Q. Carrier Modulation by Digital Signals?

Digitally modulated signals with low-pass spectral characteristics can be transmitted directly through baseband channels (having low-pass frequency-response characteristics) without the need for frequency translation of the signal. However, there are many communication bandpass channels (telephone, radio, and satellite channels) that pass signals within a band of frequencies (that is far removed from dc). Digital information may be transmitted through such channels by using a sinusoidal carrier that ismodulated by the information sequence in either amplitude, phase, or frequency, or some other combination of amplitude and phase. The effect of impressing the information signal on one ormore of the sinusoidal parameters is to shift the frequency content of the transmitted signal to the appropriate frequency band that is passed by the channel. Thus, the signal is transmitted by carrier modulation. There are several carrier-modulation methods. However, we shall limit our discussion to the following, assuming only binary modulation in all cases:

• Amplitude-shift keying (ASK)

• Phase-shift keying (PSK)

• Differential phase-shift keying (DPSK)

• Frequency-shift keying (FSK).