3 High-speed subscriber lines according to G.fast standard/div>
3.2 Utilization of phantom circuits

Phantom circuits previously used for long-distance telephone lines to increase transmission capacity. The principle is to establish a signal to transformer centers of two lines and thus create additional circuit. The use of this principle is limited towards increasing frequency impossibility of precision balancing symmetrization transformer and increasing crosstalk between the majors and the phantom circuit. Mentioned phenomena can be eliminated by suppressing of crosstalk through modulation VDMT and achieve e.g. summary speeds up to nearly 500 Mbit/s in both directions at a length of cable 400 m.

Principle of phantom circuit

Transmission speeds for high-speed xDSL systems modeled on a cable with a length of 400 m
Model for length of cable 400 m | Frequency plan VDSL2 up to 17 MHz | Frequency plan VDSL2 up to 30 MHz
downstream [Mbit/s] | upstream [Mbit/s] | downstream [Mbit/s] | upstream [Mbit/s]
2 lines plus phantom | 150 | 67 | 176 | 80
2 lines | 166 | 92 | 226 | 118
2 lines plus phantom with crosstalks cancellation | 382 | 243 | 493 | 449

Outside the phantom circuits can be realized even more so called as superphantom with 4 lines. So you can get a total of 7 circuits: 4 basic, 2 phantoms and among them one superphantom.

Besides the phantom lines has done experiments with transmission in unison mode, where the individual wire pairs are taken as separate transmission paths. The resulting strong crosstalk can be compensated by also using VDMT modulation. This solution, however, an advantage to prepare symmetrical conduction, i.e. a significant resistance against ambient interference. For reasons of considerable mutual interference between the wires and the environment, therefore this kind of practice did not catch on.