3.6.1 Amplitude-Shift Keying (ASK)

ASK is the digital counterpart of amplitude modulation. In its simplest form, known as on-off keying (OOK), the carrier is switched between two amplitude levels—typically “on” for a binary 1 and “off” for a binary 0—as illustrated in Figure 3.21.

Figure 3.21. ASK illustrating: (a) the original digital waveform and (b) the time-domain waveform.

While ASK is simple to generate and detect, it inherits the principal weakness of analog AM: susceptibility to amplitude noise and fading. Variations in received signal strength can be mistaken for symbol transitions, making ASK relatively unreliable in noisy wireless environments without additional coding or error control.

Nevertheless, ASK remains useful in applications where simplicity and low cost are paramount, such as optical intensity modulation, RFID systems, infrared remote controls, and some short-range wireless links. For high-reliability radio communication systems, however, phase- or frequency-based digital modulation is generally preferred.

3.6.2 Frequency-Shift Keying (FSK)

Frequency-shift keying (FSK) is the digital analog of frequency modulation. In binary FSK, two distinct carrier frequencies are used to represent the binary symbols “0” and “1.” These frequencies may be symmetrically spaced about a nominal carrier frequency or may occupy two distinct frequency slots within an assigned channel, as illustrated in Figure 3.22. The time-domain representation of FSK is illustrated in Figure 3.23.

Figure 3.22. FSK frequency spectrum showing the carrier shifted to either a mark or a space position, below or above the assigned frequency.

Figure 3.23. FSK, illustrating (a) the original digital waveform and (b) the FSK time-domain waveform.

The frequency separation between the two symbol states is called the frequency shift. In traditional telegraph systems, common shift values were 85 Hz (narrow shift) and 850 Hz (wide shift), the latter originally chosen for robust HF radio transmission. Wider shifts improve noise immunity but increase occupied bandwidth; narrower shifts conserve bandwidth but reduce robustness.

FSK is well suited to transmission over legacy analog FM radio systems because data can be conveyed by modulating the audio input of an FM transmitter. More advanced digital radio systems, however, typically employ phase-based modulation techniques, which achieve higher spectral efficiency and improved power efficiency.