TEST & MEASUREMENT: Interference Nightmare

You are investigating reports of dropped calls, noisy connections, lost
channels and poor reception in one of your base station coverage areas. The
equipment at the station checks out, but something is still corrupting the
communication channels. There is a long list of possible offenders capable of
creating signals that accidentally or intentionally interfere with wireless RF
signals.

Where does the interference come from?

Most of the interference is unintentional–a by-product of some legitimate
activity. Here are a few of the most common sources to give you some ideas of
what to look for in a field situation. Notice that most of the sources are
external to the base station and beyond your direct control:

Improperly Configured Transmitter: Another operator is transmitting on
your frequency. Most often this is the result of a fault or an incorrect
setting; and the operator of the transmitter would be happy to correct it to
restore his own service level.

Special Considerations for Tracking Interference in Mobiles

Interference in mobile equipment poses some problems other than what we find in base station testing. While most of the same techniques used at the BTS can be used in the field, we need to work around some limitations. In a mobile environment, it is not as easy to disable a channel to clear out intended signals, so testing time may be lengthened while waiting for channel traffic. Most interference is relatively localized, so the mobiles will have acceptable call quality everywhere except in the trouble area, where they experience a high rate of dropped calls. That can force us to move around in the call area before we can even pick up the interference. When trying to locate the source of interference from ground level, it is often difficult to see past local obstructions. To find the true direction of an offending signal, sightings may be needed from several locations due to the many reflections from local obstructions.

Unauthorized Transmitter: In this case, the operator is intentionally
operating in the same frequency band — usually because he has no license at
all. He probably started his operation when he found no transmissions in the
band and assumed that nobody was using it. The governmental agency that issues
licenses is usually helpful in getting rid of these interlopers.

Cell Overlap: A cell from your own network, or others, exceeds
specified coverage in one or more channels. Incorrect antenna tilt, excess
transmitter power, or a change in the environment can cause overlap (for
example, someone may have cut down a forest of trees that had been blocking the
signal from that site.)

Intermodulation from Another Transmitter: Intermodulation interference
can be the result of one or more external radio signals getting into the antenna
feeder coax and entering the offending transmitter’s nonlinear final amplifier
stage. The external signals mix with each other and with the transmitter’s own
signal, creating intermodulation products that appear as "new" (and
often very undesirable) frequency components in the communications band. It is
also possible for two other external signals to produce an interfering signal
when neither one is from the offending transmitter, they just use its nonlinear
stage to mix together. In this case neither of the two signals that are mixing
together is at fault — the transmitter is the culprit. The solution to this
problem is a bit difficult, since it may require changes to a transmitter that
appears to be functioning properly. A narrowband filter can be added to
attenuate the outside signals as much as possible along with a ferrite isolator
that lets RF pass from the transmitter to the antenna and attenuates signals
coming back from the feeder. Tower owners at shared sites where many different
frequencies are in use often require the installation of such filters and
isolators in all transmitters.

Intermodulation in a Rusty Fence, Roof: Transmitters are not the only
breeding ground for intermodulation products — the nonlinear junction could be
a nearby rusty tin roof or fence. In the presence of high-power radio
transmissions, the rust between the individual roof sections can act as a
nonlinear diode. The intermodulation effects from physical structures such as
these are difficult to pin down, since they vary with weather conditions — as
wind presses parts of the rusty metal together and apart and rain alters the
characteristics of the rust. Seriously offending structures must be repaired or
replaced in order to restore reliable communications.

Intermodulation in Antennas or Connectors: Sometimes, even minor
corrosion in a coaxial connector or the antenna itself will create a problem.
While not enough to cause signal loss or a VSWR problem, the corrosion can act
like a very poor diode and cause just a little bit of intermodulation. In an
environment of several nearby high-power transmitters, the resultant
intermodulation can be strong enough to interfere with the weaker signals from
mobiles trying to communicate with the base. The most difficult part of finding
this culprit is the fact that if you loosen a connector in the antenna system to
see if the problem is at that point, you disturb the oxide and stop the problem.
It may not come back for several months. In this situation you may want to use
extra time to carefully note which connectors you are re-loosening or
re-tightening and test for a while after each.

Then if the problem goes away after touching a particular connection, it
might be the one. Only time will tell.

Overload From a Legitimate Transmitter: Occasionally, strong signals
from a transmitter at any frequency can overload a neighboring system. The only
solution is to install a filter on the receiver antenna cable that will pass the
intended signals and attenuate the overload signal.

Adjacent Channel Power from a Neighboring Transmitter: As the
allocated spectrum becomes more crowded, competing radio services are being
given frequencies closer together — increasing the risk of noise sidebands
from the transmitting channel of one system appearing in or blocking the
adjacent receiving channel of another.

If the transmitter is meeting required specifications, you may need to change
channels or increase the separation between the transmitter and receiver.

