Analysis of The Current Status of Low-Orbit Satellites (LEO)
As we know that satellite communication system is kind of open system and the main usage is to transmit signal. It means that satellite communication system is easier to be interfered by some outside factors. Let us show you more details of causes and solutions about satellite communication interference.
satellite communication system
In general, common interference include polarization interference, power interference, forwarding interference, spurious interference, FM interference, frequency sweep interference, intermodulation interference, and the use of frequency bands. Today, we will focus on the first three interference types, including polarization interference, power interference and forwarding interference.
Before introducing these three interference methods, let us first discuss the possible causes of interference. Based on our years of experience, there are 4 main reasons as follows.
1.Why does interference occur?
- Uplink station.
The uplink signal is transmitted to the satellite, which is called the uplink station. The uplink station itself carries interference (the frequency of two or more uplink stations is the same, the signal transmitted by the satellite will be interfered), and the worst is artificial embezzlement.
- The transmission path of the signal.
Radio waves transmit up to 36,000km and down to 36,000km, and the distance is long. On the way, other signals of the same frequency band are inserted in series, causing interference to the link. The most common problem is adjacent satellite interference.
- The satellite itself.
For example: the equipment on the satellite fails; the satellite is subjected to electromagnetic interference in space; other satellites are too close to the satellite orbit.
- Downlink station
The equipment of the receiving station is faulty, or the surrounding environmental factors are in series with the receiving signal (equipment), causing interference.
2.Interference introduction and its solutions
2.1 Polarization interference
What is polarization interference? To understand polarization interference, we must first know what polarization means?
The electric field direction of the electric wave is horizontally polarized wave, and the electric field direction of the electric wave is perpendicular to the ground.
The same frequency can use horizontal and vertical polarization at the same time without interfering with each other. Therefore, in satellite communication, satellite companies can use polarization for reuse. When designing satellites, dual-polarization design is adopted to double the satellite capacity.
The polarization method in satellite communication depends on the polarization of the antenna. Signals in a satellite communication system require at least three antennas: an uplink station antenna, an antenna on the satellite, and a receiving station antenna. (If there are two antennas for sending and receiving on the satellite, it is at least four antennas.) The polarization of the three antennas is matched to achieve the effect of co-frequency multiplexing without affecting each other.
For satellites, when designing a satellite, specific index requirements are put forward for the antenna on the satellite. The polarization isolation in the coverage area is more than 30dB, and the main service area reaches 40dB. After the satellite is launched, the attitude basically remains unchanged and maintained in a good state for a long time.
For the uplink station and the receiving station, since the earth is a sphere, some changes will occur in the horizontal and vertical directions of the ground, and the angle of the antennas at different locations must be adjusted to match the satellite.
- Uplink Polarization Interference
The polarization of the uplink antenna can not meet the requirement, or the polarization angle does not match the satellite. For example: if you use horizontal polarization, because the polarization angle is not adjusted properly, part of the signal leaks to the vertical polarization, which will cause interference with the vertical polarization of the same frequency, as shown in the figure below.
This spectrum shows that the polarization angle is not adjusted properly
To avoid uplink polarization interference and use the satellite in compliance, please contact the satellite company, designate the frequency band to send a single carrier, guide the rotation azimuth, elevation and polarization angle, and record the angle. If the location changes, contact the satellite company to re-calibrate the polarization access network, see the following picture.
Spectrum when adjusting polarization of a transmitted single carrier
- Downlink polarization interference
The polarization of the receiving antenna is not up to standard, and the main polarization and anti-polarization signals are received at the same time, and the same frequency interferes with each other.
For example, it was intended to receive the horizontal polarization signal forwarded by the satellite, but the polarization Angle of the receiving antenna was not properly adjusted. When it received the horizontal polarization signal, it received the normal vertical polarization signal, which resulted in the interference between the received signals and could not be used.
To avoid downlink interference is mainly to adjust the polarization of the downlink station. (Specific method: use the spectrum instrument and other equipment to directly connect the polarization of the receiving antenna, to receive a beacon corresponding to the satellite polarization, rotate the polarization Angle, the beacon energy modulation is maximum. If conditions are available, connect the receiving antenna to another polarization, the receiving satellite should be relative to the polarization beacon, rotate the polarization Angle, and adjust the beacon energy to the lowest; If you do not have a spectrum analyzer and other equipment, use demodulator, beacon machine and other equipment to ensure the best state of receiving signals. If the antenna has uplink capability, you can also contact the satellite company to calibrate the polarization)
Polarization interference can also cause interference over time, and indicators need to be re-measured if the earth station is moved. Long time not to the satellite, pointing error is too large. Polarizer, duplexer shifted, out of position, damage to the feed, feed filling with other substances can cause interference as well.
