Most of the radios Wireless Internet Service Providers use in their networks utilize Quadrature Amplitude Modulation (QAM). How does it work? Why higher MCS rates need higher signal to noise ratio? Find out in this Inside Wireless episode! Learn more: https://rfelements.com/
QAM stands for Quadrature Amplitude Modulation and it’s the most common modulation modern digital radios use to encode information onto RF waves.
The RF waves have three main properties we can control to encode information on them. Amplitude, frequency, and phase. Digital communication systems work with ones and zeros 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. With phase modulation, shifting the signal by 180 degree does the trick.
QAM modulation is a combination of amplitude and phase modulation. At the input of QAM modulator is digital data processed by symbols, or groups of bits at once. 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 let’s us encode more information onto the signal compared to any 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 which helps us easily understand the QAM modulation - check the video to see the constellation diagram.
With growing QAM depth, we pack more information into the same signal but also increase the demand on acceptable SNR level - the deeper the modulation, the higher required SNR.
Considering the 802.11ac standard and 20 MHz channel, we need at least 5 dB SNR for QPSK, 11 dB SNR for 16QAM, 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.
0:00 Intro
0:22 Modulation types
0:48 QAM modulation
1:22 Constellation diagram & QAM noise immunity
2:31 MCS rate explanation
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