Introduction and solutions about interference problems in satellite communication

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.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
Signal transmission route
The satellite itself.
Downlink station
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.

polarization angle

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.

spectrum shows that the polarization angle is not adjusted properly

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.