Digital Modulation Schemes

Digital Modulation Schemes

Digital information is binary in nature in that it has only two possible states 1 or 0. Sequences of bits encode data (e.g., text characters). Digital signals are commonly referred to as baseband signals. Digital modulation is a process by which we transmit digital data via analog carrier signal. So our input or message signal is digital in nature but the carrier is still high frequency sinusoidal signal. If we draw a comparison between analog and digital modulation we can say that instead of modulation that is proportional to a continuous signal, digital schemes use discrete values. To distinguish between analog and digital modulation,we use the term shift keying rather than modulation
Shift keying operates similar to analog modulation.Instead of a continuum of possible values, digital shift keying has a fixed set of values. For example, AM allows the amplitude of a carrier to vary by arbitrarily small amounts in response to a change in the signal. In contrast, amplitude shift keying uses a fixed set of possible amplitudes.

Amplitude Shift Keying (ASK)

In ASK the amplitude of carrier wave is changed in relation to the incoming digital symbol. These amplitude values are not random but chosen from a pre-defined set of values depending upon the bits or symbols to be transmitted at that instant (see the figure below) . The frequency or phase of the carrier signal stay constant through out the data transmission. Although the bandwidth requirements for ASK are low but it is susceptible to noise and interference.

Binary Amplitude Modulation is shown in figure, where we have only two amplitude values [0,1] but it can be expanded to M-ary ASK with multiple different amplitude values.

Frequency Shift Keying (FSK)

FSK is essentially same in operation as Frequency Modulation. Here the amplitude of the modulated signal stays constant for all the different frequencies. In FSK, the frequency of the carrier is changed as a function of the modulating signal (data) being transmitted. In binary FSK (BFSK or 2FSK), a “1” is represented by one frequency and a “0” is represented by another frequency. see the figure below. The major drawback of FSK is requirement of a large bandwidth if we want to transmit a high bit rate. 

FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states

 

Comparison between BPSK and BFSK

Phase Shift Keying (PSK)

As the name implies here we play with the phase of the carrier signal. PSK is thought as an alternative to imposing the modulation onto the carrier by varying the instantaneous frequency is to modulate the phase. This can be achieved simply by defining a relative phase shift from the carrier, usually equi-distant for each required state. Therefore a two level phase modulated system, such as Binary Phase Shift Keying, has two relative phase shifts from the carrier,0 or 90 degrees.

The drawback of PSK is that rapid amplitude change occurs between symbols due to phase discontinuity, which requires infinite bandwidth. Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and BFSK

BPSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states

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