PCB News - Reasons for RF interference in mobile communication systems

Nowadays, the possible causes of radio frequency interference are increasing, some are obvious and easy to track, others are very subtle and difficult to identify. Although careful base station design can provide some protection, in most cases the interference signal can only be controlled at the source. This paper discusses the various possible causes of radio frequency interference, which will help engineers to measure, track and eliminate the sources.

Radio Frequency Interference (RF) signals can cause many problems for mobile communication within the coverage area of a wireless communication base station, such as telephone dropouts, noise in connection, channel loss, and poor reception of voice quality. The various possible causes of interference are increasing at an alarming rate.

Today's complex telecommunication technology must coexist in a complex environment with old mobile communication systems, such as private wireless communication or paging, and most of these old systems will continue to be used for several years to come. At the same time, other wireless RF devices, such as digital video broadcasting and wireless local area networks, will generate new signals that may interrupt communication services. Due to the growing environmental constraints, many new businesses are competing to occupy limited cellular sites, which makes cellular signal transmission towers full of antennas. As we increasingly connect via mobile phones, watch multimedia shows and trade on the Internet, and even soon our cars, refrigerators and ovens will use RF signals to communicate with each other, the communication sky will become more crowded.

Causes of RF interference

Most of the interference is unintentional and is a by-product of other normal operations. The interference signals only affect the receiver, and even if they are physically close to the transmitter, the transmission will not be affected. Here are some common sources of interference to let you know where to start in practice. Note that most sources of interference come from outside the base station, that is, outside your direct control.

_Emitter is not configured correctly

Another service provider also sends a signal on your frequency. In most cases this is due to malfunction or incorrect setup, and the conflicting transmitter service provider will be more anxious to correct the problem in order to restore its service.

_Unlicensed transmitter

In this case, other service providers intentionally launch on the same band as you, usually because he has not received a license at all. He may not find a signal on a band, so assume that no one is using the band and use it without authorization. Licensing government agencies often help drive away such unlicensed operators.

_Coverage overlap

Your network or other network covers more than one channel or channels. Incorrect antenna tilt, high transmission power, or environmental changes can cause overlapping coverage areas, such as someone cutting down a forest or pushing down a building that would otherwise block signals from another location.

_Self-signal intermodulation

When two or more signals are mixed together, a new modulated signal is formed, but it is not any desired signal. Common intermodulation is a cubic signal, for example, two signals with an interval of 1 MHz will produce a new signal at 1 MHz above the original high frequency signal and a new signal at 1 MHz below the low frequency signal. If the original two signals are in the 800 and 801 MHz bands, then three signals will occur at 799 and 802 MHz, respectively.

_Intermodulation with another transmitter signal

Intermodulation interference may also be caused by one or more external wireless signals feeding coaxial cables through an antenna and then entering the conflicting transmitter's non-linear terminal amplifier, where the external signals are mixed with each other and the transmitter's own signals to form a "new" frequency intermodulation signal that looks like a communication band (often undesired).

It is also possible that the interference signal is generated by two external signals, and the signal of the conflicting transmitter itself is not participating, and the external signals are only mixed up by just using the non-linear level of the transmitter. In this case, none of the two signals mixed together has a problem, and the perpetrator is the transmitter.

Solving this problem is a bit difficult because it requires changes to the transmitter that appears to be working properly. A narrow-band filter is needed to attenuate the external signal as much as possible, plus a ferrite insulator to enable the RF to transmit from the transmitter to the antenna and attenuate the signal returned from the feed line. Owners often require all transmitters to install such filters and insulators on transmission towers that use multiple different frequencies at the same time.

_Interactions caused by rusty walls/roofs, etc.

Transmitters are not breeding grounds for intermodulation signals, and non-linear connections can also be nearby rusty white iron roofs or walls. When the wireless transmission power is high, the rusted parts of the roof will act as a non-linear diode. Interactions such as these from the physical structure are difficult to prevent because the wind will press the rusted parts together or separate them depending on the weather conditions, and the rain will change the rust characteristics. Communication seriously affected must be repaired or replaced to restore a reliable communication connection.

_Intermodulation in antenna or connector

Sometimes even a slight corrosion of the coaxial cable or antenna itself can cause problems, although it is not enough to cause signal loss or VSWR problems, but the corrosion can cause fine intertuning like a poor quality diode. If there are several high-power transmitters nearby, the intermodulation generated will be strong enough to interfere with the weak communication signal between the mobile phone and the base station. The hard part to find out is that loosening a connector in the antenna system can disrupt oxidation and temporarily suspend the problem, so you have to spend more time carefully recording which connector is loosened or tightened and test it after each step to determine if it is the culprit.

_Regular transmitter overload

Any strong frequency signal emitted by the transmitter will overload the adjacent system. The solution is to install a filter on the receiver antenna cable so that the desired signal can pass through and the overloaded signal will be attenuated.

