Bandwidth-efficient QAM schemes for Rayleigh fading channels

Quadrature amplitude modulation (QAM) is a bandwidth efficient transmission method for digital signals. It is expected that the available spectrum for the proposed personal communications network (PCN) will soon be at a premium as the number of subscribers increases, and changing from binary modulation to QAM may significantly ease the problem. The severe amplitude and phase changes introduced by the fading channels, however, make low error transmission of QAM difficult to achieve, unless procedures are introduced at both the transmitter and the receiver to combat the fading. The authors describe ways to improve the bit error rate (BER) of QAM by using various forms of coding on an increased symbol set. This means that the data throughput, symbol rate and transmission power are unaffected, although the transmitter and receiver are made considerably more complex. They consider Rayleigh, rather than the less severe Ricean fading channel to get worst case performance estimates of mobile radio communications