Patent Application
20150009340
The present invention relates to characterization of cable networks, and in particular to equipment and methods for characterization of a downstream signal path in a cable network.
A cable network delivers services such as digital television, Internet, and Voice-over-IP (VoIP) phone connection. A cable network has a controlling center or “head end”, which controls video and data traffic in the network by generating or distributing video and data signals. The signals are delivered over a tree-like network of a broadband coaxial cable called “cable plant”. Digital television signals are broadcast from the headend to a trunk of the cable plant, and delivered to subscribers' homes connected to branches of the cable plant. In going from the headend to subscribers, the signals are split many times, and are attenuated in the process. Accordingly, a strong downstream broadcast signal is required, so that the signal level at the subscribers' premises is strong enough to be reliably detected. Upstream signals from the subscribers' homes carry phone and Internet traffic. The upstream signals propagate from the branches of the cable plant towards the headend of the network.
Upstream and downstream signals occupy separate frequency bands called upstream and downstream frequency bands. Downstream information channel signals co-propagate in the downstream frequency band, and upstream signals co-propagate in the upstream frequency band. The frequency separation of the upstream and the downstream signals allows bidirectional amplification of these signals, which propagate in a common cable in opposite directions. In the United States, the upstream spectral band typically spans from 5 MHz to 42 MHz, while the downstream spectral band typically spans from 50 MHz to 860 MHz.
The upstream and downstream signals are prone to impairments and interference. Oxidized connectors can act as electrical diodes distorting the downstream signals by generating frequency harmonics, which can negatively impact both upstream and downstream signal paths. Aging equipment, such as signal boosters and amplifiers, can also distort the signals and add harmonics and “ringing” at unwanted frequencies. Another source of impairments is external electrical interference, termed “ingress noise”. Despite electrical shielding of the cable, outside signals may find their way into, and become guided by the cable. Shielding punctures, especially at customers' premises, improper installation, interference from closely placed high-current electrical equipment, etc., all contribute to accumulation of ingress noise. Furthermore, a cable plant can act as a receiving radio antenna. Thanks to its large size, a cable plant can pick up signals from otherwise unlikely sources, such as aviation radars.
The impairment situation worsens as new customers are added to an existing cable network. The cable plant is extended by adding more splitters and connectors, amplifiers, and long runs of coaxial cable to new locations. When a cable plant is expanded, a probability of downstream and upstream signal impairments increases. Accordingly, growth of extent and functionality of cable based networks must be matched by a growing effort to assure quality of existing services via periodic testing and maintenance of the networks.
Due to multitude of signals and signal formats being used in a typical cable network, testing cable network performance includes measuring multiple characteristics such as a frequency spectrum of RF signals, an in-band frequency response and a group delay, and other characteristics. By way of example, Chappell in U.S. Pat. No. 6,961,370 discloses an apparatus for determining a frequency response of a cable plant. The apparatus of Chappell includes one or more testers each having a tuner, digital demodulation circuitry, and a controller that measures an absolute power level at the tester location for a particular channel and that measures a relative frequency response for the channel based on the tap weight coefficients from the digital demodulation circuitry. The absolute and relative measurements are combined and then recorded by each tester. The combined values of two or more testers are compared to determine the total frequency response of the communication system.
Cable network performance is also evaluated on higher logical levels of data transmission. For instance, to evaluate quality of digital TV signal transmission, number of tests can be performed. A technician travels to various locations of the cable plant, and at each location, uses a specially constructed and programmed digital tester to evaluate digital signal quality for each channel of interest. Measured are such parameters as carrier level or amplitude, modulation error ratio (MER), bit error rate (BER), ingress under carrier (IUC), and other parameters. The measurements are usually performed on channel-by-channel basis, each channel diagnostic data being summarized on a separate screen or data page viewed by the technician on the tester's visual display.
Interpretation of the data collected for the purpose of maintenance and troubleshooting a cable network has long been a challenge for field technicians, which are typically trained on the job, and often lack a fundamental knowledge of digital signal generation, propagation, and processing. To overcome this challenge, Pangrac et al. in US Patent Application Publication 2003/0134599 suggested using an electronic device configured as a digital assistant for a field technician. The digital assistant presents on its display a network-specific information, in combination with generic technical information to assist the technician to identify and resolve network problems. In essence, the digital assistant functions as an electronic reference and guide. The technician assistant application may further incorporate a design module that assists a field technician with creating a new network or network extension.
The complexity of the processes involved in data reception and processing, as well as the sheer amount of data, present a certain difficulty for a technician tasked with solving a particular technical problem in a short amount of time. Digital references and guides of the prior art cannot replace an analytical step of selecting a most likely impairment of the network. Interpreting the diagnostic data requires analytical skills and knowledge of technology that field technicians are simply not trained or educated for. Accordingly, there is a significant value in presenting cable network test data in a simple and easy to understand format.
