Tracing foreign interference in the cellular band Almost every interference problem can be solved, but first you must identify its source. Observations made with test equipment and radio direction-finding are among techniques used by investigators.

Cellular telephone service providers deal with radio frequency interference problems on a daily basis. A sudden increase in noise, crosstalk or dropped calls in one cell normally causes cellular engineers to assume that there is a problem with co-channel or adjacent-channel interference from another cellular base station or a mobile subscriber unit in the network. Do not forget, though, that the radio spectrum also is occupied by broadcast, paging, specialized mobile radio (SMR) and other transmitters, any of which can introduce harmful interference into the cellular band.

Foreign interference (that which is caused by non-cellular radio transmitters) can occur anywhere, but it happens more frequently where cell sites are co-located with other transmitters. It has been one of the trickiest problems to solve, simply because methods of identifying the interferers are not well-established among cellular system providers. Actually, the task is easier than you might think. You can track down just about any source of foreign interference, not just in the cellular band, but for other radio services as well, as long as you have patience, persistence and a spectrum analyzer.

Gather information Gathering information is the first and most important step in solving an interference problem. Find out the symptoms: for example, noise in the audio path or dropped calls that cannot be traced to hardware failures or cellular frequency reuse. Determine whether the problem occurs within a large geographical area or whether it is limited to a particular cell site’s coverage area. Determine whether the affected cell sites are co-located with other transmitters. Also, find out which channels are affected by the problem and which operating band is affected–the mobile transmit band or the base station transmit band.

With the necessary background information in hand, visit an affected site for a first-hand look with a spectrum analyzer. Where in the spectrum should you start to look? Unless you have prior information that indicates otherwise, start in the mobile band. Interference is much more likely to be a problem in the mobile band where the interferer easily competes with and overpowers the relatively weak signals (typically 0.6W) from cellular subscriber units.

Sample what the cell-site transceivers are seeing by connecting the spectrum analyzer to the cell-site receive multi-coupler. Set the spectrum analyzer to display the entire mobile band and adjust the resolution bandwidth to about 3kHz. First, observe the noise floor. Does it rise and fall as you watch? Is it reasonably flat, or is it higher at either band edge? Noise floor irregularities may indicate faulty cell-site hardware or an interferer close to the receive antenna.

As the next step, perform an over-the-air test, both inside the cell site and at a reasonable distance from the cell-site antennas. Connect a passive, portable antenna to the spectrum analyzer and view the affected band. Compare this unfiltered and unamplified view with the one seen through the cell-site receive multicoupler. An over-the-air test often reveals a distinct interfering signal, whereas the view from the receive multicoupler may show intermodulation or broadband noise.

For example, when our company received customer complaints of loud background noise in calls made near a particular cell site, our first step was to view the affected band through the cell-site receive multicoupler. No distinct interfering signals were observed, but the noise floor could be seen spontaneously rising above -75dBm, then falling to a normal level. Later, an over-the-air test performed about 100 feet from the antenna tower made the problem evident. An odd-looking spurious emission was being produced by a co-located SMR base station transmitter, and was being radiated from its antenna on a shared tower. (See Figure 1 at the left.) This spur, being radiated close to our receive antennas, had been overdriving the cell-site preamplifiers and causing strong receiver inter-modulation. The problem might have never been identified without the over-the-air test.

Some engineers do not like to perform the over-the-air test, citing the need for greater antenna height and for increased receive sensitivity provided by the cell-site preamplifier. My advice is to first test for strong interferers before looking for the weak ones. If you suspect the interferer is lurking in the “grass” near the bottom of the spectral display, decrease the resolution bandwidth on the spectrum analyzer. This will lower the displayed average noise level and expose any weak signals. To provide a height advantage for your antenna, use an elevated-feed antenna mounted on an extendible pole. Any way you work it, perform the over-the-air test. It is critical.

