IMPROVEMENTS IN, OR RELATING TO, MULTI-CARRIER TRANSMISSION SYSTEMS

摘要

The invention provides a multi-carrier transmission system, for example, a DMT based VDSL system, using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, said system including digital receiver and transmitter units, said receiver unit including a symbol detection unit, characterised in that said system is adapted to determining a parameter for each single carrier, said parameter being indicative of a deviation of a received signal from a corresponding constellation point; comparing the parameter with an upper and lower limit; and, if the parameter is outside said limits, changing the constellation used to modulate the carrier to a neighbouring constellation. The symbol detection unit may be used to determine the parameter which may be a ratio d<2>/ sigma <2>, where d is the shortest distance between neighbouring constellations, sigma is a standard deviation, and sigma <2> is the variance of the deviations of the input and output signal values of said symbol detection unit.