio Units (RRU) and Distributed Antenna Systems (DAS). The switchover is due to the fact that fiber optic cabling has less signal loss and supports higher bandwidth over greater distances than coax, providing a way to extend the distance from the amplifier or between components.

Working with Fiber - What You Need to Know

If you are a technician who has worked mostly with copper, be aware that optical fiber requires very different tools and procedures. This is particularly true in regard to testing
fiber optic cables and cleaning the connectors on optical feeds.

This article will familiarize you with the general tools and procedures for working with fiber, as well as provide you with the specific steps for testing optical feeds. The examples below are based on the Ericsson RBS 3418/3518 radio base station with RRU 22 xx40 remote radio units. The process should be similar for other models and vendors as well as distributed antenna systems.

Testing and Inspection Tools

1. Power Meter and Light Source
These devices connect to both ends of the fiber link and provide the most accurate measurement of optical power loss within the link or cable. They are typically the least expensive, however testing requires access to both ends of the fiber which may not be possible due to the mounting location of the RRU. They also cannot identify where the problem is located along the fiber link. For that, you’ll need the following device.

2. OTDR
This device is the workhorse of optical testing. An OTDR uses signal reflectance, similar to radar technology, to graph fiber characteristics. An OTDR provides details about the characteristics of connectors, bending stress, etc., as well as the fiber itself. An OTDR is also a sophisticated fault locator that can identify the location and type of optical faults, even over great distances. An OTDR does not require access to both ends of a fiber for testing. OTDRs are not created equal! It is important to select an OTDR that features very high resolution, short dead zones and is easy to use. The Anritsu Network Master MT9090A shown in The Tool Box on page 22 in this magazine is one such tool.

3. USB-Based Video Inspection Microscopes
These scopes provide a large image of the fiber endface within the connector ferrule, making this an effective tool for ensuring that the endface is clean and free of defects. The USB computer interface enables the image to be saved for review by managers or for inclusion in reports. Video scopes are also safe to use. Because the connector endface is viewed indirectly on a video screen, there is no danger of accidentally exposing the eye to harmful ultra violet (UV) light . Never view a fiber endface directly. UV light is invisible, so you won’t know if you are exposing yourself to harmful light.

4. Miscellaneous Items
Working with fiber also requires various connector adaptors, cleaning supplies and fiber optic patch cords (jumpers) for connecting test equipment to the fiber.

 

Locating Optical Feeds

With most radio base stations (RBS), the optical feed cable is a duplex cable that plugs directly into one of the sector ports of the transmission card. NOTE: there may be multiple RRU cables plugged into each RBS so care should be taken not to disconnect an RRU already in service. Proper cable labeling is recommended.

Identifying Optical Connector Types

Several connector types are available for optical feeder cables. The most common are shown below. Each connector type is available in either SIMPLEX (one fiber) or DUPLEX (two fibers) configurations.

It is estimated that up to 70% of all errors in fiber optic systems are the result of dirty or damaged connectors. When dealing with optical fibers, connector condition and cleanliness are two of the most important factors. Dirty or damaged connectors will reduce the quality of transmissions and cause large amounts of light to be reflected back that may potentially damage transmitters. By using proper cleaning and inspection techniques, system turn up errors can be virtually eliminated and network reliability greatly increased.

Typical testing procedure using a USB Video Inspection Probe (VIP):

NOTE: The small, lighter-colored dot in the center of the fiber is the fiber core. This is the most important area of the fiber, so pay particular attention when inspecting this area to make sure it is free of defects and debris.

Verifying Fiber Properties - Loss, Length and Placement

Since optical fibers are made of glass, greater care must be taken during the handling and placement compared to traditional coax. If a fiber is spooled too tightly or creased, it may break or at least result in high optical loss - much like when a garden hose is kinked. The excessive bending or kinking of an optical fiber is typically known as a macrobend.

Optical Loss - A loss of signal strength can greatly affect the quality of data transmitted. Therefore, the amount of loss should be kept to a minimum. Since most of the optical feed cable lengths are short (<1000ft), the majority of the loss will come from the connector matings. A typical rule of thumb is that each connector pair mating can have 0.5dB of loss. As such, a typical short optical feed will have an acceptable loss of 1.0 to 1.5dB (2 connectors x 0.5db each + the fiber loss). Since these cables are pre-terminatedfrom
a continuous strand of glass, there should be no other points of loss in the fiber cable.

Typical testing procedure using the MT9090A:

If optical feeder cable features a simplex connector, connect it directly to the OTDR port.

If optical feeder cable features duplex connectors:
* You can unclip one of the leads and connect directly to the OTDR, OR:

If the OTDR screen shows “UNDER RANGE”, then you are ready to test since no transmission signal is detected. If a value is shown, the system is active and should NOT be tested.

Figure 1: UNDER RANGE conforming no signal is present

Figure 2: A signal level displayed confirming transmisison
signal is present - DO NOT TEST

Press CONTINUE to verify connection check level.

NOTE: a minimum fiber length of about 15ft is required for connection check to operate.

Press F2 (CONTINUE) if fiber is shorter than 15ft. Connection Check can also be deactivated in the setup menu.

Figure 3: Connector is either dirty or not seated properly

Figure 4: FAIR to GOOD is OK to test

The progress will be updated on the screen and test results clearly displayed upon completion.

The PASS/FAIL threshold default is 1.50dB which should be an acceptable value for fibers up to 1000ft. If possible, verify with a network engineer or system vendor.

The PASS/FAIL threshold value can easily be set by pressing F2 (Total Loss Threshold) and adjusting the value. Press SET when finished.

Figure 5: Fiber PASSED based on the threshold

Figure 6: Fiber FAILED based on the threshold

After results are viewed, they can be stored to either internal memory or an external USB memory stick if desired.

Typical testing procedure using the Anritsu ACCESS MASTER MT9083:

 

Conclusion

With the right tools and testing procedures, proper installation and maintenance of optical feed cables can be achieved with fewer errors and improved Quality of Service.