Introduction

In baseband transmission the transmitted signal power lies at low frequencies, typically around zero. It is desirable in many digital communication systems, for the same reasons as in analog communication systems, for the transmitted signal to lie in a frequency band toward the high end of the spectrum. As an example satellite communication is normally conducted in the 6–8 gigahertz band, while mobile phones systems are implemented in the 800 megahertz–2.0 gigahertz band.

The digital information is encoded as a variation of the parameters of a sinusoidal signal, called the carrier signal. Typically, as for analog modulation systems, the carrier frequency is much higher than the highest frequency of the modulating signals (or messages). Digital passband modulation is based on variation of the amplitude, phase, or frequency of the sinusoidal carrier, or some combination of these parameters.

Amplitude-shift keying (ASK) was probably the first type of digital modulation to be practically applied. In its simplest form it has been used for radio telegraphy transmission in Morse code. Another name for ASK is “on–off keying” (OOK), since a binary “1” corresponds to the sinusoid being transmitted while a binary “0” suppresses the carrier. Phase-shift keying (PSK) is an efficient, in terms of signal power, digital modulation method. It is widely used in modern digital communication systems, such as satellite links, wideband microwave radio relay systems, etc. The digital information is encoded in the phase function of a constant-amplitude carrier signal.