By Tom Williams, Cable Television Laboratories Inc.; Cable Television Laboratories Inc., Cable Television Laboratories Inc; Larry Wolcott, Comcast; Jason Rupe, Ph.D., CableLabs
Home coaxial wiring and drop cable are expected to contain corrosion diodes created between dissimilar metals and metal oxides. Generally, corrosion diodes have not been a serious problem up to this point because continuous composite downstream radio frequency (RF) levels inside homes were not sufficient to force the corrosion diodes into hard conduction, which would create nonlinear distortion. DOCSIS® transmitted signals can be strong enough to force the corrosion diodes to conduct, but due to sub-split upstream frequencies, damage to downstream signals has been limited. It should be technically possible to automatically detect passive intermodulation (PIM) products at the headend due to their correlation with the main transmission and the spectral location of the nonlinear distortion. This paper discusses the mechanism for creating PIM in sub, mid and high split plant using orthogonal frequency division multiple access (OFDMA) transmissions. The paper also reports on PIM impairments created in our lab and analyzed using digital signal processing (DSP), discusses the potential impact of nonlinear noise on full duplex (FDX), and provides techniques to tackle this until now invisible plant impairment.