On Combining Adaptive Modulation and Unbiased MMSE-DFE Receiver to Increase the Capacity of Frequency Selective Channels

In this paper, an adaptive quadrature amplitude modulation (AQAM) scheme for an equalized system over a selective channel is investigated. To reduce intersymbol interference (ISI), a minimum-mean-squared-error decision feedback equalizer (MMSE-DFE) is used. In order to select the appropriate modulation mode, the receiver estimates the MSE at the equalizer output. This estimated MSE is then sent back to the transmitter which adjusts the modulation level. Simulation results illustrate that the MSE is accurately estimated and represents a viable switching metric. It is also shown that switching between different modulation modes does not affect the equalizer coefficients adaptation. Linear region of practical transmitter amplifiers is limited. Thus when switching from one modulation scheme to an another, it is important to control the peak power.