Modulation

Abstract

In the broad sense, the term “modulation” implies a change in time of a certain parameter. For instance, while listening to a steady single-tone signal with constant amplitude and frequency coming out of a speaker, we merely receive the simplest message that conveys information only about the existence of the signal source and nothing else. If the source is turned off, then we cannot even say if there is a signal source out there or not. For the purpose of transmitting a more sophisticated message, the communication system must use at least the simplest modulation scheme, based on time divisions, i.e., turning on and off the signal source. By listening to short and long beeps, we can decode complicated messages letter by letter. When you think about it, smoke signals are based on the same principle. As slow and inefficient as it is, Morse code does work and is used even today in special situations, for example in a very low SNR environment. In this chapter, we study the main modulation techniques for wireless communications, which are based on the time variation of periodic electrical signals.