QAM and QPSK:
Aim:
Review of Quadrature Amplitude Modulator (QAM) in digital communication system,
generation of Quadrature Phase Shift Keyed (QPSK or 4-PSK) signal and demodulation.
Introduction:
The QAM principle:
The QAM modulator is of the type shown in Figure 1 below. The two paths to the adder
are typically referred to as the ‘I’ (inphase), and ‘Q’ (quadrature), arms.
Not shown in Figure 1 is any bandlimiting. In a practical situation this would be
implemented either at message level - at the input to each multiplier - and/or at the output
of the adder. Probably both !
The motivation for QAM comes from the fact that a DSBSC signal occupies twice the
bandwidth of the message from which it is derived. This is considered wasteful of
resources. QAM restores the balance by placing two independent DSBSC, derived from
message #1 and message #2, in the same spectrum space as one DSBSC. The bandwidth
imbalance is removed.
In digital communications this arrangement is popular. It is used because of its bandwidth
conserving (and other) properties.
