# Why do we consider zero mean for gaussian noise in a communication channel?

1. Oct 8, 2012

### dexterdev

Hi all,
I got an idea why noise is having a gaussian pdf but didnot understood why it should be having zero mean. Why cant noise contain a dc content.

-Devanand T

2. Oct 8, 2012

### dexterdev

in most cases we consider 0 mean awgn , is it so....

3. Oct 9, 2012

### FailedLaunch

DC component means that there is energy (power) permanently leaving the source, i.e. the source is "evaporating" so it would eventually seize to exist.

4. Oct 9, 2012

### dexterdev

Can you explain in simple english (Iam not good in english). I didnot got the idea of evaporate and seize etc..

5. Oct 9, 2012

### FailedLaunch

Taking it back

Actually, it looks like I'll need to take this explanation back since it doesn't appear to be to right reason while the used AWGN is zero-mean. :uhh:

6. Oct 9, 2012

### sophiecentaur

Your question is quite a reasonable one. It is largely a matter of semantics and practicalities, I think. In all analogue equipment, you can expect very low rate drifting of DC levels and these will mostly be due to other effects (due to thermal variations of gain etc.) than the normal Gaussian Noise that you can also expect. Most analogue channels are deliberately made 'AC' coupled so there is a lower frequency limit, below which we are not particularly interested and it is not very relevant, in any case. Also, if you think of noise power in W/Hz, there is very little of it in a small fraction of a Hz, compared with normal signalling rates.

7. Oct 10, 2012

### dexterdev

Thankyou for your helpful replies. but what does "matter of semantics" mean

8. Oct 10, 2012

### sophiecentaur

It refers to the meaning we attribute to words. We make 'assumptions' when we read some words but different people make different assumptions. Mostly, we take it for granted that channels are AC coupled because DC is too difficult. (I can't think of an example, in fact. Now that's a challenge for you all.)

9. Oct 11, 2012

### dexterdev

thanks again sir