Discrete multitone

Discrete Multitone Transmission ( DMT ) is the name of the modulation method used in ADSL and VDSL2 .

Procedure

DMT is a multi-carrier process, it divides the frequency band assigned to it into many sub-channels. With ADSL there are up to 256 carrier frequencies for data, each with a bandwidth of 4.3125 kHz. The bit information is modulated onto the individual carriers via QAM such as 4-QAM (QPSK) or 16-QAM. The serial data stream that is to be transmitted is combined into a number of bits at DMT and mapped onto complex sub-symbols that are sent in parallel on these carriers. To do this, they are simultaneously modulated onto the available carriers, whose sum signal is then sent.

DMT is based on the same principles as orthogonal frequency division multiplexing (OFDM). In contrast to OFDM, with DMT the individual carriers can be operated with different spectral efficiency . In other words, with the same symbol rate for all carriers, the number of bits coded per symbol per carrier can be changed. For example, a 16-QAM with 4 bits per symbol can be used in channels with a good signal-to-noise ratio (SNR), whereas only a 4-QAM with 2 bits per symbol is used on carrier frequencies with poor SNR. In extreme cases with very poor SNR, individual carriers can also be blocked completely. It must be negotiated which channels use which bit rate (in this case e.g. 128-QAM) or are not used, since constellation diagrams are not compatible with lower ones and even if they were, the purpose of error avoidance would not be fulfilled. If negotiation is no longer possible (e.g. due to a very bad SNR), the connection is broken off and re-established. When connecting, a low bit rate is often used first to avoid errors when establishing the connection.

Another advantage of DMT is that the carriers can be set to different levels of transmission power. This makes it possible to compensate for the non-ideal frequency response of specific telephone lines .

Application DSL

The frequency range for these carrier frequencies (also called bins - from English bin = container, to bin = store) begins at the lower end above the cutoff frequency of the splitter . The German Telekom uses in its splinters because of 4B3T line codes of the ISDN - basic access a cutoff frequency of 138 kHz ( Annex B ), so the bins are generally not used below 33rd This simplifies handling (only one type of splitter), but it wastes transmission capacity, since the larger bandwidth of ISDN is always kept free for telephony, even with analog connections that require less bandwidth for telephony. If an analog connection is interconnected with ADSL, a number of relatively low-frequency carriers could also be used for broadband transmission, which would increase the range by around half a kilometer with otherwise the same line ( Annex A ).

With ADSL the frequency range ends with bin 255 at 1.104 MHz, with ADSL2 + at 2.2 MHz and with VDSL2 (the current DSL advancement status), in which DMT is also used, at 30 MHz. The directional separation is usually regulated using a frequency division multiplex method: the lower bins are used for upstream transmission, the upper bins for downstream transmission.

With ADSL, a pilot tone is transmitted at 414 kHz (channel 96) .

literature

• John G. Proakis, Masoud Salehi: Digital Communications . 5th edition. McGraw-Hill, 2008, ISBN 978-0-07-126378-8 .