_Adjacent channel power on adjacent transmitter

As allocated frequencies become more crowded, competing wireless services are allocating closer frequencies, increasing the risk of one system transmitting channel noise sideband appearing or preventing another adjacent receiving channel. If the transmitter meets the technical specifications, you need to change the channel or increase the physical separation between the transmitter and receiver.

_Radio transmitter harmonics

High-power sources, such as commercial radio stations, can produce high-power signal harmonics, such as a 5MW transmitter, which can easily produce 5W harmonics, enough to interfere with nearby mobile communications. If the transmitter complies with all specifications and government regulations, the solution may be to migrate the communication antenna to avoid developing the transmitter, or to reassign the frequency scheme so that the communication base station near the conflicting transmitter uses a channel that is not affected by its harmonic energy.

Master STL users

Before the advent of cellular systems, the 900MHz and 1,400-2,200MHz bands were usually assigned to studio-to-transmitter connections (STLs) for radio stations. The government has redistributed these frequencies to cellular operators, but they often continue to operate on conflict-free frequencies without restricting older users. When new cellular services are launched in these bands, those transmitters should turn to new frequencies, but some need to be "alerted".

_Audio rectifier

In rare cases, the base station controller is still transmitting analog audio input to wireless output, which can be affected by strong signals from nearby AM broadcasts or short-wave stations. AM signals may enter the audio circuit and be rectified to mix broadcast audio signals into telephone conversations. Good shielding around the audio portion connected to the base station should solve this problem.

Understanding the type of interference source

Interference can be categorized by its own characteristics or by its impact on base station and mobile phone communication. Conflict frequency is a commonly used indicator to show sources and results of interference.

_Extra Frequency Interference Source

This is a major interference, including some strong signals that are close to and different from the receiver frequency and are strong enough to affect the input. These signals are usually very close to the intended frequency because the receiver input filter filters out other signals that are too far apart.

Let's look at two effects on the receiver. One is front-end blocking, which is caused by a strong signal entering the receiver and completely saturating the stage (preamplifier or mixer) overload, which makes the stronger signal unacceptable. Another effect is the alleviation effect, in which the nearby signal enters the receiver and is discovered by the AGC (Automatic Gain Control) or the limiter circuit is activated, resulting in a loss of gain. The receiver behaves as if it is less sensitive, so weak signals are lost and the signal-to-noise ratio for strong signals is reduced.

_Intra-Frequency Interference Source

The second type of interference consists of signals of the same frequency (strong or weak) as the intended communication signal and is usually caused by the following:

Normal cell phone signal is out of its intended range

. The transmitter is malfunctioning or misconfigured

- Signal harmonics from normal transmitters

Unintentional interference signals emitted by other electrical devices

_Intra-Frequency Effects from Extra-Frequency Interference Sources

Such sources are difficult to track and appear to be signals in frequency, but there is no obvious source of interference, such as an intermodulation signal formed by the mixing of two or more signals that are perfectly normal at their own frequencies within a non-linear element.

_Intentional interference

Unpleasant intentional interference usually occurs within the signal frequency and behaves more like an improperly configured transmitter. We classify it separately because it is often particularly elusive and harmful.

An example of intentional interference is a long-range attack on a two-way wireless transponder system somewhere on a jungle hill. The system initially receives a very weak signal at its input frequency (where the correct audio decoding activates the transponder), which only appears at night and remains in the air, invalidating the transponder timeout relay and paralysing the system until the morning signal disappears. The source of interference is particularly difficult to find because the signal is too weak to detect and it only emits at night. When it is found, the source of interference is a micro transmitter with a small solar panel located on the top of a tree near the antenna pole of the transponder. The transmitter shuts down during the day and its solar panels use it to charge the battery.

_Harmonic

Some of the above refer to relatively clean raw signals. In practice, there are also strong basic frequency harmonics in the signals that can cause interference. For example, a U.S. VHF TV transmitter requires a filter to be installed to reduce its harmonics to at least 60dB of the primary carrier. The troublesome harmonic is the third harmonic because it is easily generated by small non-linear elements in the transmitter. A 5MW TV signal transmitter operating at 621.25MHz has a third harmonic of 1863.75MHz, even though the third harmonic is 5W below 60dB (after filtering)! Sending this frequency and power signal from above the city can easily cause great damage to the city-wide cellular mobile communication signal.

Harmonic signals have another feature that makes them more difficult to recognize. The multiplication process that produces harmonics changes the spectrum, multiplying the width and deviation by a factor equal to the carrier frequency. For example, the 10th harmonic of a bidirectional wireless FM signal at 13kHz width at 157.54MHz is 130kHz wide, and only 5kHz offset of the base wave will become 50kHz at 1575.4MHz. If the transmitter shares a tower with a base station, its 10th harmonic will completely cover the GPS receiver, paralysing the base station. For a 100W FM transmitter, a total attenuation of about 195dB is required to avoid this interference, and antenna isolation and filter suppression are required to achieve this.