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Engineering-Wise: The intelligent cable network

Wed, 03/06/2013 - 5:18pm
Daniel Howard Senior vice president of engineering and CTO of the SCTE

Besides the capacity and robustness improvement,  there’s something else about DOCSIS 3.1 and its use of orthogonal  frequency division multiplexing (OFDM) that excites me:  the network intelligence it enables. We already use DOCSIS  devices as network health probes, but OFDM will essentially  turn our equipment into multifunction test generators that can  be used to measure most of the key RF performance parameters  and diagnose the HFC network automatically in order to  optimize performance and maximize capacity.

Let’s start with what we can do now. The DOCSIS specification  instructs us to calculate the codeword error rate RC as  a proxy for post-forward error correction (FEC) bit error ratio  (BER) if the latter cannot be measured directly: RC equals  the number of uncorrected codewords divided by the total  number of codewords received over some interval of time. For  those of you preparing to take the SCTE’s upcoming DOCSIS  certification exam, I just helped you prepare for it.

But more recently, we’ve gone beyond BER-type measurements  and can now do the equivalent of time domain reflectometry  (TDR) using the adaptive equalization process built  into DOCSIS. For engineers, you get essentially a discrete  form of the impulse response of the cable network going to  individual homes. This measurement can then be triangulated  with geographical information to identify the magnitude, type  and location of typical cable network impairments, such as  cracks/breaks in the coax and the connectors, water flooding,  and so on. For technicians, think TDR measurements every  20 seconds or so with cable modems that used to require field equipment and truck rolls.

CableLabs calls this the Proactive Network Maintenance tool, Comcast calls it Scout Flux, but  the point is that MSOs are rolling this out throughout the world. If you want a good tutorial  on the process, an appendix in the recently published “SCTE Measurement Recommended  Practices for Cable Systems” explains it very well.

The latest addition, which CableLabs calls InGeNeOs, adds spectrum analyzer-type functions  using the built-in sampling and FFT capabilities of modern cable modem chips. Now  we can do a mini-sweep and TDR from the headend! Due to its narrow, multi-tone nature,  OFDM in DOCSIS 3.1 will allow the same kind of measurements, only with greater precision,  accuracy and flexibility in measurement configuration.

But much more is also possible. To characterize the non-linearity in the actives, SCTE 115 is  the age-old two-tone intermodulation distortion (IMD) measurement for cable networks, where  two test tones are injected into the network, and the IMD products at sums and differences of  the test tone frequencies are examined. Last year, the SCTE began exploring the use of the multicarrier  capabilities of DOCSIS 3.0 modems to effect the same measurement with up to six tones,  but it would require customized diagnostic cable modems, and only one vendor had them.

But with OFDM in DOCSIS 3.1, we can generate up to thousands of tones to probe the network  for IMD and can look for IMD products just  about anywhere we want to, on both upstream  and downstream. Further, we can conceivably  perform the more rigorous noise power ratio  (NPR)-type measurement (also described in the  SCTE Measurement Recommended Practices),  where an RF channel is loaded fully except for a  notched-out portion of the spectrum where the  IMD is measured.

And that’s not all. Back in the analog carrier  days, we looked for common path distortion  (CPD) primary tones at multiples of 6 MHz  on the upstream, or if it was strong enough,  we could see it in the triplet pattern of tones  around those primary tones. With OFDM,  we could use pilot tones in the downstream  to emulate the old analog carriers, including  locking the distributed pilot tones in phase to  coherently add to enhance the effect. Notching  out a configurable portion of the spectrum in  OFDM allows us to measure the CPD. It’s a  test engineer’s dream setup, and it will allow us  to characterize the RF plant health long before  it degrades service and fix those problems with  surgical precision.

What about leakage measurement? There  are several new RF leakage detection systems  designed to increase the sensitivity for detecting  RF leakage in all digital plants – either  by much higher antenna gain or by time  domain processing gain via correlation with  the downstream QAM signal – and another  more traditional method that uses markers.

The innovation is required because QAM  signals have flat, lower-power spectral densities  and also lower transmit power levels than  analog carriers. But we could also use OFDM  pilot tones as unmodulated carriers to look  for leakage using traditional low-cost techniques.

Or combine this with the higher sensitivity  innovations for a double enhancement.

We may need it when even small amounts of  LTE signal ingress into our networks could  degrade our network capacity.

So start thinking now about how you  could innovate with OFDM in your systems  to proactively maintain the health and  maximum capacity of your HFC networks.

For a deeper explanation on this, please  visit http://www.scte.org/OFDM to review  numerous resources on the topic.

dhoward@scte.org

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