Methods described in the Spectrum Balancing section essentially assume that the different line signals are uncoordinated. Uncoordination is the practice in current DSL systems because different service providers independently place their modulated physical-layer signal directly on copper pair lines leased from the incumbent local service provider at the central office. Instead, DSL will likely evolve to fiber-fed remote Digital Subsciber Line Access Multiplexers (DSLAMs) where service providers can lease bit streams from a single common carrier who manages all signals on the cable. In this case, DSL can consider the channel as a Multi-Input-Multi-Output (MIMO) system and employ joint signal processing over all the signals at either the receiver side or the transmitter side. Such methods will lead to better performance than the uncoordinated case.
Our research shows that application of a vectored Discrete Multi-Tone (DMT) system delivers considerable benefits in DSL systems constrained by far end crosstalk (FEXT). An example of such a system is Very-High-Speed DSL (VDSL). VDSL uses large bandwidth to deliver high speed content over short loops, which results in a severe FEXT problem. Note that as DSL services evolve, line lengths will get shorter, bandwidths will increase and more and more local loops will fall under this scenario. Our T1E1.4 contribution "Example improvements of Dynamic Spectrum Management" gives a few examples validating our claims. Also refer to our other T1E1.4 contributions to see how these schemes fit in the overall view of DSL evolution.
detailed information on Vectored DMT for VDSL and associated issues please
refer our journal and conference publications on
vector transmission methods.