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Brief analysis of satellite communication using high frequency

Author: Shenzhen Science and technology time: September 23, 2019 source: egibl.com
Abstract: in general, satellite communication uses high frequency. What is the reason? We will take you to have a brief understanding today. FM broadcasting frequency is 88 ~ 108MHz, covering hundreds of kilometers to thousands of kilometers. ...

Satellite communication generally uses high frequency. What is the reason? Today, we will take you to have a brief understanding.

FM broadcasting frequency is 88 ~ 108MHz, covering hundreds of kilometers to thousands of kilometers.

 01-fm communication.jpeg

The frequency range of the cellular network is from 3MHz to several tens of kilometers.

 02 - cellular communication.jpeg

The communication frequency of WiFi network is 2.4 ~ 2.5GHz and 5.2 ~ 5.8GHz, covering a range of several meters to hundreds of meters.

 03-wifi communication.jpeg

The satellite communication frequency is 5GHz ~ 40GHz, and the communication distance is 36000km.

 Jpeg.04

As can be seen from the above pictures, the lower the frequency of electromagnetic waves, the farther they can travel freely in the air. So why are satellites so far away from the ground that they don't use low frequency communication? Why can the communication frequency as high as 5GHz ~ 40GHz transmit such a long distance (36000km)?

To explain this question, we need to understand five aspects of knowledge.

1. The earth's atmospheric ionosphere

The ionosphere is an ionized region of the earth's atmosphere. The ionosphere extends from about 50 km above the ground to about 1000 km in the upper atmosphere of the earth. There are quite a lot of free electrons and ions in the ionosphere, which can change the propagation speed of radio waves, produce refraction, reflection and scattering, produce the rotation of polarization surface and be absorbed in different degrees.

 05 ionosphere.jpeg

Long wave (b) and medium wave (c) are directly absorbed by the ionosphere and can only travel through the ground wave;

Short wave (d) can be reflected by the ionosphere and can only propagate in the atmosphere below the ionosphere;

Microwave (a) is not affected by the ionosphere and can penetrate the ionosphere directly.

Therefore, the satellite communication must use the signal above the microwave frequency band to realize the satellite ground communication.

2. Free space attenuation of electromagnetic wave

Electromagnetic wave will have loss when penetrating any medium, so when electromagnetic wave propagates in the atmosphere, there will be loss as well.

 06 - Electromagnetic wave.jpeg

The transmission loss of electromagnetic wave in atmospheric free space is directly proportional to the frequency and transmission distance of electromagnetic wave

 07 - electromagnetic wave calculation formula.jpeg

Seeing this, I guess you must be thinking: look, the higher the frequency, the greater the space loss. My understanding is correct. But why do satellites use frequencies above 5GHz? Don't worry, let's go on. The third knowledge point will answer your doubts.

3. Calculation of received power link in wireless communication

As we all know, a complete wireless communication system includes transmitter, transmitting antenna, transmitting medium, receiving antenna and receiver. Then, how much useful signals the receiver can finally receive depends on the four units before it.

 09 wireless communication link.jpeg

The link calculation formula of wireless communication receiving power is listed as follows:

 10 - wireless communication receiving power calculation formula.jpeg

Seeing this, you may have the answer in mind. Yes, as long as the antenna gain is increased, it is possible to make up for the loss of free space. But how?

4. High gain antenna

As for the knowledge of antenna, we don't give too much explanation here. We focus on how to achieve high gain of antenna. As we all know, radio and television need to do one to many communication. The antenna is usually built on high mountains or high buildings, in order to cover more users. So this kind of antenna is omni-directional antenna, they emit electromagnetic waves in all directions, the wider the electromagnetic wave is dispersed, the faster the energy consumption is.

What if these scattered electromagnetic waves are concentrated and emitted in one direction?

Yes, if the antenna can only transmit in one direction, the antenna gain will be greatly improved. This is the origin of directional high gain antenna.

This paper introduces a classical directional high gain antenna, Cassegrain antenna, also known as parabolic antenna

 Parabolic antenna.jpg

The empirical formula is given directly

 Calculation formula for 11-parabolic antenna.jpeg

It can be concluded from this formula that:

For the same size parabolic antenna, when the working frequency is doubled, the antenna gain will increase by 6dB!


Interested spectators can calculate by themselves, isn't this truth?

Now, let's do a simple calculation


The conditions are given first

Communication distance:

The geostationary orbit is 35786km,

Round d = 36000km communication frequency:

Size of 3 GHz and 30 GHz transmitting and receiving antennas:

They are all parabolic antennas with a diameter of 10 m

Transmitting power:

100W=50dBm

And then we start to calculate

01. The free space attenuation is calculated first

·Because it's a GHz frequency

·So put it in the formula

L = 92.4 + 20lg (f) + 20lg (d) · set the distance between satellite and ground d = 36000km

L3=92.4 +9.5+91.1=193

·30 GHz spatial attenuation:

L30=92.4+29.5+91.1=213

02. Calculate the gain of transmitting and receiving antennas

·The formula of paraboloid antenna is DBI

·The results are as follows

·The wavelength of 3GHz is 0.1M

·Then the antenna gain is:

Gt3

= Gr3

= 10 lg [4.5 x (10/0.1)2]

= 46.5

·The wavelength is 30 GHz

·Then the antenna gain is:

Gt30

= Gr30

= 10 lg [4.5 x (10/0.01)2]

= 66.54

03. Calculate the actual link power

·Actual power received by receiver:

Substituting PR = Pt + gt-l + GR,

·Transmitting power

It can be calculated as follows: PW · 50dbw = 100dbw

·Signal power received by 3GHz receiver:

Pr3

= 50dBm + 46.5 - 193 + 46.5

=Signal power received by - 50dbm · 30ghz receiver:

Pr30

= 50dBm + 66.5 - 213 + 66.5

= -30dBm


The results are clear

The higher the frequency, the higher the efficiency of the wireless link, regardless of weather factors (such as rain, cloudy days, cloud thickness)!


5. High frequency, wide bandwidth, high speed

Satellites need batteries to work in space, and battery energy is limited. Therefore, shortening the communication time is a good way to save battery energy. Under the condition that the amount of information to be transmitted is constant, the transmission rate must be increased in order to shorten the communication time. So how to improve the transmission rate? Grandfather Shannon showed us a clear way Increasing the communication bandwidth can improve the information transmission rate.

Big bandwidth brings more information

On the earth, the radio frequency band below 5g has been occupied by various communication equipment. There is no way to spare a complete large bandwidth for satellite communication, and there are relatively complete spectrum resources above 5g. Therefore, satellite communication chooses above 5g to facilitate data transmission.

 12 - transmit more data with large bandwidth.jpeg

Conclusion: OK! I don't know. I don't know how long it is


In a word, we can conclude that there are several reasons why satellites operate at frequencies above 5GHz

It can pass through the ionosphere smoothly without being reflected or absorbed

The high-frequency transmission of antenna will not bring high power loss.

High frequency means large bandwidth. According to Shannon formula, the wider the bandwidth is, the higher the information transmission rate is, which is helpful for saving electric energy and improving the service life of satellites.

There are relatively complete spectrum resources above 5g, so it is not easy to be interfered.



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