Quadrature Atnplitude Modulation

Quadrature Atnplitude Modulation

QAM is a popular analog signaling technique that is used in some wireless standards.
This modulation technique is a combination ofASK and PSK. QAMcan also be considered
a logical extension of QPSK. QAM takes advantage of the fact that it is possible
to send two different signals simultaneously on the same carrier frequency, by
using two copies of the carrier frequency, one shifted by 900 with respect to the other.
For QAM, each carrier is ASK modulated. The two independent signals are simultaneously
transmitted over the same medium. At the receiver, the two signals are
demodulated and the results combined to produce the original binary input.
Figure 6.10 shows the QAM modulation scheme in general terms. The input is a
stream of binary digits arriving at a rate of R bps. This stream is converted into two
separate bit streams of R/2 bps each, by taking alternate bits for the two streams. In
the diagram, the upper stream is ASK modulated on a carrier of frequency Ie by multiplying
the bit stream by the carrier. Thus, a binary zero is represented by the absence
of the carrier wave and a binary one is represented by the presence of the carrier wave
at a constant amplitude. This same carrier wave is shifted by 900 and used for ASK
modulation of the lower binary stream. The two modulated signals are then added
together and transmitted.