this is the one of the important subject for the all PSU's, IES as well as GATE examination. the questions in the communication is mostly asked from this side. this documents are very useful for the attending that questions and very useful for the IES conventional paper, and also contribute a big role in the interviews in the various IITS, and ISRO, BARC like organization they asked very basic questions but in indirect manner.
the links given below for the downloading the pdf documents.
40 Lectures on Digital Communication
Electronics and Communication Home Page