By Robert L. Howald, Ph.D., Director, Transmission Networks Systems Engineering, Motorola Broadband Communications Sector
The impact of phase noise on digital communications systems has been an ongoing challenge for system designers as modulations have become more complex, error corrections schemes have become highly sophisticated, and the channels under consideration become more varied. The HFC channel as used by CATV providers is a unique one from the phase jitter perspective, for a couple of key reasons. First, CATV is one of very few commercial uses of very higher order QAM, such as 64- QAM and 256-QAM, primarily because of the inherently high SNR and linearity due to the needed to support video. As a corollary to this point, as one of the few implementers of QAM, CATV is perhaps the only common example of a system that does so at RF frequencies. Secondly, precisely because the CATV plant was made with analog video in mind, the traditional channel was not designed with QAM in mind, including the equipment made for RF frequency generation. The result is upconverters and downconverters adequate for one application, and being asked to support another. While there is much to be thankful with respect to how digital signaling provides advantages all the way around, there are some possible “gotchas,” and some overall confusion with respect to the phase noise topic. This paper is meant to describe and clarify these issues.