Although QAM appears to increase the efficiency of transmission for radio communications systems by utilising both amplitude and phase variations, it has a number of drawbacks. The first is that it is more susceptible to noise because the states are closer together so that a lower level of noise is needed to move the signal to a different decision point. Receivers for use with phase or frequency modulation are both able to use limiting amplifiers that are able to remove any amplitude noise and thereby improve the noise reliance. This is not the case with QAM.
The second limitation is also associated with the amplitude component of the signal. When a phase or frequency modulated signal is amplified in a radio transmitter, there is no need to use linear amplifiers, whereas when using QAM that contains an amplitude component, linearity must be maintained. Unfortunately linear amplifiers are less efficient and consume more power, and this makes them less attractive for mobile applications.
QAM vs other modulation formats
As there are advantages and disadvantages of using QAM it is necessary to compare QAM with other modes before making a decision about the optimum mode. Some radio communications systems dynamically change the modulation scheme dependent upon the link conditions and requirements - signal level, noise, data rate required, etc.
SUMMARY OF TYPES OF MODULATION WITH DATA CAPACITIES
MODULATION BITS PER SYMBOL ERROR MARGIN COMPLEXITY
OOK 1 1/2 0.5 Low
BPSK 1 1 1 Medium
QPSK 2 1/√2 0.71 Medium
16 QAM 4 √2/6 0.23 High
64QAM 6 √2/14 0.1 High
Typically it is found that if data rates above those that can be achieved using 8-PSK are required, it is more usual to use quadrature amplitude modulation. This is because it has a greater distance between adjacent points in the I - Q plane and this improves its noise immunity. As a result it can achieve the same data rate at a lower signal level.
However the points no longer the same amplitude. This means that the demodulator must detect both phase and amplitude. Also the fact that the amplitude varies means that a linear amplifier si required to amplify the signal.
Source: http://www.radio-electronics.com/info/rf-technology-design/quadrature-amplitude-modulation-qam/what-is-qam-tutorial.php