2.2 Uplink station power exceeding the standard interference
Non-compliant use, or equipment failure. Power calibration is also required for the first use of the service to meet power bandwidth balance and link communication requirements, and record and maintain the service for a long time.
If there is a problem and you want to greatly increase the power, you should contact satellite company in advance, check and evaluate whether it can be increased and to what extent the power can be increased. If the user does not contact Satcom company for power calibration after the service is opened, decides the power level without authorization when using it, or increases the power without consultation with the satellite company when he finds that the quality of his service signal is declining, or if the user uses the same power amplifier to uplink multiple carriers, the backoff is insufficient. Or the uplink power is abnormally increased due to device faults.
Excessive power will not only affect the quality of its own service signal, but also affect the same transponder or other services. In addition, it will cause hidden dangers to the security of the satellite transponder.
2.3 Forwarding interference
It affects one or several forwarders in a wide range
Cause: The user uplink station 70mhz/140mhz/ L-band intermediate frequency equipment connector, wrong connection, cable damage, poor contact, received signal into the transmitter, repeated forwarding, forwarding signal superposition caused interference.
When spectrum is not easy to discover, communication quality is degraded or interrupted, you need to notice forwarding interference.
Spectrum of forwarding interference
3.Q&A about satellite communication interference
Question 1: Do I still need to adjust the polarization angle if I use circular polarization?
Circular polarization is divided to LHCP and RHCP. For circular polarization, there is no need to adjust the polarization. There will be a circular polarizer at the antenna end. The polarization index is mainly determined by the circular polarization converter. No matter how you adjust it, it won’t change. There is no polarization interference problem for circular pol, and if there is a problem, return it to the factory for repair and replacement.
Question 2: Based on your business experience, what are the sources of interference discovered by analyzing the spectrum?
The forwarding interference is difficult to find in the spectrum, because it is an interference signal hidden under the normal carrier, which is lower than the normal power and is on the same frequency, so it is difficult to find it through the spectrum. Some interference are easier to find. For example, the use of overpower can be seen at a glance through the spectrum of which carriers are obviously overpowered; or polarized interference, for the satellite company, the satellite operator, the satellite company’s monitoring system can simultaneously receive the positive and negative polarization from the satellite. If the satellite company put them on the same spectrum at the same time, it will directly find out whether there is any polarization leakage. These are the interference that can be found directly through the spectrum. But for the interference source problem mentioned, the frequency spectrum cannot find the interference source. The interference source is the uplink station, that is, the location of the interference needs to be checked by the satellite company. For example, if anti-polarization interference is found, the satellite company will use this frequency based on the spectrum resource management and which users will use it. Then the satellite company will contact the user to adjust the polarization for investigation. Including forwarding interference, as well as over-power use, satellite companies can find interference, but they still need to contact the user to determine the source of the interference, and then find the uplink station.
Question 3: If uplink polarization had been adjusted well, does the receiving polarization still need to be adjusted?
In general, linear polarization uplink and downlink are reversed, and the same on satellites, that is, the uplink is horizontal and the downlink is vertical. Normally, because it is a 90-degree angle, after adjusting the uplink polarization, the receiving polarization is relatively good, and there is no need to adjust it. It may be affected by equipment problems. Generally speaking After adjusting the uplink polarization, there is no need to adjust the downlink polarization. Circular polarization does not need to be adjusted, nor can it be adjusted.
Question 4: If we just receive TV signal, can we adjust the polarization by ourselves?
Yes, the receiving station is only for receiving and does not have the ability to use uplink. It only receives signals from satellites and mainly relies on its own receiving antenna. Antenna works as long as polarization is adjusted well, but the adjustment of polarization mainly depends on your own conditions. If there is a spectrum analyzer, if it can receive two polarized signals, it is the best, if not, you can use the receiver, or beacon, etc. Adjust the beacon to the maximum.
Question 5: How to quickly identify the existence of 5G interference signals in the spectrum?
First of all, the 5G interference that can be received is all C-band.
China uses part of the shared C-band as a resource for terrestrial 5G, which has a 100M guard band. 5G interference has been distinguished before, including saturation interference, co-channel interference and adjacent-channel interference. How to identify 5G interference in the spectrum? If your receiving antenna system has the ability to receive the extension frequency, you can directly set the frequency to the extension section, that is, the frequency of the ground 5G, and you can see the 5G interference, because it is a different frequency after all, if you are using a standard frequency band, and then you are interfered by 5G, which is basically caused by saturated interference. Now, many methods have been adopted, such as installing filters, shielding network, and shielding this 5G signal. If you use this This method is generally not subject to 5G interference. If you want to identify it, just set frequency to extended C band and see. This is the interference problem of your ground receiving side, so you contacted the satellite company. The satellite company will monitor from different monitoring stations. The geographic location of your receiving station and the satellite company’s receiving station is inconsistent. And from the comparison of the signals received by the satellite company , the satellite company can help you eliminate the interference problem caused by the satellite. It is obviously that there is a problem with your receiving system, or the interference problem near the ground station. As for whether the interference is 5G, you can only use the method i mentioned to judge. Whether it is the 100M guard band or the same frequency signal as 5G, if there is problem of 5G interference.
