QAM stands for Quadrature Amplitude Modulation and is the most common modulation the modern radios use to encode information onto an RF wave.

The RF waves have three main properties we can control to encode information onto them the amplitude, frequency, and phase.

Digital communication systems work with 1s and 0s, which are easy to encode.

With amplitude modulation, a simple on-off switch is enough to encode 1 and 0.

With frequency modulation, signals with two different frequencies do the job.

And with phase modulation, it's shifting the signal by 180 degrees.

The QAM is a combination of amplitude and phase modulation.

Here is the simplest QAM modulator scheme.

At the input is digital data processed by groups of bits, or symbols.

At the other input of the modulator is the RF carrier signal.

These input signals are combined in the modulator which controls the amplitude and the phase of the resulting output signal, which lets us encode more information onto the signal compared to any other modulation alone.

The length of the symbol determines the QAM modulation depth and maximum number of symbols.

The symbols can be mapped into so-called constellation diagram.

The QPSK has 4 possible symbols with 2 information bits per symbol.

The 16-QAM has 16 symbols with 4 information bits per symbol.

64-QAM has 64 symbols, and so on.

Because of the noise, the transmitted symbols fluctuate around the ideal value.

The noise limit of QAM of given depth is given by the point where the acceptable noise level areas for each symbol start to overlap.

Once they do, the transmitted symbols within the overlapping area can be wrongly identified causing errors in data transfers.

With growing QAM depth, we packed more information into the same signal but also shrink the space between the symbols, which naturally decreases the acceptable noise level and is the reason why high MCS rates need high signal-to-noise ratio levels to work.

So considering the 802.11ac standard and 20 MHz channel, we need at least 5 dB SNR for

QPSK, 11 dB SNR for 16-QAM, 18 dB SNR for 64-QAM, and so on.

For practical use, the MCS rates of the 802.11ac standard combine information about the modulation depth, number of spatial channels, and coding rate, which says what part of the transferred data is user data.

For more interesting topics from RF, stay tuned to our social media and subscribe to our channel.