a. Compare Quadrature Amplitude Modulation (QAM) and Amplitude Shift Keying (ASK

Question

a. Compare Quadrature Amplitude Modulation (QAM) and Amplitude Shift Keying (ASK) in terms of its error performance and data rate.

b. binary data is transmitted over a channel at a rate of fo bits/sec. To reduce the transmission bandwidth, it is decided to transmit this data using 16-ary symbol.

i. By what factor is the bandwidth reduced?

ii. What will be the increase in the transmitted power when the separation between pulse amplitude is same in both cases. Calculate the difference in maximum power level between ASK and 16-ary symbol in this case.

Explanation / Answer

Quadrature amplitude modulation (QAM) is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying(ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves of the same frequency, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature.

The modulated waves are summed, and the final waveform is a combination of bothphase-shift keying (PSK) and amplitude-shift keying (ASK), or, in the analog case, of phase modulation (PM) and amplitude modulation. In the digital QAM case, a finite number of at least two phases and at least two amplitudes are used.QAM is being used in optical fiber systems as bit rates increase; QAM16 and QAM64 can be optically emulated with a 3-pathinterferometer.

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