The proliferation of RF devices has benefited the world in many ways, providing readier and more convenient access to communications, entertainment and information. But it has come with one unintended drawback: a huge increase in the frequency and severity of incidents of interference.
Cellular telephone networks have been hit particularly hard: the introduction of many new cellular technologies to an ever-growing number of subscribers has made it difficult for the network operators and spectrum regulation authorities to find clean spectrum unaffected by interference. Interference in cellular transmissions increases noise, with the effect of:
- reducing the effective size of a cell
- lowering data rates delivered to user equipment
- impairing the quality of voice and data communications
As a result, radio technicians spend much time travelling around the country, hunting the sources of interference. Only once a source is precisely located can action be taken to lower the power of its transmissions or to disable it entirely, if appropriate.
Figure 1 Spectrogram showing occurrences of signals in the GSM frequency band over a period of a few minutes.
On occasion, filtering an unwanted in-band signal can solve the problem. Otherwise, the technician’s job is to monitor the spectrum, uncover the nature of an interferer (permanent or intermittent) and track it to its source.
Types of Interference Affecting Cellular Networks
The existence of interference affecting a cell is normally obvious to the operator, because it will have caused a drop in communications performance which will have been reported by the operator’s networking monitoring systems. The first step in dealing with the problem is to observe what is happening in the affected frequency band. At this stage, the technician needs to know whether the interference is continuous or intermittent by measuring the amplitude and power of the interfering signal(s). This is best done by connecting an omnidirectional antenna to a spectrum analyzer, placing it in the cell in question and leaving it to log network activity for a period of time.
The analyzer may find one or more of a number of types of interference:
- In-band interference: an unwanted signal from a different transmitter type that falls inside the operating bandwidth of the desired signal. This corrupts the receiver as it is difficult to filter.
- Co-channel interference: similar to in-band interference, except that the unwanted signal originates from a transmitter in the same network, but located elsewhere.
- Out of band interference: in a wireless system designed to transmit in a different frequency band, part of a transmitted signal’s energy can fall into the operating frequency band of the cell under test, impairing its performance.
- Adjacent channel interference: often seen in wideband transmission systems, when transmissions at the main operating frequency also generate lower-amplitude signals in directly neighboring channels (called the lower and upper channels or left and right channels).
- Uplink and downlink interference: unwanted signals affecting the receiver (uplink) or the transmitter (downlink) of a base station when it communicates with a mobile terminal.Impulse noise: created whenever a flow of electricity is abruptly started or stopped, impulse noise can affect any transmitter’s or receiver’s characteristics, with the effect of scrambling communications.
Figure 2 A Yagi antenna can isolate an interference source at a particular frequency.
Other types of interference may be found, but the most common types are those listed.
Locating the Source
In today’s cellular telephone networks, the operator will be alerted to a problem by built-in alarms or dedicated sensors distributed through the network which can time-stamp instances of interference. Now, radio technicians have to identify the cause of the problem.
If the source of the problem is an RF emitter, one way to troubleshoot the system is to monitor the frequency band of the affected transmitter or receiver. A modern handheld spectrum analyzer connected to an omnidirectional antenna can accurately monitor a frequency band over a period of time (typically up to 72 hours) by continuously logging spectrum measurements.
Figure 3 A Log Periodic antenna can capture a variety of interferers across a wide frequency spectrum.
Figure 4 A spectrum analyzer can emulate the display style of an analog meter to show signal strength.
Figure 1 shows the spectrogram display of continuous measurements. Its advantage is that the power (amplitude) of each detected frequency is color-coded, so when analyzing the measurements it is easy to see whether or not an unwanted signal appears in the frequency band under investigation. This first step will commonly identify the frequency and amplitude of the interfering signal, and the nature of its emissions (random, regular or continuous). Measurements taken from a single location are not sufficient to precisely locate the interferer.
Figure 5 A drive test records the signal strength at a chosen frequency over the course of a known route.
The next step is to attempt to locate the source with the use of directional antennas (see Figures 2 and 3). In the past, the preferred way to locate a source was to use an analog meter to measure the strength of a signal or carrier (see Figure 4). These meters gave an audible beep which rose in frequency as the signal strength rose. This allowed technicians driving vehicles with roof-mounted antennas to gauge signal strength without taking their eyes off the road. While this was convenient, it was not an accurate means to locate interference, as the meter had no embedded mapping capability.
An improvement on this method is to perform a mapped drive test. Using the same omnidirectional antenna mounted on the roof of a car, the technician drives on a route through the affected area logging the power of signals at the suspected frequency. By pre-loading the route into a software application such as easyMap Tools, a map can be hosted on the spectrum analyzer and directly displayed while driving. As Figure 5 shows, the analyzer can display the car’s location in real-time (derived from a GPS signal) and the power of the received signal at the chosen frequency (known as Received Signal Strength Indicator, or RSSI).
The easyMap Tools software allows for the car’s variable speed: the user can configure the spectrum analyzer to take a measurement at set distances, for example, every 10 m. Subsequent analysis of the drive test results might indicate the area in which the interfering signal is strongest.
To this point the interference-hunting process has narrowed the search down to a small area. But it has not precisely located the source: this means that the technician can still not identify it, so it cannot be attenuated, filtered or disabled. Finding the precise location of the interferer calls for the use of the same handheld spectrum analyzer, but now with a directional, narrowband antenna. The process of ‘direction finding’ in the suspected area is uncovered by the drive test.
Figure 7 Triangulation locates the source of interference on the instrument's map.
Figure 6 Direction finding in the field with a handheld spectrum analyzer and unidirectional antenna.
Since this normally involves going out on foot in the area under investigation while holding a portable spectrum analyzer, it is helpful to have an accessory for holding the directional antenna. For example, Anritsu provides the MA2700A, an ergonomic handle that secures the antenna via a standard connector (see Figure 6).
The MA2700A also includes a broadband pre-amplifier to boost the antenna’s sensitivity and a built-in GPS receiver to enable the precise location at which measurements are taken to be logged in the spectrum analyzer. Finally, a built-in electronic compass (magnetometer) senses the exact direction in which the antenna is pointed. The user just has to pull a trigger and turn around 360° to find the direction of the strongest signal at the frequency in question. The location and direction are displayed on the instrument’s screen.
By repeating this process from multiple locations, the user can perform triangulation (see Figure 7); the various measurements should almost always point towards a single location on the map. This is the source of the interference.
This technique can be used by any network technician, since it uses familiar equipment and easy-to-operate software tools. Normally, this technique on its own is sufficient to enable the technician to find and fix the interference problem.
There are many kinds of interference sources that affect the performance of wireless transmission systems. The handheld spectrum analyzer, combined with omni- and unidirectional antennas, is the most convenient and effective instrument for identifying and locating the position of the interferer. In addition, by integrating GPS and mapping software in the spectrum analyzer, the time taken to troubleshoot interference problems is minimized.
Anritsu S.A.
Villebon-sur-Yvette, France
www.anritsu.com