Operational Considerations and Optimization of OFDM Deployments (2018)

By Christopher Topazi & Michael Cooper, Cox Communications

With the convergence of services to IP and the continued growth of bandwidth demands, the access network is stressed to greater and greater capacity limits, forcing cable operators to find the most efficient network configurations such that their networks are providing the largest capacity possible. Such configurations require the highest number of modems operating at the highest modulation order profiles in order to maximize the capacity provided by the networks RF spectrum. The introduction of DOCSIS 3.1 is one of the latest options for cable operators to optimize the performance and maximize the capacity of their networks. DOCSIS 3.1 provides a host of new levers for improving the bandwidth offered by the network. One such lever is the introduction of much higher order modulations than previously provided in earlier versions of DOCSIS. On the downstream, the DOCSIS 3.1 CMs and CMTSs must now support modulation orders up to 4096-QAM with options to support 8192-QAM and 16384-QAM. This is a significant increase over the limited SC-QAM modulations of 64-QAM and 256-QAM required in DOCSIS 3.0 and earlier, and offers as much as a 50% capacity improvement within the same spectrum.

With Cox’s initial DOCSIS 3.1 deployments, we focused on a few select parameters in order to maximize the number of modems running the highest modulations. These parameters included:

  1. OFDM channel placement within the RF spectrum
  2. Power spectral density
  3. Windowing
  4. Adjacent channel interference and profile bit-loading
  5. Profile assignment and MER thresholds, and
  6. Channel metrics for monitoring the network.

This paper will explore the approach that Cox used to select each of these parameters and provide some details on the performance we were able to achieve with these configurations.

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