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Impact of Impulse Noise on Adaptive Pre-Equalization Part II

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impulse noise in DOCSIS network

DOCSIS Adaptive Pre-Equalization and Impulse Noise

Part II of the Cliff-Hanger

***POST UPDATE -- Updates to Mod Profile Below ***

In part I of this article I discussed adaptive pre-equalization in DOCSIS cable modems and how it can compensate for many upstream impairments such as frequency response, group delay and micro-reflections.  I also left off with a cliff-hanger, which is also the title of the article; impulse noise has an impact on adaptive pre-equalization, but how and what is it?  I'll cover what impulse noise is and why and how it impacts adaptive pre-equalization.  Further, in working with a colleague of mine, we have a recommended solution of how to make your DOCSIS networks more immune to this potential issue.  So read on.

Impulse Noise

The definition of impulse noise as described by CableLabs:

Noise that is bursty in nature, characterized by non-overlapping transient disturbances. May be repetitive. Generally of short duration–from about 1 microsecond to a few tens of microseconds–with a fast risetime and moderately fast falltime. Seeing the time-domain picture of impulse noise above, you can get some idea of what impulse noise looks like.  It is very bursty and short-lived.  It is often hard to see on a spectrum analyzer unless it is set to "peak hold".  Impulse noise often comes from the electro-mechanical interference of heavy machinery and household appliances such as hair dryers.  So while it can get into an RF plant near an industrial part it can also enter from every home connected to the plant.  Further, because the electrical equipment must be running to generate impulse noise, it is an impairment that can come and go depending on the time of day.

Impulse Noise and Adaptive Pre-Equalization

To understand how impulse noise can impact adaptive pre-equalization in DOCSIS cable modems let's first review how the CMTS sets the equalization parameters in the cable modem.  During station maintenance, which occurs typically every 20 seconds for each cable modem, the cable modem sends a RNG-REQ message.  The CMTS evaluates the signal quality of the RNG-REQ and sends back equalizer coefficients in the corresponding RNG-RSP message to the cable modem.  This is the station maintenance process, also further detailed in the Station Maintenance article.

If impulse noise exists in the RF plant and a pulse happens to hit the RNG-REQ message as it is in route to the CMTS, this will result on the CMTS making bad equalizer coefficient adjustments based on the signal quality of an erroneous message.  The cable modem will receive the bad equalizer coefficients, adjust its equalizer accordingly and pre-distort its transmitter to compensate for an impairment in the RF plant that does not exist.  Now when the cable modem's signal reaches the CMTS and no impulse noise is present, the CMTS may have difficulty demodulating the subscriber data if at all.

This scenario has been duplicated at one major MSO, CableLabs and in my lab.  The results from the testing in my lab follows:

The upstream channels consisted of a bonding group of  four 64-QAM, 3.2 MHz DOCSIS channels.  No other impairments were present in the plant as this was a test environment.  The upstream channels can be seen in figure below, which already had the impulse noise injected, they are so infrequent it is difficult to see.

64-QAM 3.2 MHz DOCSIS Bonded Upstream Channels

64-QAM 3.2 MHz DOCSIS Bonded Upstream Channels

The impulse noise was  generated by a piece of electronic equipment (specifically a Dewalt industrial drill) with three wraps of #18AWG wire wrapped around it.  The wire was used to inductively pick up the impulse noise generated by the brushes in the AC motor of the drill.  The wire was directly connected to an F-connector, passed through a 19 MHz bandpass filter with a 3 MHz passband and into a step attenuator.  This allowed the impulse noise to gradually be increased just until it had minor impact on the cable modem signals being transmitted.

With adaptive pre-equalization ‘on’ all upstream channels, impulse noise is applied to upstream channel U0 centered at 19 MHz.  TCP/IP traffic was generated on the upstream at a rate of approximately 25 Mbps.  When the impulse noise was applied, the traffic dropped intermittently to lower levels as shown on the figure below:

Impulse noise impacting adaptive pre-equalization causing TCP/IP data to slow down

Data rate drops from 25 Mbps to 10 Mbps with Impulse Noise

In the case of the figure above, the drop was down to roughly 10 Mbps, which is more than a 50% decrease.  The packet generator showed a large amount of re-transmitted packets and the CMTS was also showing high levels of un-correctable codeword errors.  These are indications that some thing was wrong with the bursts of data that the cable modem was transmitting to the CMTS.  For some reason the CMTS was not able to demodulate the data.

It was time to look at the cable modem's MER (CMTS SNR) on the upstream before and after the data slow-down occurred.  Below is a screen shot from the CMTS command line interface (CLI), which shows what is occurring:

Impulse noise degrading cable modem MER due to incorrect adaptive pre-equalizer setting

Degraded Cable Modem MER (SNR) Due to Impulse Noise Impacting Pre-EQ

Just as predicted when traffic is being transmitted at 25 Mbps, the cable modem MER (SNR) is 36.12 dB.  However when the traffic takes a substantial drop to 10 Mbps, the corresponding cable modem has an MER (SNR) of 26.72 dB.  This is close to the edge of a CMTSs ability to demodulate a 64-QAM signal and is the result of the lower traffic rate.  The cable modem will stay in this state until the next station maintenance cycle when it sends a RNG-REQ message, which is typically 20 seconds.  Also note that only one cable modem was impacted (MAC address 0026.2482.9dc4).  There were two other cable modems online that were not impacted because their RNG-REQ arrived at the CMTS without being hit by impulse noise - this time.  On subsequent tests every cable modem was eventually impacted.

