Crosstalk cancellation in VDSL systems

The existing copper line telephone network is used successfully for very high speed digital subscriber line (VDSL) transmission systems. There is a major limitation in the resulting data rate by far-end crosstalk (FEXT), which can be mitigated by zero-forcing crosstalk cancellation procedures in upstream and precoding procedures in downstream. In order to obtain the channel state information (CSI) which crosstalk cancellation and precoding rely on, this paper presents a pilot-based channel estimation technique. Additionally a channel adaptation procedure is proposed, applying orthogonal pilot sequences. To avoid any pilot overhead, the pilots are continuously transmitted during the sync frames. In downstream, the precoder coefficient update is based on the feedback of the normalized error sample. The presented techniques lead to a higher efficiency and increased performance of VDSL systems. Simulations show that 95% of the data rate obtained with perfect CSI can be achieved in upstream and downstream.