3 Advanced aspects of optical modulations in achieving high bit rates and immunity to signal degradation
3.8 VDMT – advanced vectored modulation – principle, application

The principle

Discrete Multi-Tone (DMT) is a multicarrier modulation. It is implemented by a DSP. Sub-channels widely use PSK or QAM, similarly to OFDM. On the contrary to OFDM, DMT however allows using different modulation schemes or even modulation types in each sub-channel of orthogonal multiplex.

Application

Vectored Discrete Multi-Tone (VDMT)

For downstream, VDMT in Digital Subscriber Line Access Multiplexer (DSLAM) is common for xDSL lines in a metallic cable.

  • In DSLAM there is information about all the symbols to be sent to the metallic line (there is a vector of the DMT symbols – the term “Vector DMT” is created after this fact).
  • In DSLAM, there is information about the parameters of particular symmetric pairs and crosstalk relations between them; it is then possible to adjust the DMT symbols based on the environment in which they are to be transmitted.
  • Synchronisation of all DMT symbols is necessary.
  • In the upstream direction, it is possible to adjust the transmitted symbols at the terminal equipment of a user.

Advantages

  • VDMT is extension of DMT, it solves the problem of Multiple Input Multiple Output (MIMO) processes and eliminates Far End Crosstalk (FEXT).
  • Near End Crosstalk (NEXT) is eliminated due to frequency multiplexing.