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DOCSIS and Cable Modems – How it works :: Tutorial Wrap Up

Post 169 of 182
SCDMA

DOCSIS S-CDMA - Synchronous Code Division Multiple Access - AKA SCDMA

If you have followed the "DOCSIS and Cable Modems - How it works" tutorials this far, congratulations!  You now have a basic foundation of how DOCSIS networks operate and the ability to pick up the DOCSIS specification and read and comprehend it - this is hard to do for the novice.  If you are just finding this blog for the first time, then I recommend that you go to the DOCSIS Tutorial Series section of the blog and start at the beginning before proceeding.

For everyone else, there are a couple final items that I want to cover in this DOCSIS tutorial wrap-up blog before I move onto other cool topics.  They are Synchronous-Code Division Multiple Access (S-CDMA) and Adaptive Signal Cancellation Algorithms, both were introduced in the DOCSIS 2.0 specification and are carried over into DOCSIS 3.0.

S-CDMA

Synchronous-Code Division Multiple Access indicates that multiple cable modems can transmit simultaneously on the same RF channel and during the same TDMA time slot, while being separated by different orthogonal codes (ref. DOCSIS 2.0 RFI pg. 40).  Whoa!  That is a mouthful, but it can be immensely powerful for a number of reasons.  First, one can have up to 128 cable modems transmitting simultaneously, since there are 128 spreading codes.  How is this possible?  Just by using some math and digital signal processing.  Before the data is transmitted it is randomized in a unique method such that each burst of data is "spread" out using one of the 128 code words.  After the data is spread, it looks more like noise than the usual TDMA signal.  The code words and spreading algorithm is created in such a way that when the data is received, the patterns have an orthogonal (or mathematically perpendicular) arrangement such that the receiver can isolate and demodulate one cable modem from another.  Simultaneous transmission means less time waiting for REQ-MAP cycles.

Much more importantly, S-CDMA signals are significantly more immune to certain types of ingress noise.  Specifically impulse noise, which is most often associated with the low frequency band of the RF spectrum, i.e. 5 to 20 MHz.  For this reason, S-CDMA is ideally suited for use in this region.  Why is S-CDMA more immune to impulse noise?  Because the data in an S-CDMA signal is spread out by the spreading code so in the event that a burst of impulse noise does impact a modem transmission, it will randomly catch bits which are not adjacent to each other in a single packet.  This makes it easier for the Reed-Solomon error correction to repair any damaged bits once the packets are re-assembled on the receiver side.

A couple of draw backs associated with S-CDMA are incompatibilities with earlier modems already in the field, significantly increased DOCSIS MAC overhead when mixing both S-CDMA and TDMA modems together on the same upstream channel, and potential return path laser clipping in the event that one actually allows 128 modems to talk at the same time - this can create a lot of total RF power, over-driving lasers.  The ideal use for S-CDMA is in the 5-20 MHz range, usually with a low modem count and ideally for a small to medium business deployment.

Adaptive Signal Cancellation Algorithms

When DOCSIS 2.0 chipsets arrived in the market place with S-CDMA, they also added features to TDMA to make it more resistant to ingress.  Specifically, adaptive signal cancellation algorithms in the DOCSIS 2.0 (and now D3.0) chipsets focused on quickly identifying and cancelling out any coherent interfering signals that could be present under a DOCSIS channel.  Depending upon the chipset and type of interferring signal, as many as 12 signals under a TDMA upstream DOCSIS channel can be identified by a chipset and eliminated.  The impact on a DOCSIS network is that signals which would normally cause an outage to many subscribers are suppressed in the CMTS, allowing cable modems to stay online, often error free.  Note that this is meant to be a temporary solution to keep subscribers online while plant technicians troubleshoot the root cause of the problem.

The following picture shows a DOCSIS channel just below 30 MHz with an upstream that has high level CPD only about 10 dB below the peak of the DOCSIS channel.  Notice that one of the CPD remnants falls directly under the DOCSIS channel.  You will have to take my word for it, but during the testing of this event there were no errors in the upstream as these cable modems were communicating with a DOCSIS 2.0 CMTS.  Had it been a DOCSIS 1.x CMTS substantial errors would have been present since the CNR was only 10 dB on a 3.2 MHz 16-QAM signal.

DOCSIS Channel with CPD

Laboratory testing has demonstrated that adaptive signal cancellation is capable of removing coherent interferers sticking out of a DOCSIS channel as high as 10 dB.  This means one has a -10 dB CNR (for a signal carrier).  As more interferers are added the cancellation algorithm becomes less effective.  Additionally, as the interferers become wider in bandwidth, such as one would have from a modulated carrier as an FM or FSK signal, the less effective cancellation is, but nonetheless still much better than no cancellation at all.

Wrap-up

So I hope that you have found the DOCSIS 101 tutorials valuable or at least some parts of it relevant to your career.  As I often do, I urge you to visit www.cablelabs.com and take the time to read the DOCSIS 2.0 RFI as you should now have a solid basis to now understand much of this highly technical document.  This is my recommended reading before diving into  DOCSIS 3.0, however if you are brave and willing to get up to speed with the latest technology, follow the blog to begin on the DOCSIS 3.0 Tutorial series.

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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