Adaptive QAM modulations with complex spreading for high-speed mobile multimedia communications

This paper considers an adaptive QAM modem with complex spreading and noncoherent detection combined. The adaptation is performed by varying the modulation level of QAM, depending upon received signal strength which reflects instantaneous channel conditions. The adaptation mechanism allows a 2- or 3-bit increase at a time, if the channel condition is considered to be good. Simulation results show that, for a flat fading channel, the effect of spreading was minimal and the average number of bits per symbol (ABPS) for each symbol block transmitted is higher than 5.0 with a symbol rate of 32 kbaud. In case of a frequency selective fading channel, two approaches, complex spreading and an oversampling technique, were compared. It was found that the oversampling technique outperforms complex spreading. ABPS for frequency selective fading channels is approximately 5.5 with a SNR value of 40 dB.