Harmonics from Broadcast Transmitters: High-powered sources, such as
commercial broadcast stations, can produce substantial energy in harmonics of
their signals. For example, a 5-megawatt transmitter can easily generate 5 watts
of harmonics — more than enough to interfere with nearby mobile
communications. If the transmitter in question meets all of its specifications
and government regulations, the only practical solutions may be to move the
communications antenna to shield it or to reallocate the frequency plan so that
the cell sites near the offending transmitter use channels not affected by the
harmonic energy.

"Grandfathered" STL Users: Before the advent of cellular
systems, the 900 MHz and 1400 to 2200 MHz bands were often assigned to
studio-to-transmitter links (STL) for broadcast stations. Governments have
largely reassigned these frequencies to cellular operators, but frequently
"grandfathered" the old users, allowing them to continue operations.
Those transmitters were supposed to move frequencies when new cellular
operations are established, but some may need a "reminder."

Audio Rectification: In rare cases, where the base station’s
controller side still uses analog audio input to the radio, the site can be
affected by a strong signal from a nearby AM broadcast or shortwave station. It
is possible for the AM signal to get into the audio circuitry and be rectified,
adding the broadcast audio in with the phone conversation. Better shielding
around the audio connections to the base radio should solve this problem.

Recognizing Types of Interference Sources

Interference can be categorized by its own characteristics, as well as by
its effect on the desired communications. It can be found at the base station
and in the air interfaces with the mobiles. Interference signals only affect
receivers–even when they are physically close to a transmitter, the
transmission will not be affected. The frequency of the offender is the most
common indicator of the source and consequences of the interference.

A major class of interference includes strong signals that are not at the
exact receiver frequency, but are strong enough to affect its input. These
signals are usually very close to the intended frequency, since the receiver’s
input filter should eliminate the rest. Look for one of two effects in the
receiver. Front end blocking is caused by a strong signal entering the receiver
and overloading the first stage (either a preamplifier or a mixer) to complete
saturation. This type of interference usually prevents even fairly strong
signals from being received. The other effect is desensitization. A nearby
signal gets into a receiver and is detected by the AGC, or activates the limiter
circuitry, reducing the gain. The receiver acts as if it were simply less
sensitive–weaker signals will be lost and the signal-to-noise ratio will be
degraded for stronger ones.

On-Frequency Sources: The second class of interference includes
signals (weak or strong) that are at the same frequency as the intended
communications signals.

These are most commonly caused by:

• Legitimate cellular signals that are extending beyond their intended
  range.

• Malfunctioning or misconfigured transmitters

• Other electrical devices that emit unintended
  interference.

• Harmonics of legitimate transmitter signals.

Off-frequency Sources that Produce On-frequency Effects:
The most difficult to track down, this class of interference appears to be a
signal that is on your frequency with no apparent source. The signals that
create it are at substantially different frequencies, but are being combined
into a new signal at your frequency. An example of this type is an
intermodulation product created from two or more signals that are properly at
their own frequencies, but are mixing in a nonlinear element.

Intentional Interference: Intentional malicious
interference is usually on-frequency, acting a lot like a misconfigured
transmitter. We give it its own class because it often has especially elusive
and pernicious characteristics.

An extreme example of this class was one that attacked a
two-way radio repeater system in a remote forested mountain location. The system
began to receive a very weak signal on its input frequency (including the
correct tone coding to activate the repeater), but only during the hours of
darkness. The signal stayed on the air constantly, which tripped the repeater’s
time-out relay and disabled the system until morning, when the signal
disappeared. It was particularly difficult to locate the source because the
signal was too weak to be detected at ground level and it was only transmitting
at night. When it was eventually located, the interference source was found to
be a miniature transmitter with a small solar panel that had been placed in a
treetop near the repeater antenna mast. The transmitter had been shutting down
during the day while its solar panel recharged the batteries.

Harmonics: The preceding classes describe clean
original signals. In real-world situations the signals may contain harmonics of
the fundamental frequency that are strong enough to cause interference. For
example, a UHF TV transmitter in the US is required to have a filter to reduce
its harmonics to at least 60 dB below the main carrier. The most troublesome
harmonic is usually the third, since it can easily be generated by very small
non-linearities in the transmitter. A 5 Megawatt TV transmitter operating on
621.25 MHz, has its third harmonic at 1863.75 MHz. Even at 60 dB down (after
filtering) the third harmonic would still produce 5 Watts! A signal at this
frequency and power level radiating from a high point overlooking a city could
easily wreak havoc on cellular signals throughout the city. Harmonic signals
have another characteristic that makes it more difficult to identify their
source. The multiplication process that creates the harmonic alters the spectrum
signature as well — its width and deviation are multiplied by the same factor
as the carrier frequency. A 13 kHz wide 2-way radio FM signal at 157.54 MHz
would have a 10 th harmonic that is 130 kHz wide. In addition, a 5 kHz deviation
of the fundamental would become 50 kHz at the harmonic frequency of 1575.4 MHz.
If such a transmitter were sharing a tower with a base station, the harmonic
could completely cover up the GPS receiver–disabling the station. For a 100
Watt FM transmitter, a total of about 195 dB of attenuation would be required to
avoid interference, which could be achieved with a combination of antenna
isolation and filter suppression.

