ADVANCED TECHNOLOGY CPD POINTER PNM ENABLED CPD DETECTION FOR THE HFC NETWORK WHITE PAPER 185 AINSLEY DRIVE SYRACUSE, NY 13210 800.448.1655 I WWW.ARCOMDIGITAL.COM
The continued evolution of Proactive Network Maintenance ADVANCED TECHNOLOGY The capabilities of PNM platforms continue to evolve. The latest evolution comes in the form of a new process and technology that allows for the PNM software to report the presence of Common Path Distortion (CPD) detected within a node. Existing broadband capture modems are used to detect a pilot carrier inserted at a specific frequency that marks the presence of CPD. The carriers are generated from a small, low power CPD detection device installed in the optical node the CPD Pointer 1 Module. The module provides a method for remote CPD detection and leverages the PNM platform as a reporting means. Importantly, this technology does not require the addition of any hardware in the headend, it is easily scalable such that deployment can be made incrementally on a node by node basis, and the technology can be implemented within any PNM software package. What is Common Path Distortion There are several types of nonlinear distortions that can negatively affect the performance of the HFC network. The first is the traditional second and third order distortion products created from overloaded amplifiers in the forward referred to as composite intermodulation noise (CIN) in the context of digital channels. A second type of nonlinear distortion is identical to the first but occurring in the return path, due to overloaded return amplifiers or optical nodes. And the third type of nonlinear distortion is caused by forward signals propagating a corrosion point in the network where a diode effect creates new intermodulation (IM) signals. These IM products appear across the entire return band and are what is referred to as CPD. Due to the noise like characteristics of forward QAM and OFDM signals from which the CPD is created, the CPD manifests in the return path simply as an elevated noise floor across the band, difficult to distinguish from broadband noise when using traditional test equipment. This is especially relevant because due to signal level characteristics of the network, CPD can only be generated in the hardline, as compared to broadband noise typically generated from the home areas of the plant that are maintained quite 1 Patent Pending 2
ADVANCED TECHNOLOGY differently by different levels of technicians. A method to detect CIN utilizing PNM techniques was proposed by CableLabs i in 2013, and prior to the introduction of the CPD Pointer Module, no practical way to leverage PNM techniques to detect CPD had been offered. Background - Xcor Passive Radar Technology The use of passive radar technology as a technique to detect and range CPD within the HFC network was introduced by Arcom Digital in the past decade. This technology marketed as Xcor, was previously available in the Quiver handheld passive radar field test meter and in the Hunter radar and return path monitoring system. The Xcor technology is based on a signal correlation process, where reference samples are created from captured samples of forward channels processed through a CPD emulator. These reference samples are then cross-correlated with samples of live return path signals to calculate the magnitude of the CPD impairment and the time distance to the source. An important capability of this technology is that the noise floor of the test equipment is approximately 30dB better than the noise floor of the cable network, providing visibility to low level impairments and making it extremely easy to see and track CPD sources that continuously change amplitude throughout the day. While this technology has proven extremely effective and been implemented by numerous MSO s in many hundreds of systems throughout the world, one of the implementation challenges with the Hunter Xcor platform is that dedicated hardware in the form of a return path switch matrix, radar, and server need to be installed in the headend. Additionally, there is no practical way to do an install in a hub incrementally. The CPD Pointer Module is a fresh implementation of this core Xcor technology and provides a simple way to overcome these limitations. To start, the CPD Pointer Module can be installed in the lowest performing nodes, and then over time additional modules can be installed. 3
How It Works ADVANCED TECHNOLOGY The CPD Pointer Module is installed within the optical node, with jumpers connecting to power and to forward and return test points. The test points provide the necessary signal samples for CPD detection. Forward signals are used to create the CPD reference, and this reference is correlated with live return samples. If CPD is occurring anywhere in the node, it will be detected and the amplitude and time distance to the impairment will be calculated. But a challenge is presented in how to transfer this valuable information on detected CPD to consumers of the data monitoring the network and dispatching repair personnel. One way would be to install a cable modem within the node, but this requires additional space, significant power consumption, and of course cost. The CPD Pointer module solves this problem by leveraging PNM and utilizing existing capabilities of broadband capture modems. A CW pilot generator is contained within the CPD Pointer Module. The module continuously switches between two operating modes: scanning for CPD and sequentially transmitting three CW carriers that essentially function as a slow FSK modulation. The three CW s indicate the presence of CPD as well as the detected amplitude and time delay, and is interpreted by the PNM software by polling broadband capture modems installed in the node. A schematic of the module, connection to the node, and an example of the pilots captured by the PNM software via the cable modem is shown in Figure 1. 4
ADVANCED TECHNOLOGY Figure 1 As indicated, the forward CPD emulator and return signals are digitized, and presented to the cross-correlation CPD detector. The controller instructs the CW pilot generator to sequentially transmit CW carriers at frequencies corresponding to the detection results. Carriers are transmitted through the same forward test point, they jump across the isolation of the test point in the downstream and remain at a sufficient level easily detected by the broadband capture modem. The transmit frequency range is configured in the module upon installation and requires only 270kHz of unused bandwidth anywhere in the forward spectrum. This can be in the FM band, guard band, roll-off, or any convenient and available location. The schema related to how the CW carriers are assigned and interpreted is shown in Figure 2. This information is provided to the PNM server through an API along with the configured center frequency such that interpretation of results is automated. When no CPD is detected, all three carriers are placed in the center of the assigned spectrum. This provides confirmation that the module is installed in the node, and it also provides a means for field technicians equipment to confirm that a problem was fixed after mitigation effort. 5
ADVANCED TECHNOLOGY Figure 2 When CPD is detected, F0 is placed at the center frequency, F1 is placed to the left of the center frequency at a frequency corresponding to the detected delay, and F2 is placed to the right of the center frequency indicating the detected CPD level. It takes 15 seconds for the pointer module to detect CPD and place all three CW carriers, so this is the time duration required for the PNM software to pull CM spectrums from the broadband capture modem. This entire process makes it such that the amplitude of the carrier detected by the broadband capture modem is not relevant and not a factor in interpreting results. Obviously, the signal level of the CW carriers will vary significantly within the node depending upon associated tap value, cable loss, and various other insertion losses within the network. i Alberto Campos, Belal Hamzeh, Tom Williams, Testing for Nonlinear Distortion in Cable Networks, October 2013 6
Easy Integration with Operations ADVANCED TECHNOLOGY The CPD Pointer module can easily be integrated into any operational structure. Arcom Digital has created an API which is open and can be added to any PNM platform either the Arcom Digital PNM+ platform, other commercially available PNM platforms, or MSO internally developed systems. An example of the GUI that displays the detected CW carrier placement and interpretation of CPD delay and amplitude is shown in Figure 3. The solution is scalable and can be incrementally deployed across nodes as make business and budget sense. Lastly, it is not necessary that PNM software is installed in order to test and do a trial run of the CPD Pointer Modules the CMTS can be accessed via a VPN and data can be pulled from the IP address of DOCSIS 3.0 modems resident in the node. Figure 3 7
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