Variable rate DFE-QAM and OFDM systems for maximum capacity in multipath frequency selective fading channels

Two adaptive modulation systems to counteract frequency and time selective fading in digital transmission over multipath radio channels are analyzed: a single carrier QAM system with decision feedback equalization (DFE-QAM) and an OFDM system. In order to maintain a target BER, an adaptive modulation level of the single-carrier in the DFE-QAM system, or each subcarrier in the OFDM system, which tracks the channel fluctuations is used. The spectral efficiencies of both systems are compared to the theoretical channel capacity. Various impairments such as inaccurate or delayed channel estimation and a limited-rate feedback channel have been considered.