Quadrature Amplitude Modulation

In an effort to better understand other methods of signal modulation, I decided to read up on Quadrature Amplitude Modulation. QAM is a method of transmitting a signal in a more efficient way. While Amplitude Modulation varies the amplitude of a signal to encode data, Frequency Modulation changes the frequency of the signal, and Phase Modulation modifies the phase, Quadrature Amplitude Modulation uses a mix of two of the three. It uses two carrier waves, a sine and cosine (in other words, one wave phase shifted by 90 degrees), and modulates using both amplitude and phase variations. A scheme like this is more efficient than simply Amplitude Modulating a signal. By having two phase shifted carrier waves, there is another channel to transmit information. It can also allow multiple bits to be encoded per symbol, which allows for schemes like 16QAM. It's not without drawbacks, though - things can get pretty confused if there is too much noise. Still, a scheme like this is pretty cool - QAM is even used for LTE for phones.

An example of bit sequencing mapping (16QAM).


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