Why uplink freq.is always greater than downlink freq.?

[Bhanumurthy]

difference between uplink and downlink

Hi,
Kindly explain why uplink frequencies should be higher than downlink frequencies in satellite communications.For example, 6GHz and 4GHz are the uplink and downlink frequencies used in C-band.Similarly, 14GHz and 12GHz are the uplink and downlink frequencies used in Ku-band.
bye
Bhanumurthy.

 

[AdvaRes]

difference between uplink and downlink frequency

This question was already asked please search it.

 

[Bhanumurthy]

difference between downlink and uplink

Hi,
I've searched for that but the replies were not satisfactory.My question is abt the use of freq.in satcom.
bye
Bhanumurthy.

 

[jaywant007]

why uplink is greater than downlink

see the reason is that....

1) On the Earth Station we have to penetrate the atmosphere to reach the satellite station..so we need a lot of power to be generated...This is only possible with the help of High power transmitter circuits....which cannot be installed on the satellite....so uplink is at higher freq with higher power.

2)Now as the frequency is increased the amount of interference caused by the atmosphore reduces...as higher frequency signal has more signal energy in it....so it can penetrate the atmosphere more easily...(the same reason why we use modulation with higher frequency)

3) Unwanted distortion like rain (main cause of attenuation), fog, other frequency in neighbourhood, etc cause distortion....so we have to count to these factors and increase the frequency band to 100-1000Ghz range...

Now the Satellite is a small piece of equipment and needs to be light weight...so it cannot have high power amplifiers on board so by default the freq is lower than uplink...

i hope this is informative....if it is press thanks...Cheers...

 

[Bhanumurthy]

why is uplink frequency greater than downlink

Hi,
I appreciate the pains u've taken to answer my question.However, i have few doubts.Firstly, how freq. and power are related? Secondly, when the signal with high power that is uplinked reaches the satellite which is located at a height of say,36000km, the signal becomes very weak.So, even if u use a high freq.signal say 6GHz, power of the transmitted signal received at the satellite receiver input would be very weak.
Hence, we use a LNA or RF amplifier at the input to amplify the signal and then downcoversion is done from 6GHz to 4GHzThe final stage of the satellite transmitter before the signal is fed to the antenna will be a High Power Amplifier.Ofcourse, the powers used at the earth station and at the satellite will be different as u've rightly pointed out.
But, the reason for higher uplink freq. and lower downlink freq. in satcom still does'nt clear my doubt completely.
Thanx once again.
bye
Bhanumurthy

 

[jaywant007]

uplink frequency greater than downlink

ok let me try my best to explain....

See first u must know the difference between Signal Energy and AMplifier power gain...

Higher the frequency..higher is the signal energy which means that the signal can travel a greater distance without modulation...as we know energy is given as Energy dissipated when voltagef(t) is applied to 1ohm resistor and given by...
integral -∞ to ∞ [f(t)]^2 dt

and the power gain is provided by the amplifier sections of the earth station and has no relation to the signal power....
U can see this like this....Gain A1 * Gain A2*... (power gain)

And for the uncertainity u have i will tell u that it has been researched and then established uplink > downlink....for the reasons i mentioned earlier...

If u still have doubts u are welcome...

 

[Bhanumurthy]

uplink and downlink frequency in satellite

Hi,
Firstly,u've pointed out that power gain of an amplifier is not related to signal power.Can u kindly clarify how signal power is increased using high power transmitter circuits.Excuse me if i've asked a silly question...but i would like to get my concepts clear.
Secondly, in u'r first answer, u'r first point says that more power from earth station is needed to transmit the signal to the satellite since penetration through the atmosphere would be easy and to travel long distance.My question is "Is the same logic not applicable for the signal to be transmitted from satellite back to earth".
Remaining concepts are clear.Thanx once again.
Regards
Bhanumurthy.

 

[LvW]

downlink freq versus uplink frequency

Hi,
Kindly explain why uplink frequencies should be higher than downlink frequencies in satellite communications..

Your question implies that uplink frequencies always are higher than downlink frequencies. However, this is simply not true. There are several counter examples.
It is just a matter of international regulations and rules - of course with respect to physical properties of the medium in which communication takes place.

 

[jaywant007]

why uplink frequency is higher than downlink

ok one more try....

see i have given the mathematical formula for signal energy...signal power is given by signal energy/time period....

if u want clarification refer https://cnx.org/content/m10055/latest/

by this u should understand that....as the time period goes on decreasing or frequency goes on increasing the INHERENT signal power increases....

But in order to travel far distances we need to have MORE than INHERENT signal power so we need ARTIFICIAL boosting....i.e modulation with uplink frequency....

and for the second thing....u are correct there is a TRADE-OFF....meaning since we CANT boost the downlink so we use SUPER HIGH SENSITIVITY CIRCUITS on Earth station.....to compromise the loss associated with lower frequency....

i hope i have cleared your doubts....Cheers..!!
Dont forget the Helped Buttion....!!

 

[xulfee]

why is downlink greater than uplink

if u r taking it in sense of gsm system,it is all becasue of availability of power, bts has more power avaiable to transmit as compare to ur mobile system

 

[AdvaRes]

the difference between uplink and downlink

Hi,
I've searched for that but the replies were not satisfactory.My question is abt the use of freq.in satcom.
bye
Bhanumurthy.

Here is the link:

 

[farhantariq]

why is downlink frequency greater than uplink

Bhanumarthy Wrote:Kindly explain why uplink frequencies should be higher than downlink frequencies in satellite communications

Well I think the only reason that in uplink we need high frequency and more powerful signal is bcuz it is moving out Gravitational Fource.

 

[jaywant007]

dfference between downlink and uplink frequency

well farhantariq.....i have never ever come across this notion of electromagnetic signal being effected by Earth's Gravitational pull.....

Do u have any certain evidence of this phenomenon or u are bluffing....plz clearify

 

[LvW]

why uplink frequency is more than downlink freq.

I am forced to modify my reply from June 21st since I have learned something in between:

1.) It seems to be correct, that uplink frequencies are chosen higher than for the downlink, however, this is true only for communication satellite systems , where the receiving earth antenna normally has a much more lower gain the uplink central station.
The reason is the free space spreading loss as well as atmospheric/tropospheric losses, which rise with the frequency.

2.) For deep space missions this argument does not hold because in both cases (uplink, downlink) high gain antennas can be used on earth.

 

[errajeshgupta]

uplink frequency higher than downlink frequency

see we have two factors here
one we require the signal strength that penetrate the atmosphere and we also require narrow beamwidth to receive the signal properly at the satellite small sized antenna these both factors are of high frequency signal that is the reason we use high frequency signal

now power we require at the earth station is also veryy high than at the satellite beacuse the output of the satellite amplifier is fixed and we have to compensate the atmospheric losses at uplink and downlink so we need more power at the earth station as it can't be produced at the satellite. and the power requirement will be based on the atmospheric condition if the whether is good power require is less and if there is rainy season the power requirement is high.

 

[tabraiz]

uplink is greater than downlink

Dear Friend jaywant007
Plz explain your comments regarding this statement



"Just quote the expression which shows the relationship between the transmitted power and the frequency"