3.2. CODING

Codes can be concatenated with the basic DMT system in ADSL to improve performance, nominally by as much as 6.5 to 7.5 dB with random background crosstalk and line noise, and by a larger amount with impulsive disturbances. At the time of first ADSL standardization, a concatenated coding system (trellis code with outer interleaved Reed Solomon) was mandated. That system has considerable flexibility and thus, while implemented without full ability to understand codes of the future, fortunately can be used to provide more transmission robustness than originally anticipated. The designer needs to know only how to best decode that earliest specified system. Present-day knowledge allows high coding gain of 6–8 dB with a more complicated ...

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