  For xDSL systems, alien noise cancellation using an additional common mode
sensor at the downstream receiver can be thought of as interference
cancellation in a Single Input Dual Output (SIDO) system. The coupling between
the common mode and differential mode can be modelled as an LTI system with a
long impulse response, resulting in high complexity for cancellation. Frequency
domain per-tone cancellation offers a low complexity approach to the problem
besides having other advantages like faster training, but suffers from loss in
cancellation performance due to approximations in the per-tone model. We
analyze this loss and show that it is possible to minimize it by a convenient
post-training "delay" adjustment. We also show via measurements that the loss
of cancellation performance due to the per-tone model is not very large for
real scenarios.
