Digital Carrier Modulation Techniques

To transmit digital data over copper wires, they can be transmitted directly with two voltage levels +A and -A because digital data is a string 1's and 0's. But if they are to be transmitted through space using antenna, it is necessary to modulate the incoming data on to a sinusoidal carrier wave. The data may be the output of PCM system or digital computer output. Amplitude modulation, frequency modulation and phase modulation are commonly used Digital Carrier Modulation Techniques for an analog signal. But, in digital communication, the modulation process involves switching or keying the amplitude, phase or frequency of the carrier according to the incoming digital data. As the modulating signal consists of only two levels, the digital carrier modulation techniques are known as Amplitude Shift Keying (ASK), Phase Shift Keying (PSK) and Frequency Shift Keying (FSK).

Various Shift Keying Systems

Modulation Techniques:

In amplitude shift keying, the amplitude of the carrier is switched between two levels. ON and OFF. The resultant waveform consists of ON pulse representing binary '1' and OFF pulse representing binary '0'. The ASK waveform is shown in Figure.

In frequency shift keying, the frequency of the carrier is switched between to frequencies. The resultant wave form consists of high frequency carrier representing binary '1' and low frequency carrier representing binary '0'. The FSK waveform is shown in Figure (a).

If the carrier phase is switched between two levels, we get phase shift keying. The PSK waveform is shown in Figure (c). In PSK and FSK methods and amplitude of the carrier remains constant.

All the above systems use a pair of signals to represent binary symbols 1 and 0. So, these systems are known as Binary Shift Keying systems. (BPSK, BFSK, BASK). Generally, the modulator produces one signal of an available set of M = 2N district signals in response to N bits of source data at a time. This type of digital modulation is known as M - ray shift keying system. (M - ray PSK, M - ray FSK, M - ray ASK). Binary phase shift keying is a special case of M - ray shift keying with M = 2 and N = 1. In ASK, PSK and FSK, a single parameter of the carrier i.e., amplitude, frequency and phase of the carrier undergoes modulation. Sometimes, the amplitude and phase of the carrier are varied to produce single modulation techniques, amplitude phase shift keying (APSK). This type of techniques is known as hybrid modulation technique.