Noise Floor vs. Noise temperature

[tecknet2]

Hi all,

I am designing a satellite communications link and would like some assistance.

The receiver has a noise floor of -90dBm.

I am trying to calculate the carrier to noise ratio. I have calculated my receiver noise figure to be 4.97, therefore approximating a system temperature of 290*(NF-1) = 620K. The receiver noise power is then 10log10(K*Ts*B) = -122dB (80MHz bandwidth).

I have then calculated my carrier to noise ratio as receiver power (dB) - Noise power (dB).

My only issue is the noise power is lower then the receiver noise floor ? Does this make my carrier to noise calculations void?

thanks in advance,

J

 

[tecknet2]

Just as an example to further explain my question;

Say my received signal power is -80dBm. I would calculate a carrier to noise ratio of -80-(-122) = 42dB.

However, my receiver can only 'see' a signal above -90dBm.

Would my calculated carrier to noise ratio still stand at 42dB ?

thanks again,

J

 

[Harry Zachrisson]

I haven't gone over your calculation so I will not comment on them but I would like to point out that the signal to noise ration for a modulated signal can be very much lower than the decoded signal to noise ration.
So in many cases you only need your signal to be a few dB over the noise floor to get a perfect demodulated signal.
When calculating link budgets like you are doing one usually sets a fixed amount of dB above the noise that you want to have in your receiver and then calculate the designs around that.
How much signal margin you need is dependent on modulation type.
For FM it might be 10dB but for many digital modulation schemes it might only be 2 or 3 dB.