I am a bit confused about QAM modulation. In wikipedia, I found this:
It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves of the same frequency are out of phase with each other by 90°, a condition known as orthogonality or quadrature.
So for example, I can imagine having two carrier signals, lets say one sine wave and one cosin wave, which are modulated either by an analog baseband message signal (analog QAM), or by a digital message signal (digital QAM).
However, in another article, I found this:
The QAM transmitter first encodes bits into complex QAM symbols, which become the complex amplitudes of baseband pulses. The baseband QAM signal then modulates a digital RF subcarrier. The digital QAM signal is finally converted to an analog drive signal by a high-speed DAC.
Also, I could also find several references linking QAM to two PAM modulations. So here are my questions:
- Are these two different things which happen to have the same name? My understanding is that, in the wikipedia definition, QAM has nothing to do with pulses.
- What exactly is the meaning of "baseband QAM signal" in the second quote? Are PAM signals also baseband? My understanding is that we want to modulate a signal in order to transfer it to a higher (carrier's frequency) and to allow better communication (multiplexing, less interference, etc.).
Link to paper where I took the second reference.
Link of following quote about PAM and QAM. What confuses me is that PAM is a pulsed signal, and QAM is a sinusoid. How exactly are they associated?
To simplify the basic concept, one can think of a 16-QAM signal as being the Cartesian product of two PAM4 signals.