DSL data rates and transmission distances are limited by signal impair-
ments inside twisted copper pairs [2]. The attenuation of electromagnetic waves
on copper wires increases as the square root of the signal frequency. Therefore,
higher frequency signals attenuate faster on twisted pairs. The aforemen-
tioned loading coils on old twisted pair plants must be removed in order to
offer DSL services. Moreover, copper wire quality, bridge taps on twisted pairs,
and cross talk between neighboring twisted pairs all degrade the signal
quality.
Most of the DSL technologies today use a modulation technique called
discrete multitone modulation (DMT), which divides the whole frequency
bands into 247 channels of 4-kHz slots. Signal quality in each slot is constantly ...