Reduced state equalization of multilevel turbo coded signals

In this paper, a novel type of equalization technique, called "double decision feedback equalizer" (DDFE) is applied to multilevel turbo codes (MLTC) to improve BER performance, and the entire system is called "multilevel turbo equalization" (MLTEQ). The parallel input data sequences are encoded at each level by turbo encoders, and then the coded data sequences are mapped to M-PSK signals, where M depends on the level quantity. After these modulated signals are passed through severe ISI and fading channels, the corrupted signals are equalized by an innovative iterative double decision feedback equalization technique, which uses an adaptive LMS algorithm to estimate the channel taps with a new equalization design. Then, the equalized signals are sent to the turbo decoders. The performance of new proposed MLTEQ system is investigated under non-frequency selective fading and frequency selective fading channels. As an application, two level turbo codes are simulated using 4-PSK modulation over AWGN, Rician, Rayleigh and Proakis B channels with 800 frame sizes. The simulation results show that satisfactory performance improvement is obtained with the proposed system over severe ISI and non-frequency selective fading channels.