  With the latest technology of vectoring, DSL data rates in the order of
100Mbps have become a reality that is under field deployment. The key is to
cancel crosstalk from other lines, which is also known as multiuser MIMO
cancellation for wireless communications. During the DSL system upgrade phase
of field deployment, mix of legacy and vectoring-enabled VDSL lines is
inevitable and a channel estimation solution for the entire mix is needed
before vectoring can be enforced. This paper describes a practical method for
crosstalk channel estimation for downstream vectoring, assuming that a
vectoring-enabled DSLAM forces DMT symbol-level timing to be aligned for all of
the lines, but also assuming that the location of synch symbols are aligned
only among vectoring-enabled lines. Each vectoring-enabled receiver is capable
of reporting error samples to vectoring-DSLAM. The estimation method is not
only practical, but also matches the performance of Maximum-Likelihood
estimator for the selected training sequences.
