Delivering On The 256 QAM Promise

Ron D. Katznelson, Broadband Innovations, Inc.

Abstract

The accumulation of degradation factors in the downstream channels is reviewed with focus on headend/hub transmission-related noise aggregation due to a plurality of downstream Quadrature Amplitude Modulation (QAM) transmitters and upconverters. Substantial increase in the number of QAM transmitters based on frequency agile upconverters is expected. Unlike fixed channel upconverters, these do not have narrow band filters at their output, consequently imparting appreciable levels of broadband noise on the network when several such units are combined. Based on these factors and other noise contributions, it is shown that additional impairments due to fluctuations associated with composite distortion terms on the HFC network dictate (perhaps non-intuitively) the adoption of noise margins higher than that provided by the relative insertion levels of 256-QAM signals.