Digital modulation, ASK, FSK and PSK

There are three ways in which the bandwidth of the channel carrier may be altered simply. It is worth emphasising that these methods are chosen because they are practically simple, not because they are theoretically desirable. These are the altering of the amplitude, frequency and phase of the carrier sine wave. These techniques give rise to amplitude-shift-keying (ASK), frequency-shift-keying (FSK) and phase-shift-keying (PSK), respectively.

ASK describes the technique the carrier wave is multiplied by the digital signal f(t) Mathematically, the modulated carrier signal s(t) is:

Figure: Amplitude shift keying

It is a special case of amplitude modulation (AM) Amplitude modulation has the property of translating the spectrum of the modulation to the carrier frequency. The bandwidth of the signal remains unchanged.

The fact that AM simply shifts the signal spectrum is often used to convert the carrier frequency to a more suitable value without altering the modulation. This process is known variously as mixing, up-conversion or down-conversion. Some form of conversion will always be present when the channel carrier occupies a frequency range outside the modulation frequency range.

FSK describes the modulation of a carrier (or two carriers) by using a different frequency for a 1 or 0. The resultant modulated signal may be regarded as the sum of two amplitude modulated signals of different carrier frequency

Figure: Frequency shift keying

FSK is classified as wide-band if the separation between the two carrier frequencies is larger than the bandwidth of the spectrums of f1(t) and f2(t). In this case the spectrum of the modulated signal appears as two separate ASK signals. Narrow-band FSK is the term used to describe an FSK signal whose carrier frequencies are separated by less than the width of the spectrum than ASK for the same modulation.

PSK describes the modulation technique that alters the phase of the carrier. Mathematically:

Binary phase-shift-keying (BPSK) has only two phases, 0 and . It is therefore a type of ASK with taking the values -1 or 1 (Figure gif), and its bandwidth is the same as that of ASK. Phase-shift-keying offers a simple way of increasing the number of levels in the transmission without increasing the bandwidth by introducing smaller phase shifts. Quadrature phase-shift-keying (QPSK) has four phases, . M-ary PSK has M phases, . For a given bit-rate, QPSK requires half the bandwidth of PSK and is widely used for this reason.

The number of times the signal parameter (amplitude, frequency, phase) is changed per second is called the signaling rate. It is measured in baud. 1 baud = 1 change per second. With binary modulations such as ASK, FSK and BPSK, the signaling rate equals the bit-rate. With QPSK and M-ary PSK, the bit-rate may exceed the baud rate.