QAM
Unlike this, the M-ary PSK signal is modulated with a different phase-shifted version of the carrier signal and varying amplitude levels. Let me give an example for better comprehension.
QAM = ASK + PSK
In the above figure, 2 levels of amplitude and 4 PSK are applied. So, there is a total of 2*4=8 constellation points.
Unlike this, the M-ary PSK signal is modulated with a different phase-shifted version of the carrier signal and varying amplitude levels. Let me give an example for better comprehension.
QAM = ASK + PSK
In the above figure, 2 levels of amplitude and 4 PSK are applied. So, there is a total of 2*4=8 constellation points.
Using any of the M number of amplitude and phase-shifted carrier signals, Mary QAM modulates provided data bits. Send M data bits that have been modulated using M number of amplitude and phase-shifted carriers. Theoretically, there won't be any interference between them, and we'll be able to obtain a data rate that is M times higher than before (without modulation).
Instead of merely modifying the phase, frequency, or amplitude of the RF signal, the RF carrier's phase (or frequency) is also altered. Since the envelope and phase offer two degrees of freedom, Mary modulation methods convert baseband data into four or more different RF carrier signals. We are referring to four carrier signals because in this context, a symbol is made up of two bits or more, and two bits might represent four distinct signals. Such modulation systems are referred to as M-ary modulation.
Depending on whether the amplitude, phase, or frequency is changed, the modulation is referred to as M-ary ASK, M-ary PSK, or M-ary FSK. Because M-ary modulation techniques increase bandwidth efficiency, they are appealing for usage in bandlimited channels. Since a physical channel's bandwidth is constantly constrained, an 16-QAM system, for instance, uses the channel log16 (base 2) = 4 times more effectively than a ASK (also known as BASK) system.
To transfer signals in the form of symbols and to enhance the bit rate, M-ary PSK and M-ary QAM are both utilised. You can obtain multiple prior data rates if you send a symbol rather than a single bit at a time. To multiplex data, those mary modulation techniques are employed.
16-QAM ==>4N ('data rate' is 4 times as compared to binary ask, fsk, or psk)
32-QAM ==>5N
64-QAM ==>6N
128-QAM ==>7N
256-QAM ==>8N
32-QAM ==>5N
64-QAM ==>6N
128-QAM ==>7N
256-QAM ==>8N
Fig 1: Constellation Diagram of 4-QAM (Transmitted)
Fig 2: Constellation Diagram of 4-QAM (Received thru noisy channel)
MATLAB Code for BER vs SNR for M-ary QAM
(Get MATLAB Code)
Read about the Constellation Diagrams of ASK, FSK, and PSK
Also Read about Constellation Diagrams of M-ary PSK