Tone grouping for improved rate adaptation procedures in xDSL systems

In Digital Subscriber Line (DSL) systems, the noise perceived by a DSL line fluctuates when DSL lines sharing the same bundle are switched on/off. The concept of Seamless Rate Adaptation (SRA) was introduced to seamlessly adapt the data rate of a connection to the changing channel while in operation without any service interruption. Interleavers in communication paths impose a constraint on the amount of data rate that can be changed by an SRA procedure. In the case of large changes in the channel, multiple consecutive SRA procedures have to be executed. Moreover, to keep an equal signal-to-noise-ratio margin over all tones, it is very likely that the bit loadings of all tones have to be modified in each SRA procedure. Hence, the state of the art SRA is too slow and provides means of adaptation only to slowly changing channels and will not prevent reinitializations under certain conditions such as large rapid changes in noise. In our previous work [14], we have presented approaches that optimize the adaptation procedures and bring down the adaptation time from the order of tens of seconds to the order of seconds. In this paper, we further optimize the adaptation procedures and show that modifying the bit-loadings of the used tones in groups leads to a faster decrease in the average bit error rate (BER) and therefore to less occurred errors. We present a low complex iterative rate adaptation approach that can be used in practice by modems with limited computing power to seamlessly adapt to changing channels.