The technique of transmitting more than one information signal through a single channel is called multiplexing. There are two types of multiplexing
(a) Frequency Division Multiplexing (FDM) and
(b) Time Division Multiplexing (TDM)
When the information signal is digital and the carrier signal is Sinusoidal, modulation is called shift keying [Book03]. According to the change of the parameter of the carrier (sinusoidal) by the digital signal we have,
(a) Amplitude Shift Keying (ASK)
(b) Frequency Shift Keying (FSK) and
(c) Phase Shift Keying (PSK)
In addition, a combination of ASK and PSK is employed at high bit rates. This method is called Quadrature Amplitude Modulation (QAM) .
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