With the rapid development of computer and communication technologies, global information networks are rapidly evolving to IP-based next-generation networks (NGN). The future global personal multimedia communication broadband, mobile technology trends, coupled with flexibility, convenience of the market requirements, making seamless coverage, wireless connectivity goals are increasingly becoming a reality. At present, a variety of wireless technologies present a blossoming, a hundred technology competition situation, which accelerates the popularization of wireless applications at the same time, but also due to the wireless technology inherent frequency interference“ title=”Frequency Interference">frequency interference and the face of non-negligible problems.

RF interference signals to wireless communications base station coverage area of mobile communications brings many problems, such as phone drops, connection noise, channel loss, and poor quality of received voice, etc., and the interference caused by a variety of possible causes is growing at an alarming rate.

Today's newest and most advanced complex telecommunications technologies must also coexist with older mobile communications systems (such as dedicated wireless communications or paging, etc.) in a complex environment, most of which will continue to be used for years to come; at the same time, other wireless RF devices such as digital video broadcasting and wireless LANs generate new signals that can disrupt communications services. As environmental constraints grow, a multitude of new services are competing for the limited number of cellular sites, resulting in cellular towers filled with antennas of all kinds. And the communications sky will become even more crowded as we increasingly connect via cellular phones, watch multimedia shows and conduct business on the Internet, and soon even our cars, refrigerators and electric ovens will be using RF signals to communicate with each other.

Causes of RF interference

Most interference is unintentional and is simply a by-product of other normal operational activities. Interfering signals only affect the receiver, and even if they are physically close to the transmitter, the transmission is not affected by them. The following list of some of the most common sources of interference will give you an idea of where to start in a real-world situation. It is important to note that most sources of interference come from outside the base station, i.e., outside your direct control.

◆ Incorrect transmitter configuration

Another service provider is also transmitting on your frequency. In most cases this is due to a fault or incorrect setup, and the transmitter service provider that created the conflict will be more than eager to rectify the problem so that their service can be restored.

◆ Unauthorized transmitter

In this case, the other service provider is intentionally transmitting on the same band as you, usually because he simply doesn't have a license. He may have found no signal on a band and assumed that no one else was using it and took advantage of it without authorization. The government agency that issued the license usually helps to drive out such unlicensed operators.

◆ Overlapping coverage areas

The coverage area of your network or other networks exceeds the specified range on one or more channels. Coverage area overlap can be caused by incorrectly tilted antennas, excessive transmit power, or changes in the environment, such as when someone cuts down a grove of trees or bulldozes a building that would otherwise block signals emitted from another location.

◆ Self-signal intermodulation

Two or more signals mixed together form a newly modulated signal that is not any of the desired signals. The most common intermodulation is a triple signal. For example, two signals spaced 1MHz apart will each produce a new signal 1MHz above the original high frequency signal and 1MHz below the low frequency signal, and if the original two signals are in the 800 and 801MHz bands, respectively, a triple signal will appear at 799 and 802MHz.

◆ Intermodulation with another transmitter signal

Intermodulation interference can also be caused by one or more external wireless signals feeding the coaxial cable through the antenna and then entering the conflicting transmitter's nonlinear termination amplifier, where the foreign signals mix with each other and with the transmitter's own signals, creating what appears to be a “new” frequency intermodulation signal in the communications band (often unwanted). (often unwanted).

It is also possible for two external signals to create an interfering signal where the conflicting transmitter's own signal is not participating, and the external signals just happen to be used in the transmitter's nonlinear stage and are mixed together. 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 of a challenge because it requires changes to the transmitter which appears to be working fine. A narrow-band filter needs to be added to attenuate as much of the outside signal as possible, and a ferrite insulator needs to be added to allow the RF to travel from the transmitter to the antenna and to attenuate the signal returning on the feedline. On towers where several different frequencies are used at the same time, owners often require these filters and insulators to be installed on all transmitters.

◆ Intermodulation caused by rusty fences/roofs, etc.

Transmitters are not the only breeding ground for intermodulation signals; non-linear connections can also be a nearby rusty white tin roof or fence. When wireless transmitter power is high, the rusted parts of the roof act as non-linear diodes between the various parts of the roof. Intermodulation effects such as these from the physical structure are difficult to prevent as they vary depending on the weather conditions; wind can push the rusted parts of the metal together or apart, and rain can change the properties of the rust. Those that seriously affect communications must be repaired or replaced to restore a reliable communications connection.

◆ Intermodulation in the antenna or connector

Sometimes even a tiny bit of corrosion in the coaxial cable or antenna itself can create problems, although not enough to cause signal loss or VSWR problems, but the corrosion can cause subtle intermodulation like a very poor quality diode. If there are several high-powered transmitters in the vicinity, the resulting intermodulation can be strong enough to interfere with the weak communication signal between the cell phone and the base station. The hardest part of finding the source of this type of problem is that loosening a connector on the antenna system can upset the oxidation level and temporarily abort the problem, so you must spend more time carefully noting which connector you loosened or tightened and testing it after each step to determine if it is the culprit.

◆ Regular Transmitter Overload

Strong signals at any frequency from a transmitter can overload neighboring systems. The only solution is to install a filter on the receiver antenna cable that allows the desired signal to pass through and attenuates the overloaded signal.

