Comparison of QAM-VDSL and DMT-VDSL in an impulse noise environment

Impulse noise is a potential impairment of very-high speed digital subscriber line (VDSL) services. VDSL systems are protected from impulse noise using a combination of Reed-Solomon coding and interleaving. Since there are two competing proposals for VDSL, the first one based on the use of QAM line code and the second one using DMT modulation, it is interesting to compare their performance in terms of immunity to impulse noise. The paper presents a comparison of the behavior of QAM-VDSL and DMT-VDSL in the presence of impulse noise in real configurations. The required interleaving delay to correct all the generated errors for both systems is investigated. It is shown that when no notches are used in the radio-amateur bands, QAM and DMT systems require similar delays in order to achieve the same level of protection against impulse noise. However, when notches are implemented, DMT-VDSL achieves higher rates than QAM-VDSL. Therefore, larger error-correcting code redundancy can be used, which results in lower end-to-end delays for DMT-VDSL systems.