Open Mic - Upstream spectrum troubleshooting
What we really need is a way to see what’s happening in the spectrum within the active channel band.
Always a challenge, the upstream band in HFC networks is getting closer and closer to full utilization. Until now, it’s been a common practice to look at adjacent, unoccupied spectrum to evaluate noise/distortion performance. This technique assumes that what’s happening in adjacent spectrum is also happening in the active channel, and we all know what happens when we ass-u-me.
Still, as we consider bonding DOCSIS 3.0 channels and the return bandwidth fills up, there is less and less empty spectrum to analyze in this fashion. What we really need is a way to see what’s happening in the spectrum within the active channel band. But how can we do that and discriminate the noise and distortion from the service signal?
Meanwhile, conventional wisdom says that in order to get a good RF spectrum display for use in distortion, noise and ingress analysis, a relatively large, heavy and expensive instrument is required. The spectrum analyzer functions provided in most signal-level meters (field analyzers) usually offer less-than-stellar range and resolution, as well as relatively slow scan rates. The implementation of DSP technology in handheld field analyzers has enabled a quiet evolution in spectrum analysis capability available to most plant maintenance technicians. Those who have not investigated the spectrum analyzer function and its various features are missing out on a troubleshooting aid that has become an essential troubleshooting tool.
By using digital signal processing in the design, the field analyzer uses algorithms to instantly and precisely translate raw time-domain “snapshots” into the frequency domain represented on the spectrum analyzer. The design provides a number of advantages. It is not only fast and economical, but it is capable of processing the results to make application-specific analysis possible.
In addition to offering a wide dynamic range, numerous resolution bandwidths, and a variety of averaging and dwell settings, the field analyzer’s spectrum mode permits the viewing of spectrum characteristics within QAM channels (spectrum minus the QAM haystack). This is especially useful in troubleshooting distortion and ingress. Ingress and transient impulse noise commonly interfere with upstream signal transmission.
Up to this point, viewing interference within an active channel on a conventional spectrum analyzer has been difficult at best, because the spectrum display shows the highest signal at any given frequency, and if things are working according to design, this will be the service signal. Some may suggest using a “min hold” feature to eliminate the time-division multiplexed signal and reveal the spectrum below. The problem with this technique is that the interference will be minimized, and it might even remove the transient interfering signal from the display by showing only the minimum measured levels at each point of scan resolution.
A test mode dubbed “TraffiControl” takes advantage of the complete implementation of DSP in the field analyzer design by sorting the “snapshots” to enable a separate trace for the traffic signal, the peak noise – including distortion – and the “live” noise. The user simply sets a threshold, above which any signal could be assumed to be the service signal traffic. The system automatically sorts out the signal traffic from the noise/distortion traffic and displays both.
Another troubleshooting dimension is added when compatible return path monitoring equipment is installed in the hub/headend. With a properly configured analyzer, a technician can simultaneously view the upstream spectrum at the local test point and at the headend, enabling verification that the noise/ingress is still present at the headend and that it is present at this test point. The “TraffiControl” feature is useful in this application, as well, to enable viewing spectrum activity within active channels.
The challenge of testing loaded upstream bands to find sources of ingress and impulse noise is greatly eased with the availability of handheld field analyzers that have innovatively utilized DSP. These analyzers are capable of sorting the service signal traffic from the spectrum anomalies that interfere with data communication, enabling faster troubleshooting, increasing technician efficiency and productivity, and improving customer satisfaction.
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Next month, In-Stat analyst Michael Paxton will write about U.S. cable modem services and the real story on bandwidth.