Nowadays, the possible causes of RF interference are increasing. Some are obvious and easy to track, while others are very subtle and difficult to identify. Although a carefully designed base station can provide some protection, in most cases, the interference signal can only be controlled at the source. This article discusses various possible causes of RF interference. Understanding its root causes will help engineers to measure, track and eliminate it.
Radio frequency interference signals will bring many problems to mobile communication in the coverage area of wireless communication base stations, such as telephone dropping, connection noise, channel loss, and poor received voice quality. Various possible causes of interference are increasing at an alarming rate.
Nowadays, the latest and most advanced complex telecommunications technology must coexist with the old mobile communication systems (such as dedicated wireless communication or paging) in a complex environment, most of which will continue to be used in the next few years; At the same time, other wireless RF devices, such as digital video broadcasting and wireless LAN, will produce new signals that may interrupt communication services. Due to the increasing environmental constraints, many new services compete to occupy the limited cellular sites, which makes the cellular signal transmission tower full of various antennas. As we are more and more connected through mobile phones, watching multimedia performances on the Internet and conducting business trade, even soon our cars, refrigerators and electric ovens will use RF signals to communicate with each other, and the sky of communication will become more crowded.
1. Causes of RF interference
Most disturbances are unintentional and are a by-product of other normal operating activities. Jamming signals only affect the receiver, and even if they are physically close to the transmitter, the transmission will not be affected by it. Some of the most common interference sources are listed below to let you know where to start in the actual situation. It should be noted that most interference sources come from outside the base station, that is, outside your direct control range.
◆ incorrect transmitter configuration
Another service provider also sends signals on your frequency. In most cases, this is caused by failure or incorrect settings. The conflicting transmitter service chamber is more eager to correct this problem in order to restore its service
◆ unauthorized transmitter
In this case, other service providers deliberately transmit on the same frequency band as you, usually because they don’t get the license at all. He may not find a signal in a frequency band, so he assumes that no one is using the frequency band, so he uses it without authorization. Government agencies that issue permits often help to drive away such unlicensed operators.
◆ overlap of coverage area
The coverage area of your network or other networks exceeds the specified range on one or more channels. Incorrect antenna tilt, excessive transmission power or environmental changes can cause overlapping coverage areas, such as someone cutting down a forest or pushing down a building, which could have blocked signals from another location.
◆ self signal intermodulation
When two or more signals are mixed together, a new modulated signal will be formed, but it is not any desired signal. The most common intermodulation is the triple signal. For example, two signals with an interval of 1MHz will generate a new signal at 1MHz above the original high-frequency signal and 1MHz below the low-frequency signal. If the original two signals are in the frequency bands of 800 and 801mhz respectively, the triple signal will appear at 799 and 802mhz.
◆ intermodulation with another transmitter signal
Intermodulation interference may also be caused by one or more external wireless signals feeding the coaxial cable through the antenna and then entering the nonlinear terminal amplifier of the transmitter causing the conflict. The external signals are mixed with each other and mixed with the transmitter’s own signals to form a “new” frequency intermodulation signal in the communication frequency band (often undesirable).
It is also possible that interference signals are generated by two external signals, and the signals of the transmitter itself causing the conflict do not participate. The external signals are just mixed together by using the nonlinear 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 little difficult because it requires changes to the transmitter that appears to be working properly. It is necessary to add a narrow-band filter to attenuate the external signal as much as possible, and add a ferrite insulator to transmit RF from the transmitter to the antenna and attenuate the signal returned from the feeder. On transmission towers that use multiple different frequencies at the same time, the owner often requires all transmitters to install such filters and insulators.
◆ intermodulation caused by rusty walls/roofs, etc
Transmitters are not the only breeding ground for intermodulation signals, and nonlinear connections may also be nearby rusty white iron roofs or walls. When the wireless transmission power is large, the rusty parts between the roof parts will act as nonlinear diodes. Such intermodulation effects from physical structures are difficult to prevent because they vary according to weather conditions. The wind will press the rusty parts of the metal together or separate, and the rain will change the rust characteristics. Those seriously affecting communication must be repaired or replaced to restore reliable communication connection.
◆ intermodulation in antenna or connector
Sometimes even a small corrosion of the coaxial cable or antenna itself can cause problems. Although it is not enough to cause signal loss or VSWR problems, corrosion can cause subtle intermodulation 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 most difficult part to find out the root cause of such problems is that loosening a connector of the antenna system will disrupt the degree of oxidation and temporarily stop the problem. At this time, you must spend more time carefully recording which connector is loosened or tightened, and test after each step to determine whether it is the culprit.
◆ normal transmitter overload
Any strong frequency signal sent by the transmitter will overload the adjacent system. The only solution is to install a filter on the receiver antenna cable to pass the desired signal and attenuate the overload signal.
