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AWGN is consist of four words (Additive-White-Gaussian_Noise). Now we should ask about each of the first three words.
there are two main reason:
1- A receiver antenna is almost always a linear device, so have superposition property. the two sources with different natures (desired signal & noise) are received at the received antenna, independently. so following superposition property, the output of the antenna is the addition of signal and noise.
2- it is accommodated with the experiments.
why zero mean white noise?!
it is zero mean because the noise hasn't any polarity. the probability of the noise to be positive is exactly equal to the probability of noise to be negative. and it is white because the most power of the noise are thermal noise and this kind of noise is white in a broad range of frequency.
according to central limit theory sum of many phenomena with different natures are Gaussian that is what we have in communication systems.
so this model is well appropriate with real noises in data communications.