Free space propagation
Digital communications
This module discusses the basics of information transmission
Free space propagation6:37
Limited bandwidth3:00
Noise5:26
Conclusion2:19
-In a satellite transmission,
the physical link between the transmitter and the receiver,
what is called the propagation or transmission channel,
is simply the free space between the receiving or transmitting antenna
and the satellite.
For a transmission in free space,
we use carrier waves called radio or Hertzian waves.
Those are frequency carrier waves below 3 000 GHz.
This radio wave emitted by the antenna
is a vibration propagating through space.
The farther we go from the antenna, the more attenuated the wave is
since there are gas molecules in the atmosphere
such as oxygen or water vapor.
They will absorb the wave and thus weaken it.
This attenuation, this absorption,
will be more important when it is raining.
Raindrops will absorb and even spread the emitted signal.
The weakening, the attenuation induced
depends on the size of the raindrops
and on the wavelength of the emitted radio wave.
It is said that for absorption to occur,
the size of the raindrop
must be twice the wavelength of the radio wave.
I do not know if you know the wavelength notion,
but I assume that you have heard of it.
In the past, when we were broadcasting radio,
we did not talk about frequencies, but wavelengths.
We talked about short, medium and long waves.
Of course, waves and frequencies are linked.
They are inversely proportional.
When the wavelength decreases, the frequency increases.
When a wavelength is short, the signal is more sensitive to precipitations
since it is more sensitive to smaller raindrops.
Since wavelengths and frequencies are inversely proportional,
when the frequency is high,
the signal is more sensitive to precipitations.
To sum it up, in the case of free space propagation,
the useful signal is absorbed by gases in the atmosphere.
This absorption is even more important when it is raining
and increases depending on the carrier frequency of the signal.
We can see here the effect of rain
on a television signal transmitted via satellite.
You can see small squares appearing on the image
because we lose the signal sometimes.
We thus get this pixelization effect on the image.
If you receive satellite television at home, you must have already seen
a short message appearing when it is strongly raining
or during storms such as "no signal" or "signal level too low".
This happens because, in that case,
the signal is so weakened by the storm or the rain
that it reaches the receiver with too little power or energy
to allow us to retrieve the original binary information
from the received signal.
Of course, in the case of free space transmission,
the frequency space must be regulated.
Anyone cannot be allowed to transmit in any frequency band anytime.
Receivers would receive completely jammed signals.
So the frequency space must be regulated.
Satellite systems are global systems, so this regulation must be global.
An institution is in charge of this,
the International Telecommunication Union.
It regulates the frequency space. Regarding this topic,
I invite you to listen to Mohamed Benseddik's interview
who works at Thales Alenia Space in the frequency regulation field.
This is currently a crucial stake because more and more systems
want to emit in an ever busier frequency space,
so this space must absolutely be regulated.
Regarding satellite transmission,
there are mainly four reserved frequency spaces.
We talk about L, C, Ku and Ka bands.
As we previously saw, depending on the frequency of the carrier wave,
the attenuation will be more or less important,
especially in the case of rain. We see here that in the Ka band
carrier frequencies are higher than in the L or C bands.
The Ka band is thus more sensitive to precipitations than L or C bands.
The C band is used for satellite television broadcasting
in tropical zones since rains are heavy there,
so we need a robust transmission in the face of these precipitations.
The most used of these four bands is the Ku band
for satellite television, radio or computer data broadcasting.
Its benefit is that it allows
Since it is used a lot, it is very busy.
So when we need to insert new systems in the frequency space,
we have to find room somewhere else.
For example, there is room in the Ka band.
The disadvantage of the Ka band
is that it is more sensitive to precipitations
since the frequency is higher,
but it allows working with smaller reception antennas.
You will see during week 4
that the size of the antenna used on the receiver side
is inversely proportional to frequency.
So the Ka band allows us to have available frequency space
and reasonably-sized antennas.
It will allow giving Internet to white or gray zones.
These are zones that are not covered by ADSL.
You will see in this MOOC the example of a satellite Internet system
working in the Ka band.
You will have a demonstration regarding this topic.
The L band is rather used for mobile communications
with boats, planes, cars.