It is a goal of the invention to overcome many of the above mentioned problems and deficiencies of the prior art.
The inventors have realized that many cable network impairments have a unique frequency signature. As a result, impairments of digital data transmission impact frequency channels in a manner that often shows a recognizable pattern when presented as a frequency spectrum. Therefore, if the frequency dependence of various channel parameters, for example MER, BER, IUC of downstream channels, is presented as a frequency spectrum, the identification of impairments via visualization of various spectral features becomes easier. In other words, frequency spectra of MER, BER, IUC, and other higher-level parameters of downstream channels, when presented on a single screen of a testing device in a simple, clear, readable format, can facilitate cable network testing and troubleshooting by field technicians. Furthermore, combining these spectra on a single screen can provide a summary view of an overall “health” of a downstream signal path, allowing the technician to quickly evaluate a condition of a particular location of a cable network.
In accordance with an aspect of the invention, there is provided a method of characterizing a downstream path of a cable network, the downstream path including a plurality of downstream channels, each channel having a channel number or a center frequency, the method comprising:
(a) causing a tuner to tune in sequence to each one of the plurality of downstream channels;
(b) causing a processor to demodulate each one of the plurality of downstream channels and to determine a value of a first demodulation parameter of each one of the plurality of downstream channels tuned to in step (a); and
(c) causing a display to display the first demodulation parameter on a display in graphical form as a function of the channel number or center frequency.
In one exemplary embodiment, the plurality of downstream channels are consecutive channels spanning over a continuous frequency range, and in step (c), the first demodulation parameter is displayed as a first spectrum spanning over the frequency range. Preferably, the first parameter includes modulation error ratio (MER) or ingress under carrier (IUC).
In accordance with an aspect of the invention, there is further provided an apparatus for characterization of a downstream path of a cable network, the downstream path comprising a plurality of downstream channels within a downstream band, each channel having a channel number and/or a center frequency, the apparatus comprising:
a tuner configured to tune to a downstream channel, to provide a signal at an intermediate frequency;
a processor communicatively coupled to the tuner, configured to cause the tuner to tune in sequence to each one of the plurality of downstream channels; to modulate each one of the plurality of downstream channels; and to determine a value of a first demodulation parameter for each of the plurality of downstream channels tuned to by the tuner; and
a display communicatively coupled to the processor, configured to display the first demodulation parameter determined by the processor in graphical form as a function of the channel number or the center frequency.
In a preferred embodiment, the first demodulation parameter is selected from a group including MER and ingress under carrier. The processor can be further configured for determining, for each one of the plurality of downstream channels tuned to by the tuner, a value of a second parameter different from the first demodulation parameter, wherein the second parameter is selected from a group consisting of MER, BER, ingress under carrier, and carrier level. The display can be configured for displaying the second parameter concurrently with the first demodulation parameter in graphical form as a function of the channel number or the center frequency.
Exemplary embodiments will now be described in conjunction with the drawings, in which:
While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art.
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The tuner 11 may be configured to measure signal spectrum, and measure quadrature amplitude modulation (QAM) characteristics on a variety of downstream channels. The tuner 11 is capable of selecting a band of signals as wide as 8 MHz. Preferably, any signal that fits into a 8 MHz band may be processed by the tuner 11. The tuner 11 may be constructed to have a fast enough tuning time, e.g. 5 to 19 full range (1100 MHz) spectrum updates per second. The tuner 11 may maintain high enough signal fidelity that it can demodulate 256QAM signals. By way of a non-limiting example, the signal fidelity is such that the total phase error is less than 0.7 degrees in a 6 MHz bandwidth.
In the exemplary embodiment shown, the processor 12 may include an analog-to-digital converter (ADC) 14 coupled to the tuner 11, for digitizing the signal at the intermediate frequency for subsequent digital processing. Preferably, the ADC may have a dynamic range of at least 10.5 effective number of bits (ENOB). An FPGA accelerator 15 may be coupled to the ADC 14, for down-converting each of the plurality of downstream channels and for filtering the downconverted signal. A digital signal processing (DSP) processor 16 may be coupled to the FPGA accelerator 15. The DSP processor 16 and the FPGA accelerator 15 together may function as a demodulator. Other implementations of a demodulator are also possible, as known to persons of skill in the art.
The DSP processor 16 may be configured to cause the tuner 11 to tune in turn to each downstream channel and to determine a modulation scheme of that channel, according to techniques and methods known in the art. Once the modulation scheme is determined, the DSP processor 16 may demodulate the channel, and obtain demodulation parameters such as modulation depth, gain equalizer coefficients, and MER. Other channel parameters, such as a carrier level and in-band spectrum response, may be obtained, as will be explained below.