Look and listen Once you have observed a potential interfering signal, take a good, close look. Every type of radio emission has unique characteristics and, after some observation, you often can tell whether a signal is legitimate or whether it does not belong in the band. (See Photo 1 on page 48.) For example, Advanced Mobile Phone System (AMPS) cellular signals, as seen on a spectral display, vary in amplitude, but always fit within their 30kHz assigned bandwidth. Also, the AMPS signal’s transmission pattern is such that it normally occupies the same channel for a period of 30 seconds to several minutes. By comparison, a signal that pops up and down quickly on the same center frequency more likely originates from a push-to-talk type transmitter or a paging transmitter. A signal that stays up constantly or that occupies a wide band probably originates from a broadcast station. (See Figure 2 at the left.)

If you suspect that a signal is a foreign interferer, attempt to demodulate the signal as AM, FM and single-sideband (SSB) … and then listen. A surprisingly large percentage of interferers carry clean, uncluttered baseband information that is useful in identifying the type of station. Note whether the baseband signal is intelligible as voice or as digital signaling. Also, listen for the broadcast of station call letters, which may be transmitted in either voice or Morse code.

One of the trickier problems was an interferer that, on the spectral display, looked much like an AMPS signal. Although it was not centered on an AMPS channel, the signal was frequency-modulated with voice traffic that sounded like a cellular call. The transmission pattern also was consistent with a cellular call. The problem became apparent when two other signals on the spectral display were observed to pop up and down in unison with the targeted interferer. Tuning quickly among the three signals and demodulating them revealed that they all carried the same voice traffic. The interferer was found to be one of several strong spurious emissions generated along with a legitimate signal from our own cell site. (See Figure 3 at the left.) Although this interferer was not exactly “foreign,” it definitely was out of the ordinary, and it never would have been identified using conventional methods of cellular testing.

Direction-finding Sometimes observing interferers from a distance is not enough, and you may need to use one of the oldest troubleshooting methods, radio direction-finding. The art of radio direction-finding has been practiced for years by radio amateurs who often participate in organized transmitter-hunting contests, or “T-hunts.” Modern, specialized direction-finding gear uses microprocessor control and electrically scanned antenna arrays. In most cases, though, using a hand-held directional antenna, such as a yagi, in conjunction with a signal strength meter or spectrum analyzer can bring successful results. (See Photo 2 on page 56.)

To start, connect a directional antenna to the input port of your measuring device. For a spectrum analyzer with a digitizing display, speed up the sweep by widening the resolution bandwidth and narrowing the scan bandwidth just enough to see the full width of the signal. Signal strength meters or receivers equipped with a signal strength meter are even easier to use. When using a signal strength meter, be sure to narrow the filter bandwidth enough to reject any strong adjacent signals.

With the directional antenna mounted on a pole or even a broom handle, sweep the antenna through 360°, watching the spectral display or signal meter as you do so. Observe how the signal strength of the interferer rises and falls as the pole is rotated. Reflections or re-radiations on other antennas in the area may cause false readings, but as a rule, the strongest signal reading will consistently be seen on a single line of bearing.

Once a fix is found in azimuth, face the predominant direction of the interference source, move to your right or left, and repeat the process. Move closer to your target until you have a potential source of interference in view. Continue to move around your target and confirm your fixes in azimuth. Next, confirm your fix by sweeping the antenna up and down in elevation. This step is critical when the interferer is located high on a tower.

Keep in mind always that your target signal could be anywhere–even inside your cell site or inside your spectrum analyzer. Radio direction-finding is as useful in finding out that you have an equipment problem as it is in finding foreign interferers. If your efforts to direction-find do not yield a distinct line of bearing in azimuth and elevation, check your test equipment and cell site equipment.

Search and compare The methods described above are effective in tracking down an interferer, but the most definitive method is to witness a simultaneous transmission from a legitimate transmitter licensed to operate elsewhere in the radio spectrum. The best way to do this is to monitor the affected cellular band while using a second spectrum analyzer to search the rest of the spectrum, one frequency block at a time. Searching for a legitimate signal somewhere in the spectrum can be like looking for a needle in a haystack, but the task can be simplified if you follow a few helpful hints.

