A basic digital communication
system consists of a source, which generates digital
data. Digital data may come from a computer, voice signal,
digitized facsimile or television signal, ad communication
signal etc. These signals are required to be transmitted
over long distances by means of ratio waves. As the
carrier is sinusoidal, the digital signals must be made
to modulate this sinusoidal carrier. The signal may
modulate amplitude, frequency or phase of the carrier.
Accordingly the resulting methods are termed as Amplitude
Shift Keying (ASK) or On and Off Keying (OOK), Frequency
Shift Keying (FSK) and phase shifting keying (PSK).
Sometimes a hybrid term of modulation is used, in this
changes in both amplitude and phase of the carrier,
are combined to produce amplitude phase keying (APK).
At the receiver, demodulation is performed by coherent
or noncoherent detection. In the coherent detector,
the receiver is phase- locked to the transmitter. In
the coherent detector, the receiver has exact knowledge
of the carrier waves phase reference. In noncoherent
detection, the knowledge of the carrier waves phase
is not required. |
