How to obtain No/2 in AWGN channel?

In AWGN channel, we always assume that variance= No/2, and one sided PSD. My question is, how we obtained No/2? And why it is always one-sided instead of two-sided. Pls help me on it or any related informations?

I think you can find your answer in chapter 9 (section 9-3 & fig 9-3-4a) of this book:
Communication systems: an intriduction to signals in electrical communication
Bryce A. Carlson
4th ed. 2002[/i]

thanks. Is there any ebook for this then?

We normally defined noise in communication system as one-sided spectral density. Is it because we only have positive frequency (as No/2)? Therefore, it is just one sided spectral density instead of two.

Hi!

N0/2 comes from the expression of the power spectral density of the thermal noise. It depends on temperature and bolztman constant.
When you calculate the power of the noise with the two sided expression you must integrate from -∞ to +∞. Some authors uses the one side expression to simplify ad integrate form 0 to +∞, but their power spectral densities are multiplied by 2 (that is the same of using the two sided exp. normally).