What is QAM?
[top] Quadrature amplitude modulation (QAM) is a method for encoding data on a single carrier frequency. The modulation encodes data (or bits) as discrete phase plus amplitude changes of a carrier tone. The phase vectors are arranged in a pattern of points called a constellation from which the transmitted point is selected based on the data to be sent. The modem sends the symbols as abrupt changes in phase and amplitude, but only as what emerges from a sharp cutoff filter which carefully limits the bandwidth. The transmitted signal occupies slightly more than 1/2 the modulation rate either side of the carrier frequency. The excess bandwidth, perhaps as much as 10%, is required for recovering symbol timing within the remote receiver. The receiver has to pick which point was transmitted with great reliability. It may employ adaptive equalization or other methods to reduce inter-symbol interference to levels which are acceptable for discriminating the received point. The background noise
