This book is designed as a textbook for courses on communication systems. These courses are taken by undergraduate students of computer science, electronics, communication, instrumentation and electrical engineering. It also suits the needs of postgraduate students -physics, computer science and electronics. The text is clear and easy to understand. It is supported by practical circuits which will help the reader understand the application of the subject and will help him study the circuit in the laboratory. A large number of solved examples are provided at each chapter end. Unsolved examples are provided. Their answers are given at the end of the book. The last chapter of the book consists of a bank of multiple choice questions with answers. These will help the reader test his knowledge and prepare for various competitive examinations.