Question 6: Are there more interference problems in the C-band or more in the Ku-band?
C and Ku are different frequency bands designed for satellites, so the frequency bands used by users are also different. Regarding the technical characteristics of these frequency bands, it can be concluded from the operating experience of satellite companies for so many years that the interference of Ku is slightly more than C. For The main reasons, satellite companies have also done some simple analysis. The frequency of the C-band is low and the beam is wide. The requirements for the user’s uplink station are relatively high. Larger diameter antenna is needed, the users are mainly fixed stations, and the number of users is not that large. The possibility of interference is much lower than Ku. On the contrary, for Ku users, the Ku-band frequency is high and the beam is narrow. Most uplink stations are small diameters. Even now, many mobile and portable stations can be as uplink from a few tenths of a meter, and the number of the user group is very large, so there are more possible interference factors.
Secondly, many of C band services are radio and television services, most of which are receive-only, so the uplink station is easy to manage, and the possibility of interference is relatively small.
Question 7: I have an antenna that I want to connect to the network, how to adjust the polarization, and what is the process?
To establish the service, you need to contact the satellite company for polarization adjustment and power calibration. As for how to adjust the polarization angle, the earth is a sphere. According to the angle at which the satellite irradiates the earth, this polarization angle is generated only after the horizontal and vertical angles to the ground have changed. This polarization angle can actually be calculated according to the formula, if you are interested, you can discuss with us later. The calculated angle is used as a reference. When the actual adjustment is made, duplexer needs to be rotated. If the duplexer and the frequency converter are together, LNB needs to be rotated . If there is equipment, you can intuitively see that the rotation is actually the horizontal and vertical angles, which is actually relatively simple.
Question 8: What should we do if we encounter interference?
First of all, if the user finds the problem, don’t worry for the first time. If you have quality requirements for the business, you can immediately increase the power, but you cannot greatly increase the power. If you want to greatly increase the power, you must first follow the instructions: Contact the satellite company. If your business is more important and urgent, you can slightly increase the power by 1-2dB and there is no problem in a short time. If it is not the problem of forwarding interference, you can ensure the normal use of your business. Next, you need to contact the satellite company, and the satellite company will judge the type of interference based on the satellite company’s spectrum monitoring and a series of other means. If it is judged that it is interference from reverse polarization, the satellite company will contact the reverse polarization user. If the satellite company judges that it is caused by other users’ power exceeding the standard, the satellite company will contact them to drop it. If the satellite company judges that it is interference from a neighboring satellite, the satellite company will find out which satellite is, and go to the country’s operator responsible for the satellite to file a complaint and ask them to adjust. This is a follow-up series of disposals. If the satellite company judges that the interference signal is caused by yourself, because there is no such interference signal on the satellite company’s satellite, and it is not transmitted through the satellite company’s satellite, it is a problem of your own system, then you need to go carefully. Check a status of your upstream system or receiving system. At this time, there are certain requirements for your technical ability, including this integrated device. I believe that all users can find out very well and quickly, and cooperate with the satellite company to solve this interference.
Question 9: There is a free band resource in the process of using resources. How can I prevent others from misappropriating this resource, and how can I ensure that my resource is vacant and no one will use it arbitrarily?
The idle frequency band seen may not be really idle, because the satellite company may have already allocated it to the user, but it may be that the user has not used it at this point in time. If all users of the satellite company use it in compliance with the regulations and apply for the frequency band allocated to you by the satellite company, this frequency can be well guaranteed to avoid such interference problems, such as wrong sending or stealing. . This is one of the most important means of securing this frequency.
Secondly, if this frequency is very important to the customer, and the customer needs the satellite company to guarantee this frequency, then the satellite company itself has the capability of this uplink signal and the frequency occupancy system, and the satellite company can use the satellite company system or uplink. The ability of the signal helps the user to occupy the idle frequency in advance when he has not yet used the service. This prevents other users from temporarily using it, sending it wrong, or stealing it. When the user who rents the frequency of the satellite company in compliance with the law really wants to use this frequency, the satellite company will turn off the signal.
Finally, satellite companies can locate and investigate those who illegally steal signals to promote legal and compliant use by users.
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