Adaptive nondisruptive measurement of harmonic distortion for voiceband data transmission

Volterra functional expansions are power series with memory that explicitly characterize the input/output relationship of a finite-memory single-valued nonlinear black box whenever the latter is time-invariant and stable. The purpose of this paper is to 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, QAM data communication system. Our investigation was originated as part of a request for new nondisruptive impairment measurement algorithms that would reliably characterize in real-time the performance of voiceband modems over both D1 conditioned private lines and the direct-distance-dialing (DDD) public switched network. Though, contrary to earlier studies, we do not contemplate nonlinear distortion compensator design, experimental evidence obtained by implementing the simplest of our nondisruptive algorithms on a digital signal processor chip verifies that over a certain range of signal-to-second- and third-order intermodulation distortion ratios our measurements are consistent with those obtained via the disruptive digital four tone test under a variety of voiceband telephone channel impairment conditions. Unfortunately, our experiments also indicate that neither the digital four-tone test nor any of our nondisruptive techniques have chances to prove useful for greater than 5% of the existing voiceband channels. However, this is entirely due to the special nature of the telephone channel environment and is not likely to dilute the importance of our proposal for other types of nonlinear environments such as radio channels or satellite links