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QAM-Quadrature Amplitude Modulation | 16-QAM, 64-QAM, 256-QAM

This page covers basics of QAM i.e. Quadrature Amplitude Modulation technique. It compares 16-QAM vs 64-QAM vs 256-QAM and mentions difference between 16-QAM,64-QAM and 256-QAM.

QAM stands for Quadrature Amplitude Modulation. 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. 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.


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. In 16-QAM each symbol represents 4 bits as mentioned in the constellation diagram above.
For example if the input is 1010 then the output is (-3-j*3)*KMOD.
Typically KMOD is 1/root (10) for 16-QAM.

In 64-QAM, each symbol is represented by 6 bits and 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.

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 64-QAM 256-QAM
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)

Applications

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

What is Difference between QAM, BPSK, QPSK, OQPSK

512QAM vs 1024QAM vs 2048QAM vs 4096QAM
BPSK vs QPSK -Difference Between BPSK and QPSK modulation techniques.
QPSK vs OQPSK vs pi/4QPSK-Difference between QPSK,OQPSK and pi/4QPSK modulation techniques

MATLAB CODE

BPSK QPSK 16QAM 64QAM modulation matlab code

Modulation Related Useful Links

what is modulation
MSK and GMSK modulation
8-PSK modulation
QPSK modulation
BPSK modulation
BPSK vs QPSK -Difference Between BPSK and QPSK modulation techniques.
QPSK vs OQPSK vs pi/4QPSK-Difference between QPSK,OQPSK and pi/4QPSK modulation techniques
Differential Encoder and Decoder

RF and Wireless Terminologies


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