3 High-speed subscriber lines according to G.fast standard
3.4 Vector Modulation – VDMT

DMT modulation is used by the xDSL for the most effective use of the potential of metallic symmetrical pairs and allows you to respond appropriately to the existence of narrowband interference in the frequency channel. For xDSL with DMT modulation, due to the nature of the transmission environment shared metallic cable, can further increase the transmission speeds achievable using so-called as vector modulation VDMT (Vectored DMT).

The dominant component of the interference, which limits the transmission speed of xDSL systems is crosstalk interference.

Crosstalk is signal transmission capacitive and inductive relationships between pairs. Thus a transmitted signal, for example, from the first pair to the other pair operates in the second pair of negative interference and decreases the SNR (Signal-to-Noise Ratio) and thus the resultant transmission rate. Given that prevent cross talk is not possible due to the structure of a copper cable and the properties of electromagnetic waves, it is necessary to eliminate the effects of crosstalk on the transmitted useful signal. Elimination of crosstalk ensure the improvement of the SNR and thus the higher the achievable transmission rate. According to the site crosstalk measurement, we distinguish between NEXT (Near End Cross Talk) and FEXT (Far End Cross Talk). NEXT crosstalk can be eliminated by using a frequency division transmission directions. Size of FEXT crosstalk is therefore crucial for xDSL connections.

Modulation principle of VDMT uses sophistication of mathematical apparatus describing the characteristics of symmetrical pairs of crosstalk and mutual ties between couples. It is possible to differentiate the direction of transmission for their elimination.

In the downstream direction VDMT exploits the fact that the device DSLAM is a common element for all xDSL connections operated in harness copper cable. Thus in the DSLAM we have a single point of information on DMT symbols to be sent in the next moment to stretch metallic lines (i.e. we vector values ​​of DMT symbols - hence vectored modulation DMT). Given that in the DSLAM also known parameters of the various symmetrical pairs of crosstalk and the links between them (known is a process of establishing a connection), it is possible to modify the transmitted DMT symbols with respect to transmission media, which will be transmitted. On the receiving side modem end user will then have the symbols theoretically ideal course and parameters suitable for error-free detection. Problems synchronization transmission is also easily manageable since it is the central element of the DSLAM. Synchronization of transmission of DMT symbols is needed. Symbols are transmitted at each other through crosstalk bonds have a defined cause, as calculated during their preparation.

In the upstream direction it is not possible to adjust the transmitted symbol in the terminal equipment of a subscriber. That would place increased demands on equipment and computational efficiency of terminal equipment. In addition, the terminal equipment can never have information about the symbols that are broadcast by other connections in metallic cable. Therefore, it is necessary to modify only the received signal again in the DSLAM (the central element) using a special block crosstalk canceller. An important question is also necessary to synchronize the transmissions from the subscribers towards the end DSLAM with respect to different length loops.

VDMT is an extension of DMT modulation in a multi-user environment, that solves the problem of MIMO type and type eliminates crosstalk FEXT. Crosstalk NEXT crosstalk is eliminated by frequency division transmission directions.

With regard to co-location of modems in DSLAM it can provide synchronous broadcast DMT symbols in the downstream direction. Ensure synchronous transmission of symbols from the users to the access multiplexer DSLAM is due to the different lengths of the individual connections more complicated, but can use a method such as FDD Zipper.

Synchronization in the upstream direction is parallel to the procedures for medium access control in a shared environment point - a lot of points, such as passive optical networks and radio networks LDMS (Local Multipoint Distribution System). Broadcasting in subscriber modems must be timed with respect to different times of propagation of signals along different lines from remote participants.

FDD Zipper method uses CS (Cyclic Suffix). This is unlike the CP (Cyclic prefix) is inserted per DMT symbol. The length of CS must be equal to or greater than the maximum difference in propagation delay of the signal channels. FDD Zipper method also eliminates the residual crosstalk NEXT. The disadvantage of this method is a reduction of transmission rate.

The principle of synchronization of symbols at xDSL connection in the upstream direction.