Fundamentals of Quadrature Amplitude Modulation

Quadrature amplitude modulation (QAM) is a widely used method for transmitting digital data over bandpass channels. It can be viewed as a generalization of PAM to bandpass channels. All current telephone line modems based on the ITU-T V series recommendations for transmission at rates of 2400 bps or more use QAM or include it as an option. These include recommendations V,22 through V.92. This series includes FAX modems. Recommendation V.90 modems normally use PAM in the downstream direction from the server to the client modem and always use QAM in the upstream direction from the client to the server. V.90 modems can choose to use QAM downstream if a digital link from the server to the codec in the local office on the client side does not exist. V.92 modems normally use PAM in the downstream and upstream directions but can choose to use QAM based on line conditions. QAM is also used in high speed cable, multi-tone wireless, 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).