Use 1,000x more spectrum, get 1,000 more speed. The proposed terabit DSL can use 300 GHz+ of spectrum, "submillimeter waves." Current DSLs use 100-500 MHz. Higher frequencies just wouldn't make it through a copper wire. The signals get weaker (attenuate) very quickly in copper. 500 MHz can only carry about 30 meters. Gigahertz, even less.
Cioffi proposes using the space between the plastic insulated wires as a "waveguide." Fiber optic, glass or plastic, "guides" the waves; why couldn't the plastic insulation do similar? (The waves are very, very small. They can easily fit in the gaps.) The signal is carried through the air between the wires, not on the copper wires..
5G millimeter wave wireless runs at 26 GHz (Europe,) 28 GHz (U.S.) and 39 GHz. WiGIG is 60 GHz and commercial microwave often 70 GHz - 90 GHz. 300 GHz and 500 GHz is only used in a number of research labs and perhaps military applications. It's not available commercially, one reason building a demonstration unit will be a major task.
Multiple miniature antennas will direct the beam, in a way similar to massive MIMO. They will require massive processing power. One researcher believes the latest generation of GPUs might be sufficient. Others assume another iteration of Moore's Law will be necessary. Fortunately, Moore's Law has at least two more steps. Many, including Marconi Prize winner Henry Samueli, believe the future of Moore's Law is cloudy beyond the 5 nanometer generation.The chips will be approaching the size of single atoms.
Cioffi's presentation is at http://www.assia-inc.com/wp-content/uploads/2017/05/TDSL-presentation.pdf. Co-authors are Ken Kerpez, Chan Soo Hwang, and Ioannis Kanellakopoulos. He's shared it with numerous respected engineers. They tell me they are impressed, although nothing is certain until the hardware for testing is built. Several dozen engineers have reviewed the work,some identifying pratcial problems that would need to be solved. No one, as far as I know, found fundamental flaws in the theory.
In 1993, the best engineers in the field believed DSL modems could not go beyond 1.5 megabits. John Cioffi invented DMT line coding and delivered a modem running at 6 megabits down. Two hundred million homes are now connected with DMT ADSL A few years later, John again went beyond the state of the art, using Dynamic Spectrum Management to deliver more reliable speeds over DSL. In 2002, he introduced "vectoring," essentially noise cancelling for DSL. The chips of the day couldn't keep up with the necessary calculations. John hoped that chips would catch up by 2010; in fact, it took until 2013. Since then millons of lines have shipped and tens of millions are promised.
That's three near-miracles Cioffi has delivered. He's hoping for a fourth.