Modulation and Channel Coding

Abstract

Communication always involves a source, a receiver, and a communication channel which is the physical means through which the signal is transmitted. A conversation, a radio broadcast, a cable TV program, a person reading a book or typing on a computer keyboard, and a computer transferring information to a hard disk are all examples of communication. In each case, information is transmitted by modifying something physical that can interact with both the sender and the receiver. For instance, speaking causes small systematic pressure variations in the air between the speaker and the hearer, which the hearer’s ear translates or “decodes ” into a stream of nervous impulses that are transmitted to the brain. The pressure variation of air is the communication channel for transmitting the speech signal. The air between the speaker’s mouth and the hearer’s ear can be thought of as the “carrier ” of the speech signal; the small pressure variation caused by speaking “ modulates ” the carrier to encode the information signal on it.