Intermodulation: Intermodulation has unexpected
characteristics. Several signals may be mixed and create new modulation products
that do not necessarily follow any of the expected types. The most common
intermodulation products are third order. Two signals separated by 1 MHz would
produce one new product located 1 MHz above the higher of the original two, and
the second new one 1 MHz below the lower (originals of 800 and 801 MHz would
have third order products at 799 and 812 MHz). When measuring intermodulation
interference, be sure that the measuring instrument is not contributing its own
distortion. Any spectrum analyzer or other RF measuring tool has its own dynamic
range or "third order intercept." If the measuring tool is causing
intermodulation, it will be unable to tell which it is seeing — incoming
products or its own products. The solution for this problem is to use an
external filter that passes only the intermodulation product that you are
looking for and rejects the strong signals that made it.

Preliminary Inference: There is a long list of
possible offenders capable of creating signals that interfere with the wireless
RF signals and plague cellular communications systems. What you have understood
until here is about the described interference sources and the places within
communications systems where the interference causes problems and Interactions
between multiple offenders.

The Solutions lies in Hunting for Sources of Interference in
Mobile Networks

Recognizing Clues and Gathering Evidence with Measurement
Tools

Moving forward now, let us try and better recognize those
interference sources and gather "evidence" to prove your findings to
others.

Test Tools and Techniques Selecting, Connecting and Using
Test Tools

Today’s technician is responsible for maintaining many more
base stations than ever before. When a BTS is plagued with interference, you
need a tool that helps you identify and find the source of the offending signal
quickly. The field transmitter and interference tester such as the Tektronix
NetTek YBT250, includes automated measurements specifically designed to speed
the process of identifying the causes while greatly reducing the complexity of
testing on site. Among the automated measurements are spectrum display, signal
modulation identification, spectrogram displays, signal strength indication, and
unattended signal logging. For instance, the NetTek YBT250 also features high
sensitivity (with a built-in preamplifier), battery operation, accessory
antennas and results storage — all in a compact, rugged, weather resistant
package.

Measurements at the Base Station Receiver: The best
way to hunt for interference in the base station receiver is to disable traffic
on the channel to be measured. This will assure that no calls are assigned to it
and the only signals that you will see are the interfering ones. This is a
routine matter when the interference is so severe that the channel or the entire
face (sector) has already been completely shut down. If you need to leave the
channel active, you should disconnect one of the two diversity receivers. While
you are looking at the signals from one receive antenna, the remaining receiver
will handle legitimate call traffic. In this case, you must wait for a quiet
time between active calls to get valid measurements of the interference. This
procedure is more difficult and time-consuming, and should only be used if
absolutely necessary.

Using Automatic Spectrum Analysis: Many of the
normally complex settings of a spectrum analyzer can be automatically set for
you by the BTS Tester. There is no need to adjust controls such as resolution
bandwidth or video bandwidth, they are automatically set internally. If the
signals are constantly changing amplitude (as with GSM, for example), you may
not catch the signals at their maximum when you press AutoLevel. If the levels
increase beyond range, a warning banner on the screen will ask you to decrease
the reference level. Tap the "Ref Lvl" button and the
"increase" arrow, or enter a reference level 10 or 20 dB higher using
the numeric keypad. This will allow more headroom for those highly variable
signals. Tap on the trace of the suspect signal in the display to place the
Measurement Marker (round red icon) on the trace. Tapping the button next to
"AutoLevel" moves the measurement frequency to the center of the
screen, and the "Decrease Span" button allows you to focus the screen
on the suspect signal.

Finding Overlap in Your Mobile Network: One of the
more difficult problems to resolve is overlapping coverage from your own system.
If a BTS is located on high ground, its coverage may unintentionally exceed the
engineering plan. If the coverage is so great that it overlaps a cell that has
the same control channel (BCCH for GSM), then the mobile can no longer
distinguish between the two and is put out of service in the area of overlap. A
directional antenna will let you determine the source of each of the overlapping
signals. Moving in those directions will confirm which of the base stations is
causing the extended overlap.

Interference Testing: Now that we have become familiar
with the tools and techniques, let’s finally look at some examples of typical
signal types with descriptions of some of the possible sources. Any of these
signals, or related ones, may show up as interference to your system. These
descriptions may also be generally useful in simply identifying your RF
neighbors.

Analog Signals

Broadcast signals have some telltale characteristics. FM signals for voice
or music vary in width. During quiet times, FM appears as a CW carrier without
modulation. This changing width is its most distinguishing feature. While AM
also varies, it is narrower than FM. The best way to confirm this type of
interference is to demodulate it and listen to it. The blue trace is the
spectrum of the signal when the user was talking loudly and with lots of high
frequency energy (such as the spoken "S" sound.) This type of FM
communications signal will be continuously varying between the two widths shown
and will have no steady state.

These have only been some selected example signals that you
may see in everyday use while you look at all of the FM/ RF signals that
surround your wireless system. These examples can help you identify what is and
what is not a problem for you. And when you identify the problem, your life in
handling interference in mobile networks is a lot easier.

Viraj S Pradhan, business manager South Asia
(communications business unit) Tektronix India