Partial Crosstalk Cancellation for Upstream VDSL

Abstract

Crosstalk is a major problem in modern DSL systems such as VDSL. Many crosstalk cancellation techniques have been proposed to help mitigate crosstalk, but whilst they lead to impressive performance gains, their complexity grows with the square of the number of lines within a binder. In binder groups which can carry up to hundreds of lines, this complexity is outside the scope of current implementation. In this paper, we investigate partial crosstalk cancellation for upstream VDSL. The majority of the detrimental effects of crosstalk are typically limited to a small subset of lines and tones. Furthermore, significant crosstalk is often only seen from neighbouring pairs within the binder configuration. We present a number of algorithms which exploit these properties to reduce the complexity of crosstalk cancellation. These algorithms are shown to achieve the majority of the performance gains of full crosstalk cancellation with significantly reduced run-time complexity

Joint spectrum management and constrained partial crosstalk cancellation in a multi-user xDSL environment

“…In a 8-user case, the observations reduce the number of crosstalk cancellation configurations from 2 56 to 2 6 . If there are 2 line cards, each having 4 lines, the number of crosstalk cancellation configurations is reduced from 2 24 to 2 5 .…”

“…If user n is cancelling crosstalk originating from user m on tone k, thenh n,m k = 0. We refer to [6] [7] where procedures are explained for cancelling individual crosstalk channels, based on particular DSL channel characteristics (row/column-wise diagonal dominance).…”

“…The cancellation of the crosstalk from one crosstalker on a tone is done by one cancellation tap [6] [7]. The number of cancellation taps that can be used is constrained by the cancellation tap constraint C tot [12].…”

“…Because most of the crosstalk originates from a limited number of lines on a limited number of tones, a fraction of this complexity suffices to cancel most of the crosstalk. This is called partial crosstalk cancellation [6] [7]. Crosstalk cancellation requires signal-level coordination at either the transmitter or receiver, i.e.…”

“…In [10], partial crosstalk cancellation based on resource allocation [6] is combined with Iterative Waterfilling (IW) spectrum management [11] in an iterative fashion. However, IW tends to be highly suboptimal in near-far scenarios.…”