June 26, 2015
There are many interference sources that field technicians must deal with on a daily basis. Simply, interference is anything that makes a radio receiver’s job more difficult – signal noise, EMI, radios, even multipath propagation in some cases. The list of potential interferers is even longer and can include other radios, Wi-Fi routers, CATV, transformers and anything connected to electricity. Intentional sources such as jammers are also a common cause.
Interference often causes big problems in network performance. It adversely affects receiver performance, leading to customer complaints due to coverage drops and slower throughput. All of this can weaken a service provider’s bottom line. To make sure operators receive the best possible return on their network investment, interference sources must be found and eliminated quickly. In this post, we will outline three steps to help you efficiently locate and remove interference sources.
Step 1: Get a Good Look at the Signals
Once you suspect an interference problem, the first step is to take measurements with your handheld spectrum analyzer to get a good look at the signals. Today’s handheld spectrum analyzers offer a variety of measurement and analysis tools that a generation ago seemed unfathomable in a field instrument. These features can be used to find even the most difficult sources, including broadband noise, transient signals, and moving emitters.
For broadband noise caused by things such as fluorescent lighting ballasts, jammers and CATV signals, you can use a handheld spectrum analyzer to look in the guard band between allocated channels. Trace averaging and narrow VBW are useful to see the average power of the noise. Be sure that the noise isn’t coming from your TMA/LNA. If you can bypass that amplifier and the noise level remains high, you can be reasonably confident that the source is external.
Using the correct tools can help you find interference sources faster and more easily.
For CW and CW-like signals such as oscillating repeaters/BDAs, a spectrum analyzer with a very narrow RBW of <100 Hz can make your job easier. If the signal is at the same frequency as your traffic, the narrow RBW will drop the modulated signal amplitude for a clearer view of the interfering signals. You can also use a Min-Hold feature, which is particularly effective for CW-like interfering signals because it helps to ignore the bursty uplink. Anritsu’s mobile interference hunting system makes this even easier by automatically resetting the Min-Hold every few seconds. This is particularly beneficial when hunting in a vehicle, as it eliminates the need to repeatedly press the Reset Trace button every time you change directions.
Transient signals can be caused by repeaters that turn themselves off when oscillation is detected, as well as other bursty types caused by signals on the move such as jammers in cars, repeaters in boats or trucks, or arcing. Key spectrum analyzer capabilities that can help track these sources down include:
- Spectrogram – This can be used to gather data over multiple days, which can then be analyzed on a computer to give you a better understanding of what you might see during your hunt.
- Max Hold – A nice feature that makes it is easier to see and locate intermittent signals.
- Save-on-Event – For very long signal recording, Save-on-Event is preferred over a Spectrogram, as you can record the signal just when it’s present and exceeds a mask level.
- Burst Detect – This feature can be used to capture signals as short as 200 µs. It can be particularly effective when combined with a Spectrogram and Save-on-Event.
Getting a good early look at the signal(s) is essential, as it can shave considerable time off of your hunt. Why? Here are four reasons:
- Helps eliminate local problems such as PIM, loss or reflections caused by an antenna or feedline, as well as the radio – both transmitter and receiver.
- Determines when interference happens. Locating an interfering signal that is constant is simple. If it’s only on Sunday morning, you know when you have to look for it (and you might start by checking emissions from churches.)
- Acquire a reference trace, so you know what to look for in the field.
- Gain an initial estimate of the emitter position by comparing the power on adjacent sectors. This is explained further in step two.
Step 2: Estimate Emitter Position
Step two is to get an estimate of the emitter position. In cellular networks you can make a measurement at the base station to determine the rough direction of the emitter by comparing the amplitude in different sectors. The emitter will be in the general direction of the sector with the highest power, and closer to the direction of the next-highest-power sector than others. For networks without sectorized antennas, you may need to use a directional antenna, such as a high-gain Yagi, to determine the initial direction.
While this estimate will not pinpoint the source of interference perfectly, it will be good enough for you to drive to the area the interference source calls home. If you can compare the power from several different base stations, you can also make a simple estimate of the emitter’s distance between them. Again, the emitter will generally be closer to the base station with the higher received power.
Step 3: Locate the Source
To refine your estimate and locate the source, you can use several techniques. One is signal survey, in which you record amplitudes once you arrive at the target area to determine the strongest signal locale. Find the location with the largest amplitude and then repeat the procedure with a finer grid. Signal surveying can be very effective, especially when a mobile interference hunting solution, such as the one in Figure 1, is employed. Using such a solution will make this technique faster because it helps direct you where to go rather than surveying an entire area.
Figure 1 (MIH) – A mobile interference hunting solution can save time when on the hunt for interfering signals.
Triangulation is another technique that is usually used for long distances of at least a half mile. In this method, you acquire multiple bearings from several widely spaced positions. Using multiple bearings is especially beneficial to avoid being fooled by large reflections that are common in high multipath environments. The triangulation technique is much easier to perform with a spectrum analyzer that has mapping capability. A good tip is to verify the bearing from several nearby locations to reduce the effects of multipath and shielding caused by buildings and vegetation.
Successive approximation, also known as homing, can be the fastest technique, especially indoors or when the emitter is nearby. You find the strongest signal using a spectrum analyzer and Yagi antenna, and travel along the bearing line. When a peak in signal strength is detected, you acquire a new bearing and go in that direction. This approach is not recommended when conducting the hunt from a vehicle for two reasons. One, it’s difficult and unsafe to hold the antenna out the window of a moving truck or car. The second is because multipath and path loss are more common due to buildings and vegetation.
Spectrum analyzers also have a number of tools to help in successive approximation. A dual trace mode allows for normal and max-hold traces; you can use this to help get you pointed in the right direction. Simply rotate the antenna 360° horizontally, then continue until you see the maximum signal strength again. A signal strength meter and interference mapping tools – both of which are common in handheld spectrum analyzers – are also helpful in narrowing the area in which the emitter is located.
In the real-world, you will probably employ all three of these methods. For example, you can start by performing triangulation from several cellular base stations, or just get an initial bearing from one. Then, determine the general area of the emitter location using the signal survey process. Using mobile interference hunting software and mapping tools are particularly helpful, as they allow you to take a more “intelligent” survey. Bearings aren’t collected, but rather the software uses power measurements from many locations to help lead you to the emitter. For the final step, you may use a handheld analyzer and directional antenna to find the actual device that’s emitting the unwanted signal.
Use the Right Tools
No matter the technique employed, you can follow certain interference hunting tips to make it easier and faster to locate the source of the network issues. Always remember to look in all directions. Consider multipath, as it can provide faulty analysis and lead you to incorrect conclusions. Most importantly, using the proper test tools can make your job easier. Select a handheld spectrum analyzer that is loaded with interference analysis options such as those discussed in this post. The analyzer should also have a broad enough frequency range to see the signal, and high sensitivity and dynamic range. Antennas should also be part of your took kit. Training courses can help you be best prepared when you are out in the field, as well.
Finding interference sources continues to be one of the biggest issues for field technicians. By taking the steps outlined and selecting the proper test tools, you can locate the problem and fix it faster. To learn more, view an online webinar on interference hunting.