QAM

6/16/2001

QAM: Quadrature Amplitude Modulation. A technique for doubling of transmission bandwidth without requiring higher bandwidth components. In basic QAM, two amplitude modulated digital or analog signals are modulated 90 degrees out of phase at the same frequency. The two channels are called I and Q. They are combined at the transmitter and separated at the receiver. The resulting signal is capable of carrying twice as much data as a single AM channel in the same bandwidth. The peak power required is only twice the power required for either channel alone because maximum power in one channel coincides with minimum in the other. The average power is lower than two separate channels because the signals sometimes partially cancel each other.

In practice, AM is often combined with phase shift keying to produce rather complex waveforms. Provided that the phase shifts are confined to 90, 180, 270 and 0 degrees, the signals can be considered to be a form of QAM. A commonly used encoding technique that uses four phase variations and two amplitude possibilities to carry any of eight values per carrier cycle is often called QAM or QAM8. Similar techniques can be used with non-quadrature phase shifts to get higher data densities. These may sometimes be called QAM even though there may be no actual quadrature component.

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