  Self- and alien-FEXT disturbances on the same cable severely limit
performance in the commonly employed VDSL2 17a, i.e. 17.6 MHz, standard
profile. Then, in a multi-operator environment, VDSL2 may be unable to provide
the 100 Mbit/s speed required by the EC's policy target, unless vectoring is
adopted along with a suitable Multi-Operator Vectoring (MOV) technique. Some
vendors recently proposed enlarging the bandwidth up to 35.2~MHz (so-called
e-VDSL) as one possible solution to increase the 100 Mbit/s coverage. However,
as we show in the paper, the bandwidth advantage is illusory as an "ideal MOV"
is not implementable with today's technology, and alien-FEXT practically
destroys the envisaged data-rate increase. Therefore, we introduce the Sub-band
Vectoring (SBV) technique, as one practical solution, which turns out to be
near optimal in terms of achievable data-rate, as well as incremental for those
Countries where the NRA adopts sub-loop unbundling regulation. SBV adopts a
novel frequency division multiplexing approach, which imposes one fairness
condition to equalize data-rate values and overall bandwidth per operator at
different distances. The paper shows that e-VDSL with SBV is robust, as it can
cope with relatively high values of vectoring impairments. It also shows that
the SBV allows up to three co-located operators with e-VDSL to coexist, without
imposing any of the burdensome coordination limitations of the ideal MOV
architecture. Our results show that the SBV allows achieving up to 210 Mbit/s
per user for e-VDSL and up to 620 Mbit/s per user for a bandwidth expanded up
to 105.6 MHz, with two telecom operators. Finally, the paper shows that a
migration strategy towards usage of the G.fast standard thanks to SBV is
achieved in a simple way.
