Turbo coded modulation in ADSL DMT systems.

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Title: Turbo coded modulation in ADSL DMT systems.
Authors: Zhang, Liang.
Date: 2002
Abstract: A&barbelow;symmetric d&barbelow;igital s&barbelow;ubscriber l&barbelow;ine (ADSL) technology provides transport of high-bit-rate digital information over digital subscriber lines (i.e., common telephone lines), which were initially engineered to carry a single voice signal with a 3.4 kHz bandwidth. Sophisticated digital transmission techniques have to be utilized to overcome digital subscriber line transmission impairments due to the high signal attenuation, i&barbelow;nter-s&barbelow;ymbol i&barbelow;nterference (ISI), c&barbelow;rosst&barbelow;alk noise (XT) from the signals present on adjacent wires, signal reflections, radio-frequency noise, and impulse noise. In current ADSL standard, a multicazrier modulation technique named d&barbelow;iscrete m&barbelow;ultit&barbelow;one (DMT) is used to deal with the impairments caused by heavy ISI and signal reflections. A concatenated channel coding structure is employed to efficiently increase the transmission capability in the presence of strong additive crosstalk noise. This channel coding scheme consists of an inner 4&barbelow;-D&barbelow;imensional T&barbelow;rellis C&barbelow;oded M&barbelow;odulation (4D-TCM) and an outer R&barbelow;eed-S&barbelow;olomon (RS) code separated by an interleaver. In this thesis, we investigated the potential transmission performance improvement by using Turbo code in ADSL DMT systems. To achieve this, we first proposed a turbo coding structure that fits well in the current ADSL DMT system architecture. This structure is very similar to the existing concatenated channel coding structure, except a bandwidth efficient T&barbelow;urbo T&barbelow;rellis C&barbelow;oded M&barbelow;odulation (TTCM) is used instead of the 4D-TCM. To obtain high bandwidth efficiency, only part of the bits are turbo coded and the uncoded bits are protected by a constellation mapping similar to Ungerboeck's set partitioning in TCM. The proposed TTCM was shown to provide extremely good performance when a S&barbelow;ymbol M&barbelow;aximum a&barbelow; p&barbelow;osteriori (SMAP) decoding algorithm is employed. In spite of the good performance of this SMAP algorithm, we proposed to use a simplified TTCM decoding structure to avoid the excessive decoding complexity introduced by the possibly large number of uncoded bits. The proposed turbo coded ADSL DMT system was simulated to evaluate the achievable transmission performance in a stationary noise environment and its performance was compared to that obtained by using the 4D-TCM. Simulation results showed significant performance improvements by using turbo codes.
URL: http://hdl.handle.net/10393/6308
CollectionTh├Ęses, 1910 - 2005 // Theses, 1910 - 2005
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