Executive Summary
This document describes a method for measurement of a cable network’s upstream frequency response based upon cable modem pre-equalization coefficients. This technique allows the field technician to use a portable field instrument, incorporating an embedded DOCSIS® cable modem connected to a DOCSIS cable modem termination system (CMTS). Proactive network maintenance (PNM) technologies derive in-channel frequency response from the pre-equalization coefficients, and the resulting per-channel responses in multiple upstream SC-QAM and/or OFDMA channels are “stitched” together to provide a characterization of frequency response across a wider frequency range than just a single channel. From this result, the technician can determine proper unity gain and flatness from the node to ends-of-line locations; or, as system frequency response is purposed in our industry, show network response inequalities, imbalance, level and tilt issues. Furthermore, it allows for remotely located DOCSIS cable modems to participate in continuous upstream frequency response analysis. This population of sensors could include cable modems, field meters, and embedded DOCSIS modems or transponders in outside plant and headend equipment..
Scope
As HFC network upstream bandwidth demands increase, operators experience a decrease in the vacant RF spectrum available to help in the diagnosis of any particular return path frequency issues. A lack of available spectrum for traditional frequency response testing makes the task even harder to execute, and thus pushing cable operators to develop tools that could help in the day-to-day troubleshooting of issues without affecting service, and preferably, without affecting the customer experience. The focus is on field troubleshooting, from the node to the customer premises. With fiber deep and remote PHY technologies being deployed, the ability to verify frequency response in a non-intrusive manner is becoming increasingly desirable. The particular tools addressed in this operational practice require DOCSIS cable modem connectivity. Information flow may come directly from the CMTS. However, for various reasons, CMTS interrogation from the field may not always be an option. So, localized reading of the field equipment’s pre-equalization coefficients may be a limiting factor for the task at hand. .
Benefits
One of the interesting benefits of the pre-equalization-based frequency response measurement is that it leverages existing network equipment and readily adapts to future network topologies. The use of pre-equalization-based frequency response measurement can augment existing frequency response measurement methods and can be a cost-effective alternative in those scenarios where conventional frequency response test equipment is not currently available. The capabilities are especially valuable when integrated with an existing PNM solution to automate the detection of upstream frequency response problems.
It is also beneficial for operators to understand the frequency response within the RF spectrum which will be used by customers, rather than vacant spectrum between and around those services. When employing an upstream adaptive pre-equalizer response characterization method such as this, technicians will be provided with a high-resolution analysis of the DOCSIS 3.1 and 4.0 channels which are in use. In the case of single carrier quadrature amplitude modulation (SC-QAM) signals, it can be shown (T. Kolze) that the derived equalizer response is a superposition of some number N different filter responses (that is, the number of pre-equalizer taps), each filter with a center frequency spaced 1/(NT) Hz from its adjacent filters (in frequency, where N is the previously mentioned number of taps and T is the symbol period in seconds), and each with an equivalent noise bandwidth (ENBW) of 1/(NT) Hz. Thus, the equalizer response comprises a sum of frequency responses, where each of the N frequency responses has a resolution bandwidth (RBW) of 1/(NT) Hz, providing a very similar, but not exactly the same, response as with a conventional sweeping spectrum analyzer.
N = 24 taps
T = 1/ (5,120,000 symbols per second) = 1.95E-7 second
Solving the equation above gives 1/(24*1.95E-7) = 213.33 kHz
When considering the coefficients of OFDMA, the resolution is greatly improved. The frequency resolution of the OFDMA coefficients is determined by the subcarrier spacing such as 25 kHz or 50 kHz, respectively.
Intended Audience
The primary target audience of this document is the field technician currently involved in ensuring and maintaining system integrity, unity gain, balance and flatness of the HFC network. A secondary audience includes engineers and development personnel who would be interested in implementing a pre-equalization-based frequency response system such as the one described within this document.