Quadrature amplitude modulation (QAM) is a modulation scheme in which two sinusoidal carriers,

one exactly 90 degrees out of phase with respect to the other, are used to transmit data over

a given physical channel. One signal is called the I signal, and the other is called the Q signal.

Because the orthogonal carriers occupy the same frequency band and differ by a 90 degree phase

shift, each can be modulated independently, transmitted over the same frequency band, and

separated by demodulation at the receiver. For a given available bandwidth, QAM enables data

transmission at twice the rate of standard pulse amplitude modulation (PAM) without any degradation

in the bit error ratio (BER).