Signal Processing for Vectored Multichannel VDSL

Vectored DSL communication systems fall into the category of multiple input multiple output (MIMO) communication systems. As such, they can take advantage of the various MIMO techniques and the vast volume of MIMO research that has been published in recent years. Nevertheless, DSL systems have two unique features that significantly simplify the implementation of MIMO techniques, and hence gave rise to specific DSL research and specific DSL signal processing techniques. These characteristics are the very large coherence time and the diagonal dominance structure of the channel matrix. The diagonal dominant channel matrices have been shown to significantly simplify the interference cancellation between different DSL users. This simplification is crucial to allow MIMO techniques to be applied at the high data rates achievable by modern DSL systems over many lines sharing the same binder. The large coherence time results from the static nature of the communication lines, and also from the continuous activity of DSL links. The very long coherence time allows receivers to obtain very good channel estimates, or alternatively to apply adaptive MIMO techniques that can adapt to the specific channel matrix. In this paper we review the specific DSL research, that has taken advantage of DSL system characteristics and paved the way for very high rate vectored DSL systems.