discrete multitone (modulation) (DMT)
The multi-tone technique (DMT) is a digital transmission method for wireline and radio data transmission with which the available bandwidth can be used efficiently and thus the data rates can be increased. It is a frequency division multiplexing( FDM) technique.
In the multitone method, the available frequency range is divided into many narrow frequency bands, each of which is transmitted separately on a subcarrier. Each individual carrier frequency can be modulated in phase as well as in amplitude in multiple steps, as in the case of quadrature amplitude modulation( QAM). In the center of each frequency band is the carrier frequency.
In multitone modulation, the data stream is divided into several parallel data streams, each of which is modulated and transmitted separately with individual subcarriers. Each individual subcarrier is modulated at a low symbol rate. The resulting data rate is approximately the same as that of single-carrier modulation at the same bandwidth.
Multi-tone processes are used in DSL processes such as ADSL, ADSL2+ ADSL2+ and VDSL2, among others. In ADSL, for example, the frequency range between 20 kHz and 1.1 MHz is divided into 256 frequency channels, each with a bandwidth of 4.3125 kHz. Data is transmitted in 225 frequency channels and control information in the remaining 30 frequency channels. DMT establishes its own data connection in each of the available frequency bands, the data rate of which depends on the quality of the connection. If the connection is good, a high data rate is transmitted; if the connection quality is poor, it is reduced accordingly. The procedure is based on the bit error rate.
The data is encoded in quadrature amplitude modulation (64QAM), resulting in a total transmission speed of 6 Mbit/s, which is transmitted via normal telephone cables.