Review of Quadrature Amplitude Modulator (QAM) in digital communication system, generation of Quadrature Phase Shift Keyed (QPSK or 4-PSK) signal and demodulation.
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.
It is not used for multiplexing two independent messages. Given an input binary sequence (message) at the rate of n bit/s, two sequences may be obtained by splitting the bit stream into two paths, each of n/2 bit/s. This is akin to a serial-to-parallel conversion. The two streams become the channel 1 and channel 2 messages of Figure 1. Because of the halved rate the bits in the I and Q paths are stretched to twice the input sequence bit clock period. The two messages are recombined at the receiver, which uses a QAM-type demodulator. The two bit streams would typically be band limited and/or pulse shaped before reaching the modulator. A block diagram of such a system is shown in Figure 2 below.
QAM becomes QPSK: The QAM modulator is so named because, in analog applications, the messages do in fact vary the amplitude of each of the DSBSC signals. In QPSK the same modulator is used, but with binary messages in...