Adaptive non-disruptive measurement of harmonic distortion for voiceband data transmission

The authors introduce and report on the strengths and weaknesses of two distinct Volterra-series-type classes of adaptive nonlinear techniques for the quantitative measurement of second- and third-order harmonic distortion present in the communications channel of a possibly signal space coded quadrature amplitude modulation (QAM) data communication system. Their experiments indicate that neither the digital four-tone test nor any of their non-disruptive techniques prove useful for greater than 5% of the existing voiceband channels. This is entirely due to the special nature of the telephone channel environment