The Prevention

I had been sharing my test results with John Downey of Cisco as he was interested in the results of this testing.  We discussed and tried a number of different methods to harden the DOCSIS network against this impairment.  Some worked better than others, but ultimately his proposal of enabling upstream dynamic interleaving did the trick.

When upstream dynamic interleaving is set, the interleaver depth is chosen dynamically to achieve optimum burst robustness.   Interleaving multiplies the effectiveness of forward error correction (FEC) by aggregating multiple codewords in a block and changing the order in which bytes are transmitted through a shuffling or interleaving process of the codeword bytes so that a burst in time does not predominantly impact a single codeword. Instead, the impact is spread among all the codewords contained in a block, thereby providing enhanced protection.

Enabling upstream dynamic interleaving is performed in the CMTS by changing a one to a zero in a couple of lines of the modulation profile as follows.

The old modulation profile looked like this:

cable modulation-profile 224 initial 5 34 0 48 16qam scrambler 152 no-diff 98 fixed qpsk1 1 2048
cable modulation-profile 224 station 5 34 0 48 16qam scrambler 152 no-diff 98 fixed qpsk1 1 2048
cable modulation-profile 224 a-short 6 76 6 22 64qam scrambler 152 no-diff 64 shortened qpsk1 1 2048
cable modulation-profile 224 a-long 9 232 0 22 64qam scrambler 152 no-diff 64 shortened qpsk1 1 2048
cable modulation-profile 224 a-ugs 9 232 0 22 64qam scrambler 152 no-diff 64 shortened qpsk1 1 2048

The modified profile with the changes in red looked like this:

*** Update - This works better if the "Initial" and "Station" maintenance profiles have a longer preamble, so note the changes shown.  The old lines are lined out with the new modulation profile updated underneath. ***

cable modulation-profile 224 initial 5 34 0 48 16qam scrambler 152 no-diff 98 fixed qpsk1 1 2048
cable modulation-profile 224 station 5 34 0 48 16qam scrambler 152 no-diff 98 fixed qpsk1 1 2048
cable modulation-profile 224 a-short 6 76 6 22 64qam scrambler 152 no-diff 384 shortened qpsk1 0 2048
cable modulation-profile 224 a-long 9 232 0 22 64qam scrambler 152 no-diff 384 shortened qpsk1 0 2048
cable modulation-profile 224 a-ugs 9 232 0 22 64qam scrambler 152 no-diff 64 shortened qpsk1 1 2048

cable modulation-profile 224 initial 5 34 0 48 16qam scrambler 152 no-diff 384 fixed qpsk1 1 2048
cable modulation-profile 224 station 5 34 0 48 16qam scrambler 152 no-diff 384 fixed qpsk1 1 2048
cable modulation-profile 224 a-short 6 76 6 22 64qam scrambler 152 no-diff 64 shortened qpsk1 0 2048
cable modulation-profile 224 a-long 9 232 0 22 64qam scrambler 152 no-diff 64 shortened qpsk1 0 2048
cable modulation-profile 224 a-ugs 9 232 0 22 64qam scrambler 152 no-diff 64 shortened qpsk1 1 2048

Note that the preamble on the a-short and a-long Initial Maintenance and Station Maintenance bursts was also increased from 98 symbols to 384 symbols.   This was done to provide a longer training sequence for the CMTS to demodulate the bursts in the presence of impulse noise.

While the changes are seemingly very minor, the impact in the lab environment was substantial.  I was able to increase the impulse noise levels even further without data transmission loss or MER degradation in the cable modem.  It is important to note that I have only tested this configuration on a Cisco CMTS and with Motorola and Thomson DOCSIS 3.0 cable modems, so I cannot guarantee equivalent performance with other CMTS vendors.

Further, John Downey has confirmed that he was faced with a live scenario shortly after I shared the test results with him.  He implemented the modulation profile configuration settings that worked in the lab and found that they had an equal improvement in resolving the customer issues as seen in the field with nearly identical impairments.  So it was good to see the lab testing replicated and proven in a live field environment.  Plus it resolved a complex customer issue, making the customer happy.

I do encourage everyone to use adaptive pre-equalization and then consider implementing the updated modulation profile discussed in this article.   These two actions will go a long way to making your DOCSIS networks more resilient to network impairments and improving network availability.

Mr. Volpe has over 25 years of communications industry experience. He is focused on the cable and telecom industry with deep technical and business skills. Mr. Volpe is currently the president and chief technologist of the Volpe Firm and holds an MSEE with honors.

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