You can download Modern Digital And Analog Communications Systems 3rd edition by B. P. Lathi free in pdf format.
Lathi’s trademark user-friendly and highly readable text presents a complete and modern treatment of communication systems. It begins by introducing students to the basics of communication systems without using probabilistic theory. Only after a solid knowledge base–an understanding of how communication systems work–has been built are concepts requiring probability theory covered. This third edition has been thoroughly updated and revised to include expanded coverage of digital communications. New topics discussed include spread-spectrum systems, cellular communication systems, global positioning systems (GPS), and an entire chapter on emerging digital technologies (such as SONET, ISDN, BISDN, ATM, and video compression).
Ideal for the first communication systems course for electrical engineers, Modern Digital and Analog Communication Systems offers students a superb pedagogical style; it consistently does an excellent job of explaining difficult concepts clearly, using prose as well as mathematics. The author makes every effort to give intuitive insights–rather than just proofs–as well as heuristic explanations of theoretical results wherever possible. Featuring lucid explanations, well-chosen examples clarifying abstract mathematical results, and excellent illustrations, this unique text is highly informative and easily accessible to students.