QAM is a modulation technique where amplitude modulation is performed for two signals, one sine signal and another cosine signal. Since the signals are out of phase, the information in these signals is independent and can be fully recovered.
However, recovering the signals requires knowing the phase of the sender. If the phase is known incorrectly, it is for the receiver possible that the sine signal will be treated as cosine signal and the cosine signal will be treated as minus sine signal. Or worse, the signals can be mixed up.
How do practical implementations solve this problem and have the same phase in the sender and in the receiver?