Turbo-per-Tone Equalization for ADSL Systems
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Turbo-per-Tone Equalization for ADSL Systems Hilde Vanhaute ESAT/SCD, Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium Email: [email protected]
Marc Moonen ESAT/SCD, Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium Email: [email protected] Received 9 October 2003; Revised 27 August 2004 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, increases the achieved bitrate by 5–10%. We show that the application of the turbo principle to per-tone equalization can provide significant additional gains. In the proposed receiver structure, referred to as a “turbo-per-tone equalization” structure, equalization and decoding are performed in an iterative fashion. Equalization is done by means of a linear minimum mean squared error (MMSE) equalizer, using a priori information. 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 12–16% compared to the original per-tone equalization-based receiver structure. Keywords and phrases: ADSL, multicarrier modulation, turbo equalization.
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INTRODUCTION
Discrete multitone (DMT) modulation has become an important transmission method, for instance, for asymmetric digital subscriber line (ADSL), which provides a high bit rate downstream channel and a lower bit rate upstream channel over twisted-pair copper wire. DMT divides the available bandwidth into parallel subchannels or tones, which are quadrature amplitude modulated (QAM) by the incoming bit stream. After modulation with an inverse fast Fourier transform (IFFT), a cyclic prefix is added to each symbol. If the channel impulse response (CIR) order is less than or equal to the cyclic prefix length, demodulation can be implemented by means of an FFT, followed by a (complex) 1-tap frequency-domain equalizer (FEQ) for each tone to compensate for the channel amplitude and phase effects. A long prefix however results in a large overhead with respect to the data rate. An existing solution for this problem, currently used in ADSL, is to insert a (real) T-tap time-domain equalizer (TEQ) before demodulation to shorten the channel impulse response. Many algorithms have been developed to initialize the TEQ (e.g., [1, 2, 3]). However a general disadvantage is that the TEQ equalizes all tones simultaneously and as a result limits the performance. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
As an alternative to time-domain equalization, per-tone equalization (PTEQ) is proposed in [4]. The equalization is now carried out in the
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