why is QAM used instead of simple frequency or amplitude modulation?

Answer 1

Quadrature Amplitude Modulation (QAM) is used over the other 2 as it provides more benefits in that for a given frequency bandwidth, you can pass a greater bit rate. The same frequency is modulated by changing the phase of the signal (Quadrature reffereing to the 4 phases used) and the received signal level (Amplitude) also gives a variable.

Typically, for a burst of frequency in phase with the reference at rx level 1, the digital signal demodulated will be 000. For a signal received at rx level 1 with a +90degrees phase shift, then a binary 001 will be demodulated etc. This simple table may demonstrate better:

Phase Amplitude Demod code
0 <1 000
90 <1 001
180 <1 010
270 <1 011
0 >1 100
90 >1 101
180 >1 110
270 >1 111

Answer 2

Read the article in wikipedia.

Then decide which is most suitable for digital data transmission (given known forms of interference) and so calculate the maximum reliable information transmission rates for the various modulation schemes.