◆ Adjacent Channel Power on Neighboring Transmitters

As allocated spectrum becomes more crowded, competing wireless services are allocated frequencies closer together, increasing the risk that one system's transmit channel noise sidebands will appear on or block another adjacent receive channel. If the transmitter meets the specifications, channel changes or additional physical separation between the transmitter and receiver are required.

◆ Broadcast Transmitter Harmonics

High power sources such as commercial broadcast stations can generate high power signal harmonics, for example a 5MW transmitter can easily generate 5W harmonics, enough to interfere with nearby mobile communications. If the transmitter complies with all codes and government regulations, the only solution may be to relocate the communications antenna to avoid the transmitter, or to reallocate the frequency scheme so that the communications base station near the conflicting transmitter is using a channel that is not affected by its harmonic energy.

◆ “Vintage” STL Users

Before the advent of cellular systems, the 900 MHz and 1,400-2,200 MHz bands were typically allocated for studio-to-transmitter links (STL) for broadcasting stations. Governments have now reallocated these frequencies to cellular operators, but again they often fail to restrict older users and allow them to continue to operate on frequencies where there is no conflict. Those transmitters should be shifted to the new frequencies when new cellular services are launched in these bands, but some need to be “alerted”.

◆ Audio Rectification

In a few places, the base station controller is still using analog audio inputs to transmit to the wireless outputs, and is therefore subject to strong signals from nearby AM broadcast or shortwave radio stations.The AM signals may enter the audio circuitry and be rectified, allowing broadcast audio signals to be mixed in with telephone conversations. Good shielding around the audio portion of the connection to the base station should solve this problem.

Recognizing Types of Interference Sources

Interference can be categorized by its own characteristics or by its effect on base station and cell phone communications, with the conflict frequency being the most common indicator of the source and result of the interference.

◆ Out-of-frequency interference sources

This is a major type of interference that consists of a number of strong signals of similar but not identical frequency to that of the receiver, strong enough to affect the input. These signals are usually very close to the intended frequency because the receiver input filter will filter out other signals that are too far away.

Let's look at two types of effects on the receiver. One is front-end blocking, which is caused by strong signals entering the receiver overloading and completely saturating the first stage (preamplifier or mixer), which prevents stronger signals from being received. The other effect is a detuning effect, where a nearby signal enters the receiver and is detected by the AGC (Automatic Gain Control) or activates the limiter circuitry, causing the gain to drop. The receiver behaves as if its sensitivity is reduced, so weak signals are lost and the signal-to-noise ratio for strong signals will be reduced.

◆ Intra-Frequency Interference Sources

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

-Normal cellular phone signals outside their intended range

-Transmitter malfunction or misconfiguration

-Signal harmonics from normal transmitters

-Unintentional interference signals radiated from other electrical devices

◆Intra-frequency effects from out-of-frequency interference sources

This type of interference source is the most difficult to track and appears to be an in-frequency signal with no apparent source of interference, such as an intermodulation signal formed when two or more signals that are perfectly normal on their own frequency are mixed within a nonlinear element.

◆ Intentional interference

Unsuspecting intentional interference is usually within the signal frequency and behaves more like a misconfigured transmitter. We categorize it separately because it usually has particularly elusive and harmful characteristics.

In one such example of intentional jamming, someone attacked a two-way wireless transponder system from a distance somewhere on a jungle hill. The system initially received a very weak signal on its input frequency (in which the correct audio decoding activated the transponder), which appeared only during the night, and which stayed in the air, eventually deactivating the transponder's time-out relay and paralyzing the system until the signal disappeared in the morning. The source of the interference was particularly difficult to locate because the signal was too weak to be detected and it was only transmitting at night. When finally located, the source was found to be a miniature transmitter with a small solar panel on top of a tree near the transponder mast, which was turned off during the daytime and used by the solar panel to recharge the batteries.

◆ Harmonics

While the above still refers to the relatively clean original signal, in reality there are fundamental harmonics in the signal that are strong enough to cause interference, such as the US VHF TV transmitter, which requires a filter to reduce the harmonics to at least 60dB below the main carrier. The most troublesome harmonic is the third harmonic, as it is easily generated by small nonlinear components in the transmitter. A 5MW TV signal transmitter operating at 621.25MHz has a third harmonic of 1863.75MHz, and even below 60dB (after filtering) the third harmonic is still 5W! A signal of this frequency and power from a high point overlooking a city can easily wreak havoc with cellular cellular signals throughout the city.

Harmonic signals have another property that makes it even more difficult to recognize their source. The multiplication process that creates harmonics changes the spectrogram by multiplying its width and deviation by the same factor as the carrier frequency. For example, a two-way radio FM signal that is 13kHz wide at 157.54MHz has a 10th harmonic that is 130kHz wide, and a fundamental with only a 5kHz offset becomes 50kHz at the harmonic frequency of 1575.4MHz. if such a transmitter were to share a tower with a base station, the 10th harmonic would completely override the GPS receiver, paralyzing the base station. For a 100W FM transmitter, a total attenuation of about 195dB is required to avoid this interference, which is achieved with antenna isolation and filter suppression.

Conclusion of this paper

We have discussed the causes of common RF interference generation in mobile communication systems and suggested some troubleshooting methods. With a better understanding, engineers will be better able to apply new interference measurement tools to recognize and track interference sources.