ADSL modems rely on discrete multitone modulation (DMT). DMT divides a broadband channel into many narrowband subchannels and modulates encoded signals onto the narrowband subchannels by using the fast Fourier transform (FFT). In ADSL standards, the lowest subchannels are not used for data transmission so as not to interfere with voice and ISDN signals, and one subchannel is often reserved for a pilot tone. Each of the other subchannels either carry a QAM signal, or no message, depending on the bit allocation determined by the receiver and sent back to the transmitter. A bit allocation is determined during modem initialization. During data transmission, the receiver can request small changes to the bit allocation table, e.g. as a response to changes in the channel.
Before a bit allocation table can be created, the ADSL receiver has two different equalization tasks to perform:
The new generation of DMT-based wireline modems has been defined by three standards: ADSL+, ADSL2, and Very High Speed DSL (VDSL) standards. These three standards were approved in 2002 and 2003. VDSL is essentially a higher speed version of ADSL. VDSL provides training sequences to train the equalizer, and uses a cyclic prefix length that is 1/16th the symbol length. VDSL experiences interference from AM radio stations and amateur radio. A VDSL tutorial is available under tutorials. As in ADSL modems, equalizer design is the key to maximizing bit rate in an VDSL modem.
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