# Complex time domain signals and QAM

Is there an inherent connection between complex time domain signals and QAM?

Given a complex time-domain signal is it always implied that this is a QAM-demodulated form of a real time domain signal?

Are there any other commonly used ways of converting a real time domain signal to a complex time domain signal that don't imply QAM?

• Please let me know if the question is too vague, not sure I formulated it correctly, I'll delete it.
– axk
Jan 10, 2020 at 18:46
• The question is unclear. Are you asking how to take the complex baseband signal and convert it to QAM symbols? Jan 10, 2020 at 20:03

Given a complex time-domain signal is it always implied that this is a QAM-demodulated form of a real time domain signal?

No, we generally use complex baseband to describe all kinds of signals, or even passband systems (e.g. frequency-selective or time-variant channels). So, any bandlimited signal has an equivalent complex baseband, and it's not implied this has anything to do with QAM or even digital communication.

QAM is just one example of something that is really easy to write down in complex baseband.

Are there any other commonly used ways of converting a real time domain signal to a complex time domain signal that don't imply QAM?

I don't even know how QAM comes into this – you transform any real-valued bandpass signal to complex baseband through multiplication with its (complex) carrier and low-pass filtering. QAM really has nothing to do with it.

• When talking about converting a time domain real signal to a time-domain complex signal and QAM demodulation I refer to what an SDR like RTL-SDR would do by down-converting to baseband and multiplying with sin and cos to get the I and Q components, not sure this can be called QAM demodulation.
– axk
Jan 10, 2020 at 21:36
• um, no, QAM is usually used for something else. That's describing a square lattice of constellation points for a linear modulation. Jan 10, 2020 at 22:22
• thanks! I think the keywords I was looking for is "analytic signal", not QAM
– axk
Jan 10, 2020 at 22:37