First, use the information gained in the previous steps. If the transmission pattern or the demodulated baseband information consisted of “bursty” digital signaling, begin the search in the 929MHz-930MHz one-way paging band. If the transmission pattern resembles that of a push-to-talk transmitter, search the land mobile base station transmit band between 851MHz and 869MHz. Continue to search upward and downward in frequency away from the cellular bands. Also, try dividing the frequency of the interferer by whole numbers, and tune to the resulting frequency to determine whether the interferer is a harmonic of some legitimate signal. Once you have found a signal that appears and disappears in unison with the interferer, attempt to demodulate it, and compare it to the spurious signal in your band. Your goal is to find an exact match.

Success with this simple comparison method relies on there being a one-to-one correspondence between the interferer and a legitimate signal, but what if the interfering signal comes as the result of transmitter intermodulation, i.e., a mix of two or more signals from different transmitters? Any number of combinations of transmit frequencies could result in an intermodulation product that would appear only when all transmitters involved are simultaneously keyed. Observing such an interferer in real time would be exceedingly difficult. The large test equipment manufacturers offer automated spectral monitoring systems to help in this work, but the problem of intermodulation remains complex.

Although transmitter intermod poses a challenging problem, rarely are radiated intermodulation products strong enough to cause serious interference to cellular signals. If you do encounter such a problem in your system, keep in mind that, regardless of the mechanism by which the intermodulation was created, the responsible entity still is the licensee of the one transmitter actually radiating the interfering signal. Radio direction-finding may be the best means of tracking down such an interferer and, at the very least, it will narrow the list of candidates capable of producing the mix.

One more hint is worth considering in searching the radio spectrum. You should become familiar with other radio services licensed in the UHF band, especially those in the spectral vicinity of cellular. (See Figure 4 on page 58.) For this purpose, obtain a copy of the FCC rules found in Title 47 of the Code of Federal Regulations. Part 2 of the FCC rules contains the Table of Frequency Allocations, an extremely valuable list for identifying potential interferers. Other parts give specific frequency assignments for the various radio services and outline permissible levels for out-of-band emissions. You can purchase a copy by calling the Order and Information Desk of the Government Printing Office at 202-512-1800.

Once you are confident that you have identified the source of the foreign interference, call the FCC National Call Center at 888-FCC-TALK to obtain a point of contact for the suspected interfering station. Although the FCC generally does not have the manpower to conduct research or to hunt interferers, it still maintains the database of licensed transmitters that will help you in your efforts. Do not be shy about asking the FCC for assistance.

“Do the basic legwork first,” advises Mike Ridder at the FCC. “Investigate as much as you can on your own, and have the important information ready, specifically, the operating frequency and approximate coordinates of the interfering licensee. We will gladly provide licensing information if it exists.”

When you do contact the licensee of the suspected interfering station, be diplomatic. Identify yourself and your employer, and explain precisely why you believe that his transmitter is the cause of the problem. Be ready to describe the test methods that led you to this conclusion, and invite the licensee to an on-site demonstration of your test.

Almost every operator whom I have approached in these matters has been cooperative, professional and, in many cases, eager to resolve the problem. If, on the other hand, you encounter a system operator who is evasive, uncooperative or even unlicensed, then by all means inform the FCC, and be prepared to state your case and to present evidence. Bear in mind, though, that the law allows all transmitters to emit out-of-band emissions within specific limits, and even in cases of clear violations, the FCC generally prefers that the parties involved work out the problem among themselves.

The solution to the interference problem could come in the form of repairing faulty equipment; installing cavity filters, isolators or pre-selectors; repositioning antennas on shared towers; or in extreme cases, by re-channeling equipment. Almost every interference problem can be solved, once you have identified its source. The fact that so many different types of radio transmitters already share the spectrum and operate in a non-interfering manner should serve as an inspiration.

Tracking down and eliminating the source of foreign interference in the cellular band can be one of the most rewarding tasks you will perform, and the ability to do so will become increasingly important as the radio spectrum becomes more crowded. Always remember to be persistent, patient and, above all, diplomatic. Happy hunting!