Turbo per tone equalization for ADSL systems

In this paper, we study the equalization procedure in discrete multitone (DMT)-based systems, in particular in DMT-based ADSL systems. Traditionally, equalization is performed in the time domain by means of a channel shortening filter. Shifting the equalization operations to the frequency domain, as is done in per tone equalization K. Van Acker et al. (2001), increases the achieved bitrate with 5-10%. We show that the application of the turbo principle in this per tone equalization can provide significant additional gains. Optimal equalization methods for minimizing the bit error rate use maximum a posteriori (MAP) estimation algorithms, which suffer from high computational complexity for large signal constellations and long channel impulse responses. In this approach, as in M. Tuchler et al. (2002)|, the MAP equalizer is replaced by a linear equalizer, based on the minimum mean squared error (MMSE) criterion. We give a description of an efficient implementation of such an equalizer in the per tone structure. Simulations show that we obtain a bitrate increase of 14-17% compared to the original per tone equalization based receiver structure.