Quadrature Amplitude Modulation (QAM):

QAM is a popular analog signaling technique that is used in some wireless standards. This modulation technique is a combination of ASK and PSK. QAM can also be con- sidered a logical extension of QPSK. QAM takes advantage of the fact that it is pos- sible 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 simulta- neously transmitted over the same medium. At the receiver, the two signals are demodulated and the results combined to produce the original binary input.

Fig: QAM Modulator:

The given above figure 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 mul- tiplying 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. The transmitted signal can be expressed as follows:

If two-level ASK is used, then each ofthe two streams can be in one of two states and the combined stream can be in one of 4 = 2 X 2 states. This is essentially QPSK.

If four-level ASK is used (i.e., four different amplitude levels), then the combined stream can be in one of 16 = 4 X 4 states. Systems using 64 and even 256 states have been implemented. The greater the number of states, the higher the data rate that is possible within a given bandwidth. Of course, as discussed previously, the greater the number of states, the higher the potential error rate due to noise and attenuation.