Interference Tips For Your RF Spectrum
2019-09-29 | 5 min read
Today’s crowded spectrum is rife with interferers ranging from harmless to threatening. These intentional or accidental signals can wreak havoc on a wireless system, creating issues such as noise in the channel or even loss of service. Hard to predict and detect, interference creates difficult mitigation challenges. Overcoming interference begins with knowing what kinds of interferers exist and how to detect and overcome them.
Types of Interference
There are two basic kinds of interferers: in band and out of band. With in-band interference, a transmitter is transmitting directly on top of another signal. The legitimate signal is at a particular frequency and then someone brings in an illegitimate transmitter on the same frequency. Usually, you can identify those emitters more easily than out-of-band interferers.
With out-of-band interference, a transmitter is turning on at a different frequency. When it turns on, however, it has intermodulation distortion or harmonic mixing that causes it to mix with another nearby signal or even the receiver itself. The impacted receiver could be acting as a mixing receiver, which causes a harmful interference product to show up on the frequency.
For applications like interference detection, users perform signal monitoring—a combination of spectrum monitoring and signal analysis—over the air (OTA). Such monitoring does not take place in a chamber, but rather out in the real world. As a result, the user cannot control what the receiver is seeing, which leads to the need to record, trim, and process signals of interest.
Signal monitoring systems perform three basic measurements: energy detection, signal classification, and emitter geo-location. These functions provide guidance and assistance in issues regarding ITU/regulatory, spectrum security, or monitoring/intercept. For example, applications like the following increasingly call for spectrum monitoring: facilities monitoring, satellite/outdoor ranges, drone monitoring, and signal intelligence (SIGINT).
More signal events occur in today’s spectrum, leading to a rise in interference. Government and commercial organizations must detect, locate, and mitigate any problem signals. Finding the source of such interference is difficult. One approach is placing time-synchronized RF sensors around the area of concern. A central control facility can collect and analyze data from these sensors. Software can detect, identify, and provide a location of interfering signals using a unique RF fingerprinting algorithm. Discovery of that fingerprint anywhere in the spectrum can trigger geolocation, which leads to the emitter location.
Another possible step involves using a handheld analyzer in conjunction with the geolocation software. A geolocation algorithm can pinpoint the location of the interfering signal down to less than 100 meters, but you still need feet on the street for those last few meters. You can point a handheld analyzer with a directional wand antenna in different directions to pick up the strongest signal and walk toward it. Determining the source of a moving signal, such as something transmitting from a vehicle, is a bit trickier. Continuous monitoring and geolocation over time can provide the direction and speed of the moving signal.
Think about mitigating interference as dealing with one problem at a time. To identify a transmitter, for instance, you can make a recording of it. An IQ recording gives you all the information about that signal. When you make an IQ recording, you also can leverage software to utilize it across different applications. You can then use spectral analysis tools to identify whether it is a signal showing up in the wrong band, for example.
Keep in mind that interference is often intermittent, making it very hard to pinpoint the source. Often, an interferer no longer appears when equipment is set up to detect and locate it. To hunt down the interference, use real-time spectrum analysis tools for the most effective approach.