Understanding interference hunting

As wireless services grow, interference, once uncommon, becomes a fact of life for wireless and broadcast services. A metropolitan area of two million people may have 2000 licensed two way radios, 1,500 cell sites and 100 broadcasters. To this mix, add military, aeronautical, and emergency services. And then there are all the lower powered unlicensed signals such as Wi-Fi or wireless video cameras. If you consider that many of these services are expanding, being modified, ageing, or failing, it becomes evident that interference will be an issue.

The first indicators of interference are noisy links, for analogue systems. Legacy AM and FM systems indicate interference problems by various noises. Hiss, hum, or even voices from other transmissions can be heard. For digital transmissions, such as HDTV, cellular, or P25, interference shows up as limited range, dropped calls, or low data rate. A second indicator of interference is a high noise floor in the receive channel. Interference naturally affects reception first, where the signal levels are normally small.

Once a high receive noise floor has been identified and located, it’s time to get a spectrum analyser out and take a look. The first and best place to start looking is at the input to the receiver. If the receiver has a pre-filter, it’s best to measure the signal after the pre-filter. This will allow you to see what the receiver and the receiver’s antenna sees. It is important to get a ‘visual ID’ on the signal at this point so you can be sure you are on the same signal later.

If you are working a cellular issue and the base station has a high noise floor, you need to be looking on the uplink channels, not the downlink. If the issue is instead poor mobile reception in a given area, then you would look on the downlink frequencies, since that is what the phone receives. Two way radio and other push-to-talk systems often use the same frequency for both the uplink and the downlink, so this distinction becomes less important for them.

A key point is that an interfering signal does not need to be on the receive channel to cause interference. It only needs to be within the receiver bandwidth, which normally means that it only needs to get past the receive pre-filter. Once an interfering signal is present at the input of a receiver, it affects the receiver’s front end, causing a reduction in sensitivity. This will cause the effective carrier-to-interference ratio (C/I) to be lower and result in all the symptoms of a weak signal, except that the received signal strength measurements will be strong due to the high noise floor. It’s as if you were at a noisy party, trying to hear a soft-spoken person while the band was playing. Plenty of information is reaching your ears, but much of it is preventing you from hearing the conversation.

Once the interfering signal is spotted, it’s important to characterise the signal before disconnecting from the receiver’s signal. To characterise the signal, adjust the spectrum analyser to best view the signal by using the pre-amp, reference level, span and resolution bandwidth controls. Observe the signal’s shape, bandwidth, and behaviour. Look for frequency drift, amplitude changes, and frequency hopping. If the signal is intermittent, or turns on and off, use Max-Hold to create an envelope. For signals that are intermittent with a long time between appearances, it can be helpful to use a ‘save on event’ capability. A Spectrogram display is also useful to show how signals change over time.

Locating the source is best done with a handheld GPS enabled mapping spectrum analyser. This helps resolve the most complex cases in the quickest possible manner.

For further information or to request a free 30 page guide to Interference Hunting please call Anritsu on 1800-689-685.