The purpose of a communication systems is to transmit information-bearing signals through a communication channel separating the transmitter from the receiver. Information-bearing signals are also referred to as baseband signals. The term baseband is used to designate the band of frequencies representing the original signal as delivered by a source of information. The proper use of the communication channel requires a shift of the range of baseband frequencies into other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. For example, a radio system must operate with frequencies of 30 kHz and upward, whereas the baseband signal usually contains frequencies in the audio frequency range, and so some form of frequency-band shifting must be used for the system to operate satisfactorily. A shift of the range of frequencies in a signal is accomplished by using modulation, which is defined as the process by which some characteristic of a carrier is varied in accordance with a modulating wave (signal). A common form of the carrier is a sinusoidal wave, in which case we speak of a continuous-wave modulation1 process. The baseband signal is referred to as the modulating wave, and the result of the modulation process is referred to as the modulated wave. Modulation is performed at the transmitting end of the communication system. At the receiving end of the system, we usually require the original baseband signal to be restored. This is accomplished by using a process known as demodulation, which is the reverse of the modulation process.