The AWGN has constant power spectral density.

The method heterodyning means frequency conversion and mixing and this results in a spectral shift.

A channel can be modelled as a linear filter with additional noise. The noise comes from thermal noise source and also from interference from other signals.

The output symbol of the sampler consists of sample which is directly proportional to the energy of the received signal and inversely proportional to the noise.

Thermal noise which cannot be eliminated is caused by the motion of thermal electrons causes degradation in system.

The main causes of error performance degradation are interference electrical noise effect of filtering and also due to the surroundings.

If error correcting codes are not present, the detector output consists of estimates of the message symbol which is also called as hard decisions.

The functions of both receiving and equalizing filter can be performed by only the equalizing filter alone. Thus equalizing filter is the combination of equalizing and receiving filter.

White noise is a idealized process with two sided spectral density equal to constant N0/2 and frequencies varying from minus infinity to plus infinity. Thus the average noise power is infinity.

The optimum filter used to recover the pulse with best possible signal to noise ratio and less or no ISI is called as correlator or matched filter.