Block Siso Linear Equalizers for Turbo Equalization in Serial-tone Hf Modems

This paper considers the application of iterative equalization and decoding, also known as Turbo equalization, to high frequency (HF) serial-tone modems. The softin soft-out (SISO) equalizer required for this approach is based on a linear filter, since a trellis-based equalizer’s computational complexity is too large for this application. Linear equalization algorithms with SISO capability are rederived, including a new approach based on a minimum-mean-sum-of-squared-errors (MMSSE) criterion. The problem of incorporating channel estimation errors into the equalization algorithms is also considered. Simulations are presented for a intersymbol interference channel with or without fading, whose characteristics are time-invariant within one frame of data. In absence of fading, Turbo equalization exhibits gains up to 3,5 dB compared to a conventional decision feedback equalizer where no iterations are performed.