Fine-Granularity Loading Schemes Using Adaptive Reed-Solomon Coding for xDSL-DMT Systems
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Fine-Granularity Loading Schemes Using Adaptive Reed-Solomon Coding for xDSL-DMT Systems Saswat Panigrahi and Tho Le-Ngoc Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montr´eal, QC, Canada H3A 2A7 Received 29 November 2004; Revised 9 May 2005; Accepted 22 July 2005 While most existing loading algorithms for xDSL-DMT systems strive for the optimal energy distribution to maximize their rate, the amounts of bits loaded to subcarriers are constrained to be integers and the associated granularity losses can represent a significant percentage of the achievable data rate, especially in the presence of the peak-power constraint. To recover these losses, we propose a fine-granularity loading scheme using joint optimization of adaptive modulation and flexible coding parameters based on programmable Reed-Solomon (RS) codes and bit-error probability criterion. Illustrative examples of applications to VDSL-DMT systems indicate that the proposed scheme can offer a rate increase of about 20% in most cases as compared to various existing integer-bit-loading algorithms. This improvement is in good agreement with the theoretical estimates developed to quantify the granularity loss. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
Discrete multitone (DMT) modulation [1] has been widely used in xDSL applications such as asymmetric DSL (ADSL) [2] by the American National Standards Institute (ANSI) and the European Telecommunications Standard Institute (ETSI) and more recently for VDSL [3] by ANSI. Loading strategy is used for dynamic subcarrier rate and power allocation for given channel conditions, system constraints, and performance requirements. For a multichannel total-power constrained problem, the optimal power distribution has long been known to be the “water-filling” distribution [4]. However the derivation tacitly assumes infinite granularity while most of the known modulation schemes support the integer number of bits per symbol. It was initially observed in [5, 6] that most of the granularity losses due to the integer number of bits per symbol could be recovered by rounding off rates to integers and scaling energies accordingly after starting with a water-filling [6] or flat on/off [5] energy distribution. However the freedom for such rescaling is considerably reduced in the presence of peak-power constraint. Peak-power constraint [7, 8] arises from spectrum compatibility requirements to enable coexistence among multiple users and diverse services. When the peak-power constraint is far stricter than the total-power constraint, as is often the case in VDSL-DMT, there is hardly any room left for maneuverability (or rescaling) in the energy domain (to recover
lapses in the bit-domain) and significant losses in achievable data rates of integer-bit algorithms are observed. These losses accounting to be a significant percentage of the supported information rate compel us to tackle the integer-bit granularity problem through bit-error-rate-based jo
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