Combining Virtual Noise and Seamless Rate Adaptation for User-specific Requirements on Quality of Service in xDSL Systems

Inhalt:
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 or enter/exit the Low Power state. One approach to protect against timevarying non-stationary noise is to set the target bit rate such that it can still be achieved at times with peak noise levels at the same target bit-error-rate (BER) by using an SNR margin or by using the Virtual Noise (VN) approach. Clearly, this will lead to a data rate loss at times with low noise levels. Another approach is to adapt the target data rate to the changing channel conditions using Seamless Rate Adaptation (SRA). SRA will maximize the achievable data rate, however, a BER smaller than the target BER cannot be guaranteed at least during the adaptation times. DSL users often deploy services with different requirements on Quality of Service (QoS). In this paper, we present a iterative sub-optimal hybrid VN-SRA approach that allocates tones to be used by the VN approach for services with strict requirements on BER and tones to be used by the SRA approach for services that can tolerate temporary decreases in BER such that the overall data rate is maximized while user specific requirements on QoS are satisfied.