By: Ron Hranac, Technical Leader, Cisco’s cable access business unit
Nick Segura, Director of Technical Operations for Charter Communications, and I teamed up to co-present a paper in a Society of Cable Telecommunications Engineers’ Cable-Tec Expo technical workshop on Thursday, October 24. The paper was titled Signal Leakage and Ingress at Higher Frequencies: Understanding the Challenges.
For decades cable operators have managed signal leakage by monitoring in or near the 108-137 MHz very high frequency (VHF) aeronautical band. A relatively new problem has cropped up over the past couple years or so: Leakage and ingress in the ultra high frequency (UHF) spectrum such as the 700 MHz long term evolution (LTE) band. The impact of UHF leakage and ingress clearly poses liability and risk to the cable industry. Recent field tests by Hranac, Segura, and others corroborate previous studies that show there is little or no correlation between signal leakage field strengths in the VHF aeronautical band and at UHF, and emphasize the need for visibility into what is happening at higher frequencies. Our Cable-Tec Expo technical workshop presentation reviewed some of the common mechanisms that cause leakage at higher frequencies, helped clarify cable operators’ liability when interference to over-the-air service providers happens, and discussed the difficulties encountered when attempting to measure leaking quadrature amplitude modulation (QAM) signals. We recommended how to deal with affected LTE service providers, highlighted currently available commercial digital leakage detection and measurement solutions, and provided guidelines on “homebrew” UHF leakage detection work-arounds that can be implemented until commercial solutions are obtained for local cable system use. Leakage and ingress mitigation techniques along with proposed best practices were be included in the workshop.
One of the other presenters on the Cable-Tec Expo technical workshop panel highlighted some of the challenges of physical layer testing of orthogonal frequency division multiplex (OFDM) signals, which will be part new cable modem technology defined in the new DOCSIS 3.1 specifications for high-speed data transmission on cable networks. A third presentation covered proposed analytics for troubleshooting outside plant problems, including identifying the probable location of the impairments.