◆ adjacent channel power on the adjacent transmitter
As the allocated spectrum becomes more and more crowded, the frequencies allocated by competing for wireless services become closer and closer, which increases the risk of one system transmitting channel noise sideband appearing or preventing another adjacent receiving channel. If the transmitter meets the technical specifications, the channel needs to be changed or the physical separation between the transmitter and the receiver needs to be increased.
◆ broadcast transmitter harmonics
High power sources such as commercial radio stations will produce high-power signal harmonics. For example, a 5MW transmitter is easy to produce 5W harmonics, which is enough to interfere with nearby mobile communications. If the transmitter meets all specifications and government regulations, the only solution may be to relocate the communication antenna to avoid the transmitter, or 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 user
Before the advent of cellular systems, 900MHz and 1400 ~ 2200MHz bands were usually allocated to the studio and transmitter connection (STL) of broadcasting stations. The government has now redistributed these frequencies to cellular operators, but they often do not restrict old users and let them continue to operate on conflict free frequencies. When new cellular services are carried out in these frequency bands, those transmitters should switch to new frequencies, but some still need to be “reminded”.
◆ audio rectification
In very few places, the base station controller terminal still uses analog audio input to transmit to wireless output, so it will be affected by the strong signal of nearby AM broadcast or short wave radio station. The AM signal may enter the audio circuit and be rectified so that the broadcast audio signal is mixed in the telephone conversation. Good shielding around the audio part connected to the base station should solve this problem.
2. Knowing the type of interference source
Interference can be classified according to its own characteristics or its impact on the base station and mobile phone communication. Conflict frequency is the most commonly used indicator to display interference sources and interference results.
◆ out of frequency interference source
This is a major interference, including some strong signals similar to but different from the receiver frequency, which are strong enough to affect the input. These signals are usually very close to the predetermined frequency because the receiver input filter will filter out other signals that are too far apart.
Let’s take a look at the two effects on the receiver. One is front-end blocking, which is caused by the overload and complete saturation of the first stage (preamplifier or mixer) due to the strong signal entering the receiver, which will make the stronger signal unable to receive. Another effect is the desensitization effect. After entering the receiver, the nearby signal is found by AGC (automatic gain control) or the limiter circuit is started, resulting in the decrease of gain. The receiver behaves as if the sensitivity is reduced, so the weak signal will be lost and the signal-to-noise ratio of the strong signal will be reduced.
◆ in frequency interference source
The second type of interference includes a signal with the same frequency as the predetermined communication signal (regardless of strength), which is usually caused by the following conditions:
·The normal mobile phone signal exceeds its predetermined range
·Transmitter failure or improper configuration
·Signal harmonics of normal transmitter
·Unintentional interference signals radiated by other electrical devices
◆ in frequency influence caused by out of frequency interference source
This kind of interference source is the most difficult to track. It seems to be a signal within the frequency, but there is no obvious interference source, such as an intermodulation signal formed by mixing two or more perfectly normal signals at their own frequency in nonlinear elements.
◆ intentional interference
Malicious intentional interference is usually within the signal frequency, acting more like an improperly configured transmitter. We classify it separately because it usually has particularly elusive and harmful properties.
There is an example of intentional interference. Someone attacked a two-way wireless transponder system from a long-distance somewhere on the jungle mountain. At the beginning, the system receives a very weak signal on its input frequency (in which the correct audio decoding activates the repeater), which appears only at night. The signal remains in the air. Finally, it invalidates the repeater timeout relay and paralyzes the system until the signal disappears in the morning. The interference source is particularly difficult to find because the signal is too weak to find, and it is only transmitted at night. When it was finally found, it was found that the interference source was a micro transmitter with a small solar panel on the top of a tree near the repeater antenna pole. The transmitter was closed during the day, and its solar panel used this time to charge the battery.
◆ harmonics
The above still refers to the relatively clean original signal. In practice, there are fundamental frequency harmonics in the signal that is strong enough to produce interference. For example, the American VHF TV transmitter is required to install a filter to reduce its harmonics to at least 60dB below the main carrier. The most troublesome harmonic is the third harmonic because it is easily generated by small nonlinear elements in the transmitter. A 5MW TV signal transmitter working at 621.25mhz has a third harmonic of 1863.75mhz. Even if it is below 60dB (after filtering), the third harmonic still has 5W! This kind of frequency and power signal sent from the height overlooking the city can easily bring great damage to the cellular mobile communication signal of the whole city.
Another characteristic of the harmonic signal makes it more difficult to identify its source. The multiplication process of generating harmonics will change the spectrum, and its width and deviation will be multiplied by the same factor as the carrier frequency. For example, the 10th harmonic of a 13khz wide bidirectional wireless FM signal at 157.54mhz is 130KHz wide, the fundamental wave is only 5KHz offset, and it will become 50KHz at 1575.4mhz. If this transmitter shares a transmission tower with a base station, its 10th harmonic will completely cover the GPS receiver and paralyze the base station. For a 100W FM transmitter, a total attenuation of about 195db is required to avoid this interference, which can be realized by antenna isolation and filter suppression
