Fundamentals of Quadrature Amplitude Modulation

Abstract

Quadrature amplitude modulation (QAM) is a widely used method for transmitting digital data over bandpass channels. It can be considered to be a generalization of PAM to bandpass channels. All current telephone line modems based on the CCITT V series recommendations for transmission at rates of 2400 bps or more use QAM. These include fax modems. QAM is also used in highspeed cable, microwave, and satellite systems. It is a popular choice because it uses bandwidth efficiently and linear channel distortions can be corrected by adaptive equalization at the receiver. In addition, QAM fits in nicely with a popular combined coding and modulation scheme called trellis coded modulation (TCM) and a method for selecting transmitted symbols known as shell mapping, which is included in the ITU (the new name for CCITT) V.34 modem recommendation, whose final draft was approved in June 1994.