Designing Tone Reservation PAR Reduction
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Designing Tone Reservation PAR Reduction 1 Albin Johansson,2 and Per Ola Borjesson 1 ¨ ¨ Niklas Andgart,1 Per Odling, 1 Signal
Processing Group, Department of Information Technology, Lund University, Box 118, SE-221 00 Lund, Sweden AB, 126 25 Stockholm, Sweden
2 Ericsson
Received 20 December 2004; Revised 27 May 2005; Accepted 8 July 2005 Tone reservation peak-to-average (PAR) ratio reduction is an established area when it comes to bringing down signal peaks in multicarrier (DMT or OFDM) systems. When designing such a system, some questions often arise about PAR reduction. Is it worth the effort? How much can it give? How much does it give depending on the parameter choices? With this paper, we attempt to answer these questions without resolving to extensive simulations for every system and every parameter choice. From a specification of the allowed spectrum, for instance prescribed by a standard, including a PSD-mask and a number of tones, we analytically predict achievable PAR levels, and thus implicitly suggest parameter choices. We use the ADSL2 and ADSL2+ systems as design examples. Copyright © 2006 Niklas Andgart et al. 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.
1. INTRODUCTION With discrete multitone modulation (DMT) as the dominating modulation scheme in digital subscriber line (DSL) systems, there is a problem with high signal amplitudes. This is caused by several independent sequences adding up to a signal that approximately will adhere to a Gaussian distribution and is commonly referred to as a high peak-to-average ratio (PAR). Several methods have been presented to alleviate this problem [1–7]. We focus on the tone reservation method, which has been presented in [1, 2], with further improvements in [8– 12]. Following the constraints set up by the standards, the achievable performance is limited, and can be determined by mathematical analysis in combination with some sound engineering assumptions. Construction of a system where the designer is unaware of the limitations will likely lead to a severe violation of the power spectral density (PSD) mask, or to a worse performance than what could be expected. This is illustrated in Figure 1, where the target PAR level is of significant importance. Aiming at a too low PAR level will lead to a violation of the PSD, or to a much worse result if the PSD is somehow enforced. In this paper we explain this relationship and develop means to predict what can be done when applying tone reservation PAR reduction to a practical DMT system. The aim is to produce results that are valid without having to run extensive simulations for each individual case. Hence, we will look at a number of bounds and engineering approximations that will tell us what can be done in the complete system.
When a system designer is contemplating whether it is worthwhile to include PAR reduction in a system or not, it normally
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