Point to Multipoint Channel Estimation by "Abuse" of Receivers

With increasing bandwidths and decreasing loop lengths, crosstalk becomes the main impairment in VDSL systems. For downstream communication, crosstalk precompensation techniques have been designed to cope with this issue by using the collocation of the transmitters. These techniques naturally need an accurate estimation of the crosstalk channel responses. This paper investigates the issue of estimating and tracking these channels. It was previously thought that efficient estimation schemes must use some part of the upstream or downstream bit rates. Moreover, it was assumed that the training signals and their duration must be coordinated between the collocated transmitters and the receivers. In this paper, a new algorithm is presented that practically needs no real-time coordination between the transmitters and the receivers and only needs a very narrow-band control channel between the transmitters and the receivers. The principle is to define two different types of precoding matrices, namely estimation precoding matrices (EPM) for estimation and tracking and cancellation precoding matrices (CPM) for the actual FEXT precompensation. During the estimation period the information symbols are modulated by the estimation precoding matrices, which cause the transmitted symbols to be slightly distorted. These distorted signals "abuse" the adaptive filters at the receivers, and cause the filters to converge to parameters that are functions of the crosstalk channel responses. The proposed estimation algorithm allows upgrading of SISO VDSL systems into FEXT cancelled systems by upgrading the transmitters alone without replacing the existing receivers.