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I was discussing with a colleague with background in communication about white Gaussian (WG) channel noise.

He said noise is not added by the channel and it's just a misnomer. Noise, which is thermal noise, is actually added by receiver. Channel will just introduce fading, not noise. I was stunned, as I studied in many textbooks that channel add WG noise to the transmitted signal. Is this correct?

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  • $\begingroup$ In a compressed data stream, you are transmitting essentially random data. The degradation you speak about will lead to cross-talk in time and frequency. As these neighboring places also carry random data, you can simulate their influence as some kind of random noise. $\endgroup$ – Lutz Lehmann Jan 23 at 9:42
  • $\begingroup$ For the purpose of calculation, does it matter whether the channel adds noise, or the receiver? $\endgroup$ – user253751 Jan 23 at 12:14
  • $\begingroup$ Read this answer and re-ask your question. $\endgroup$ – Dilip Sarwate Jan 24 at 0:16
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So, it's generally right that a receiver adds noise due to physics (Johnson-Nyquist noise). So, yes, in any system, at least part of the noise in the received signal is caused by the receiver.

There's also noise that is background noise from e.g. cosmic sources that happen neither in the receiver nor the transmitter (and also aren't other transmitters interfering), but those sources are typically far less important than receiver noise – at least in the microwave bands, where modern comms happen; in LF/MF/HF bands, background noise is very observable.

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  • $\begingroup$ Is background noise in LF/MF/HF bands usually modeled by addictive white Gaussian process, which OP refers to? $\endgroup$ – AlexTP Jan 23 at 11:09
  • $\begingroup$ @AlexTP good question, I don't know – "globally" its power density falls with f, but the same actually applies to Johnson-Nyquist noise; just that if you observe a bandwidth small enough, it looks white. $\endgroup$ – Marcus Müller Jan 23 at 12:09
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In some textbooks the channel defined to be not only the propagating medium, but it is considered to include some of the components of the transmitter and receiver, usually the analog parts. For example, you might read somewhere that the "channel introduces noise, power loss, etc." and this is true when the channel includes both the propagating medium as well as analog front end or other parts.

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