Orthogonal frequency domain multiplexing (OFDM) and the profiles it creates can boost the speed and throughput of DOCSIS 3.1 to levels that rival fiber. But what building blocks make up OFDM and why are they important?

One of the interesting aspects of DOCSIS 3.1 is its backwards compatibility with DOCSIS 3.0. The good news is that very few plant upgrades need to be made to take advantage of the new version of DOCSIS. The bad news is that certain precautions must be taken or else the new DOCSIS 3.1 cable modems will resort back to DOCSIS 3.0 without warning – losing all efficiencies.

To keep this from happening, let’s take a look at the building blocks that make up OFDM. At the base is the phy link channel (PLC) that contains information on how to decode the OFDM signal. One level up is the next codeword pointer (NCP) that tells the modem which codewords are present and which profile to use on each codeword.

Next is Profile A. This is the boot profile that every DOCSIS 3.1 modem must be able to use in order to reach higher QAMs with the other profiles. Beyond Profile A are the higher order profiles – B, C, D, and more. Profiles B and above can be used to reach higher QAMs and more efficiency. Profiles beyond A, B, C, and D are up to the discretion of the CMTS and CM manufacturers, but there aren’t any limits to the amount of profiles that can be used.

Each of the building blocks mentioned make up a stack and each of the base building blocks must be working properly for the next building block to be used. Now that we know what building blocks make up OFDM, lets look at each one to see why it is important.

We know that the PLC contains the information on how to decode the OFDM signal. Without the PLC, the modem cannot find the OFDM carrier or understand how to decode it. To keep this from happening, the PLC must be locked and have no uncorrectable codeword errors (CWE).

Once the PLC is running properly, the NCP must also be locked and have no uncorrectable CWE. If there are lost messages at this point, there will be retries, or even worse, no communications at all.

Profile A is next and is the boot profile. It can be assigned lower mixed QAMs such as 64 or 16 so that all DOCSIS 3.1 modems can communicate in the worst part of the plant. Just like the two building blocks before it, Profile A must be locked and have no uncorrectable CWE. If there are uncorrectable CWE, the modem will resort back to DOCSIS 3.0 and all efficiencies will be lost.

MER/CNR Across OFDM CarrierNow that these three building blocks are working within proper levels, it’s time to look at the overall performance of an OFDM carrier. For OFDM to operate at peak performance, the average power level must be within range, MER must be good, and noise levels must be as low as possible. Noise greatly affects OFDM and can prohibit the use of higher profile modulations.

Once OFDM is performing well, the higher profiles become possible. With each profile, it is important that they are locked. The higher profiles can have acceptable amounts of uncorrectable CWE since it is not as critical as it is in the lower building blocks, but uncorrectable CWE can restrict higher profiles from performing at their peak.

With all building blocks working at optimal levels, DOCSIS 3.1 is able to take advantage of the best parts of the plant to create efficiencies that are well above anything that previous versions could obtain. Proper testing is critical for each of the building blocks as even small problems lower in the stack can create major problems for DOCSIS 3.1 overall.

For more in depth information on the building blocks, the white paper “DOCSIS 3.1: Best Practices for Peak Performance” can give more insight on proper testing. For a quick overview of how DOCSIS 3.1 works, this infographic is a great place to start.

Have you started implementing DOCSIS 3.1 in your network? Let us know about your experience with the OFDM building blocks in the comments section below.

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1 Comment

  1. I just started testing a technicolor tc 4400 3.1 fastest i can get is 550 down. I need to prove with ordinary cable tools that the modem is not resorting to 3.0 bonded channels. any ideas the modem says its aquired down stream channel a bonded channel. The aquired channel has 4000 correctables. its at 309mhz ch 38.

    thanks brian

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