Which type of modulation is characterized by phase shifts of the carrier signal?

Phase Shift Keying (PSK) is a type of modulation where the key characteristic is the variation in the phase of the carrier signal to represent data. In this method, different phase angles indicate different symbols or values in the transmitted information. For instance, in binary phase shift keying (BPSK), two distinct phases are used to represent binary digits (0s and 1s).

The fundamental idea behind PSK is that by altering the phase of the carrier wave while keeping its frequency and amplitude constant, data can be encoded and transmitted efficiently. This phase alteration allows for robust transmission with a degree of noise immunity, which is especially important in digital communication systems.

While other modulation methods like amplitude modulation and frequency modulation use variations in amplitude and frequency, respectively, they do not utilize phase shifts as their principal means of data representation. Pulse Width Modulation also does not involve phase shifts; instead, it varies the duration of the pulses to encode information. Thus, the correct answer highlights the unique aspect of PSK as the modulation technique inherently based on phase changes of the carrier signal.