By D. Stoneback, R. Howald and T. Brophy, GE and O. Sniezko , AT&T Broadband & Internet Services
This paper discusses measurement and analytical results of distortion beat characterization. This analysis received significant level of urgency recently when field tests showed that the traditional methods of characterizing nonlinear distortions (see NCTA Practicei) generated in multichannel HFC networks do not allow for an easy prediction of the behavior of digital signals in the same environment. Especially the introduction of higher, more sensitive modulation schemes (256-QAM) made characterization of the impairments of the HFC channel very valuable.
Beat distortion requirements for analog video are based on subjective video perceptibility correlated to the levels of impairments measured according to a standardized test method. For QAM, BER is a more quantifiable measure. To properly characterize the BER degradation analytically, a statistical description of the intermodulation distortion is necessary. The description required is the probability density function (PDF), an appropriate mathematical description for the evaluation of steady state BER performance. Beyond the PDF, the nonstationarity of the distortion process must be considered to capture possible variations of BER as carrier frequency drift varies the statistics.
Using these techniques, BER performance can be analyzed as a function of the variables that contribute to the distortion's parameters - channel map, analog-to-digital signal power ratios, relative carrier phasing, time, and the nonlinearity performance of the channel. This analysis and supporting measurements can be used to optimally align the forward path when the spectrum consists of a composite of analog and digital signals.