16 QAM modulation vs 64 QAM modulation vs 256 QAM modulation

"The full form of QAM is Quadrature Amplitude Modulation technique. It is digital modulation technique. This modulation technique is a combination of both Amplitude and phase modulation techniques. QAM is better than QPSK in terms of data carrying capacity. QAM takes benefit from the concept that two signal frequencies; one shifted by 90 degree with respect to the other can be transmitted on the same carrier. -----FMUSER"

#QAM Applications

● CDMA
● WiMAX-16d, 16e
● WLAN-11a OFDM
● Satellite
● DVB
● Cable modem

For QAM, each carrier is ASK/PSK modulated. Hence data symbols have different amplitudes and phases.

S(t)= d1(t) cos(2*pi*fc*t)+ d2(t) sin(2*pi*fc*t)

#QAM
Figure mention the constellation points and encoding rule, which is taken from IEEE standard 802.16-2004 to demonstrate the 16-QAM concept.Let us compare 16 QAM vs 64 QAM vs 256 QAM modulation types and derive difference between 16-QAM modulation, 64-QAM modulation and 256-QAM modulation.

#16-QAM vs 64-QAM vs 256-QAM

As mentioned for each symbol both phase and amplitudes are varied to represent different bits. There are two levels of amplitudes for each phase i.e. d1 level and d2 level . There are many variants to this technique. 

● Most popular are 16-QAM, 64-QAM and 256-QAM. The example below explains 16-QAM. The 16 qam bits per symbol are 4 (four). This means in 16-QAM each symbol represents 4 bits as mentioned in the 16-QAM constellation diagram above.

See Also: >>QAM Theory and Formulas

For example if the input is 1010 then the output is (-3-j*3)*KMOD.

As we know in digital modulation, baseband is separated into in phase(I) and quadrature phase(Q) components. The combination of I and Q is known as baseband modulating signal. It is also referred as IQ diagram. The constellation diagram represent all the possible modulated symbols which will be used by modulation technique to map the information bits. This different symbols are represented in the complex plane with their amplitude and phase informations.

● The 64 qam bits per symbol are 6 (six). In 64-QAM, each symbol is represented by 6 bits as shown in the 64-QAM constellation diagram above. 

● In 256-QAM, each symbol is represented by 8 bits. As the level increases, QAM technique becomes more bandwidth efficient but it requires very robust algorithms in order to decode complex symbols to bits at receiver.

For example 256-QAM is complex than 16-QAM. QAM is more bandwidth efficient compare to BPSK but it is less robust. Hence for better CINR in the system QAM is employed which leads better data rate. For poor CINR, BPSK is employed. CINR stands for Carrier to Interference and Noise Ratio.

See Also: >>Comparison Of 8-QAM, 16-QAM, 32-QAM, 64-QAM 128-QAM, 256-QAM 

#Tabular difference between 16-QAM, 64-QAM and 256-QAM
Following table mentions difference between 16-QAM, 64-QAM and 256-QAM modulation techniques. The purpose of KMOD here is to achieve the same average power for all the mapped symbols (i.e. average power of 1).

Specifications	16-QAM modulation	64-QAM modulation	256-QAM modulation
Number of bits per symbol	4	6	8
Symbol rate	(1/4) of bit rate	(1/6) of bit rate	(1/8) of bit rate
KMOD	1/SQRT(10)	1/SQRT(42)	1/SQRT(170)

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