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

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The continued evolution of Proactive Network Maintenance

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

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Patent Pending

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differently by different levels of technicians. A method to detect CIN utilizing PNM

techniques was proposed by CableLabsi 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.

How It Works

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.

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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.

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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.

iAlberto Campos, Belal Hamzeh, Tom Williams, Testing for Nonlinear Distortion in Cable Networks, October 2013

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Easy Integration with Operations

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

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