Chapter 6 / 6.6

6.6 DIGITAL MODULATION

Digital modulation differs from analog modulation in that the modulating signal assumes discrete values rather than varying continuously. Information is conveyed by transmitting one of a finite set of distinct signal states, each representing a symbol. In the simplest case, a symbol represents a single binary digit (bit), but more generally, one symbol may represent multiple bits. Digital modulation techniques can therefore be viewed as discrete versions of AM, FM, or PM.

ASK is the digital counterpart of AM and is most commonly implemented as on-off keying (OOK), in which the carrier is switched on and off to represent binary symbols. While ASK can be used effectively for simple signaling systems such as Morse code, it is poorly suited to modern data transmission. ASK-modulated signals are highly susceptible to amplitude noise and fading—vulnerabilities shared with all amplitude-modulated systems. Consequently, ASK is seldom used for reliable digital communications, particularly in satellite and wireless applications.

Digital modulation differs from analog modulation in that the modulating signal assumes discrete values rather than varying continuously. Information is conveyed by transmitting one of a finite set of distinct signal states, each representing a symbol. In the simplest case, a symbol represents a single binary digit (bit), but more generally, one symbol may represent multiple bits.

The principal design challenge in digital modulation is to transmit data at high bit rates within limited bandwidth and power constraints. The efficiency of a digital modulation scheme is commonly measured in bits per second per hertz (bits/s/Hz), a concept introduced earlier as spectral efficiency.

As digital modulation schemes become more sophisticated, these discrete signal states are most clearly understood in terms of their representation in amplitude–phase space, where each symbol corresponds to a distinct point in a signal constellation. This geometric viewpoint will prove central to understanding modern phase- and amplitude-based digital modulation techniques.