  In xDSL systems, crosstalk can be separated into two categories, namely
in-domain crosstalk and out-of-domain crosstalk. In-domain crosstalk is also
refered to as self crosstalk. Out-of-domain crosstalk is crosstalk originating
from outside the multi-pair system and is also denoted as external noise (alien
crosstalk, radio frequency interference,...). While self crosstalk in itself
can easily be canceled by a linear detector like the ZF detector, the presence
of external noise requires a more advanced processing. Coordination between
transmitters and receivers enables the self crosstalk and the external noise to
be mitigated using MIMO signal processing, usually by means of a whitening
filter and SVD. In this paper, we investigate the problem of finding the
optimal power allocation in MIMO xDSL systems in the presence of self crosstalk
and external noise. Optimal Tx/Rx structures and power allocation algorithms
will be devised under practical limitations from xDSL systems, namely per-modem
total power constraints and/or spectral mask constraints, leading to a
generalized SVD-based transmission. Simulation results are given for bonded
VDSL2 systems with external noise coming from ADSL2+ or VDSL2 disturbing lines,
along with a comparison between algorithms with one-sided signal coordination
either only at the transmit side or the receive side.