A measurement control processor 17 may be coupled to the DSP processor 16 for maintaining a list of channels and associated modulation parameters, and for controlling overall flow of measurements performed by the DSP processor 16, including the spectral measurements and demodulation processing. The spectral measurements generally have much idle DSP time, while the tuner 11 is settling to a new frequency. During this idle time of the DSP processor 16, the DSP processor 16 may either determine the channel type, or can demodulate signals.
Initially the measurement control processor 17 will go through a fixed list of possible downstream channels. The measurement control processor 17 may request the DSP processor 16 to determine the type of each channel, for example Analog Video (NTSC or PAL); Digital Video (Annex A, B or C); or “Unknown”. If the channel is Digital, the symbol rate and modulation depth is determined. The measurement control processor 17 may then instruct the DSP processor 16 to demodulate each of the detected channels sequentially. The first demodulation may take up to 0.5 seconds per channel, but many of the demodulation parameters are cached in order for the demodulation to be performed an order of magnitude faster in subsequent passes.
In addition to providing a list of tasks for the DSP processor 16 to perform, the measurement control processor 17 also gathers results from the DSP processor 16, and builds display data. A display processor 18 may be coupled to the measurement control processor 17 for receiving the display data from the measurement control processor 17 and generating pages of information to be displayed by the display 13, and for providing necessary user controls, for example, a frequency band or a list of downstream channels to process.
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In following optional steps 36 and 37, the DSP processor 16 checks if the parameter is inside a pre-defined operational range for each channel. When the DSP processor 16 determines that the parameter is outside of the operational range for a certain channel, that channel is highlighted on the display 13 in the step 37, to warn a user that normal transmission on that channel is impaired.
Preferably, the plurality of downstream channels span consecutively over a continuous frequency range, termed a “first frequency range”. This may allow one to display the first demodulation parameter as a continuous “first spectrum” spanning over the first frequency range. This may also allow spectra of such parameters as MER and IUC to be obtained and displayed as spectra for diagnostic purposes. The IUC data may be calculated from error vectors of individual sequential symbols of a data packet, as disclosed by Tsui et al. in U.S. Pat. No. 6,385,237, incorporated herein by reference. Furthermore, spectra of decoding parameters and high-level transmission parameters, such as bit error ratio (BER), may also be obtained and displayed. In one embodiment, the first frequency range may span an entire downstream frequency band of the cable network.
Many cable network impairments have a unique frequency signature. As a result, impairments of digital data transmission may show a recognizable pattern when presented as a frequency spectrum. Frequency spectra of MER, BER, IUC, and other higher-level parameters of downstream channels, presented on the display 13, facilitate cable network testing and troubleshooting. Two or more different parameters may be concurrently displayed in graphical form, as a frequency spectrum on the display 13, thus providing a clear and concise summary of the state or “health” of the downstream path at the current test location. For instance, the first demodulation parameter may include MER or IUC, and the second parameter can include the other of MER or IUC, as well as BER and carrier level.
In addition to displaying carrier level, demodulation, and decoding parameters in form of frequency spectra, in-band channel functions, such as spectral response and channel spectrum, can be displayed as well. By way of non-limiting examples, the DSP processor 16 may obtain in-band spectral response and/or channel spectrum for each channel being tuned to in the tuning step 31. The in-band spectral response function of a channel may be determined by performing a fast Fourier transform (FFT) of demodulation equalization coefficients, that is, tap weight coefficients of the demodulator circuitry, as described by Chappell in U.S. Pat. No. 6,961,370, incorporated herein by reference. Individual response functions may then be concatenated into a continuous spectral response spanning across multiple digital channels. Similarly, the in-band channel spectrum may be calculated by performing an FFT of a time trace of a corresponding downconverted downstream channel, as known in the art. Once an in-band function is determined for each of the plurality of consecutive downstream channels tuned to in the tuning step 31, the in-band functions of different channels may be concatenated into a second spectrum and displayed concurrently with the first spectrum, that is, BER spectrum, MER spectrum, IUC spectrum, carrier spectrum, etc.
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In this exemplary embodiment, automatic scanning of the entire frequency spectrum of a cable network may be enabled, with demodulation of each channel and displaying relevant demodulation parameters in spectral form, on a single screen page. Turning now to
The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function.
The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
The present application claims priority from U.S. Provisional Patent Application No. 61/842,592, entitled “CATV Downstream Analyzer” filed Jul. 3, 2013, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61842592 | Jul 2013 | US |
