Noise Modelling - Digital communication systems

AI Thread Summary
The discussion revolves around a teacher seeking clarification on a noise figure calculation in a digital mobile communication system. The problem involves a receiver with a noise bandwidth of 200 kHz and an input SNR requirement of 10 dB at an input signal of -104 dBm. The teacher is puzzled by the inclusion of a -30 value in their calculations, suspecting it may relate to a conversion between dBW and dBm. Participants suggest that the -30 could indeed represent such a conversion factor. The conversation highlights the importance of understanding unit conversions in signal processing calculations.
CMW328i
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Hi all, Teacher here looking for some guidance. I'm teaching a class that I took myself a few years back, and I'm looking at my solution to a problem but I have a random value in my answer (which I know to be correct) but I can't for the life of me remember where this value came from! Can anyone help?

1. Homework Statement

A receiver in a digital mobile communication system has a noise bandwidth of 200 kHz and requires that its input SNR should be at least 10 dB when the input signal is -104 dBm.
What is the maximum permitted value of the receiver noise figure?2. Homework Equations

FdB = Ps -SNR -10log (kTB)

3. The Attempt at a Solution

FdB = (-104 -30)-10 -10log (1.399x10^-23*290*200000) = 6.9 dBI can't for the life of me figure out where the -30 comes from. I even have an excerpt from the book the question came out of and it just shows the -30 in the equation without explaining where it comes from. Any ideas?

I see the input at -104, I see the bandwith, the required SNR, but no origin for -30
 
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CMW328i said:
Hi all, Teacher here looking for some guidance. I'm teaching a class that I took myself a few years back, and I'm looking at my solution to a problem but I have a random value in my answer (which I know to be correct) but I can't for the life of me remember where this value came from! Can anyone help?

1. Homework Statement

A receiver in a digital mobile communication system has a noise bandwidth of 200 kHz and requires that its input SNR should be at least 10 dB when the input signal is -104 dBm.
What is the maximum permitted value of the receiver noise figure?2. Homework Equations

FdB = Ps -SNR -10log (kTB)

3. The Attempt at a Solution

FdB = (-104 -30)-10 -10log (1.399x10^-23*290*200000) = 6.9 dBI can't for the life of me figure out where the -30 comes from. I even have an excerpt from the book the question came out of and it just shows the -30 in the equation without explaining where it comes from. Any ideas?

I see the input at -104, I see the bandwith, the required SNR, but no origin for -30
Is it a conversion between dB W and dB mW?
 
Ah ha! That may be it actually!
 
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