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Satellite is at 6/4GHz and bandwidth 72MHz. Bellow is the earth station. enter image description here I have Pt = 200 Watt , Gt=30dB (gain satellite) ,Gr=42dB, Lm=6.7dB( loss other than free space loss) and distance is d=35870km . Earth station temperature is 20 Celsius.

Pt is transmit power 
Gt is gain transmit (satelitte antenna)
Gr is earth antenna gain
L  loss

S/N of threshold is 15dB after amplifier and i need to calculate the gain of the LNA.
I can calculate Pr receive power but then i am not sure what to do. I cant calculate Teff(effective system temperature) as i dont have noise temperature of transmition line or filter. Also i dont know what has earth station temperature has to do with the system. S/N or C/N should be the same as i know but this really confuses me. All formulas calculate system temperature but i havent found how to calculate when it is between.

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Too many undefined variables, acronyms and other assumed context. – Olin Lathrop Sep 16 '12 at 23:10
@OlinLathrop, i tried to fix it. What other is assumed ?These are all the data – Parhs Sep 16 '12 at 23:16
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What do you mean by "S/N ratio is 15dB"? Are you saying your system can only tolerate that ratio or is that the SNR that your system is receiving? – Kellenjb Sep 17 '12 at 2:33
S/N of threshold is 15db after the amplifier.. – Parhs Sep 21 '12 at 19:05

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For finding LNA specification two calculation should be done. First calculation should be done by considering attenuators and amplifiers and second should be done by considering noise temperature. Then results of these two calculations should be compared and the maximum sensitivity will be selected. So first you should know signal sensitivity requirement at receiver. Then find the gain of LNA. Third by considering noise figure of other components find noise figure of LNA.You can see this link. Also if you can find receiver sensitivity and noise specification of other components i could calculate your LNA gain and noise figure.

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i am sorry but i am confused. I have figured out a solution but i am not sure if it correct. What do you mean considering attenuators-amplifier /noise temperature? Using cascade equation after antenna ?About Signal sensitivity i am not sure how to apply it here but i can calculate Received power .(the link is useful but doesnt mention about sensitivity).All i know is S/N=15db , so S=Received Power? and N= k * Teff * B (B is bandwidth,k bolzman)( maybe thats the sensitivy equation) – Parhs Sep 17 '12 at 9:57
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Without receiver sensitivity you could not find LNA gain. For example some mobile equipments have sensitivity of -110 dbm. This means if power at receiver is below this, signal will be lost. So received signal should be higher about 10 dBor 15 dB which they are used as margin. If receiver's sensitivity is -80 dBm, received signal should be higher than (-80dBm + 15 dB(margin)=-65 dBm). So first you can find your LNA gain. Noise parameter as shown in link is calculated by noise figures and gain of compenents. – mehdi Sep 18 '12 at 7:00
recieved power is easy to find. – Parhs Sep 22 '12 at 10:36

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