Defining upstream power backoff for VDSL

Very high-speed digital subscriber line (VDSL) upstream data transmission in a distributed environment will suffer from relatively strong far-end crosstalk generated by the shorter lines in the binder. This effect can dramatically reduce the upstream capacity on longer lines. To secure the capacity on lines of all lengths, shorter lines will be required to systematically reduce their transmit power. This power reduction is known as (upstream) power backoff (PBO or UPBO). This paper reviews the problem and summarizes a number of PBO techniques and results that may not be readily available in the published literature. Next, a general formulation is presented in terms of a "reference PSD" and a method is proposed to determine the optimal value of the reference PSD. This paper is mainly based on work that was done in VDSL standardization. It is an extended version of work that was previously published