By Brady Volpe, NimbleThis and The VolpeFirm
Speed. We never seem to get enough of it. This is certainly true when it comes to high-speed data. Over the past 20 years the data over cable system interface specification (DOCSIS®) publications have undergone several iterations to accommodate more and more speed. DOCSIS 3.1 introduced new technologies specifically to support even more speed improvements. Specifically, DOCSIS 3.1 introduced orthogonal frequency division multiplexing (OFDM) in the downstream and orthogonal frequency division multiple access (OFDMA) in the upstream. OFDM and OFDMA are unique in their implementation of frequency allocation because they use very narrow slices of bandwidth in the RF spectrum to transmit data, called sub-carriers. A single OFDM channel is made of many sub-carriers. For example, a single 192 MHz OFDM block can contain 7600 subcarriers. These subcarriers are only 25 kHz or 50 kHz in bandwidth. Contrast this to our traditional single-carrier quadrature amplitude modulated (SC-QAM) channel which is typical 6MHz (or 8 MHz for some non-North American systems).
Each subcarrier can have its own modulation from 16-QAM up to 4096-QAM (even high order modulations may be supported). Again, we can contrast modulation to legacy SC-QAM, which are limited to 64-QAMor 256-QAM. In the comparison of SC-QAM to OFDM we see a drastic difference between bandwidth (6MHz vs 25 kHz) and modulation (256-QAM vs 4096-QAM).
As previously mentioned, in OFDM each subcarrier can have its own modulation. This means one subcarrier can be running at 4096-QAM while its adjacent subcarrier could be running at 16-QAM. This is a very power feature when impairments are present because each subcarrier can be optimized to ensure modems can receive data from each subcarrier no matter how bad the impairment. But how could one possibly know how to configure 8000 subcarriers? It’s not possible. This falls into software called the profile management application or PMA coupled with proactive network maintenance (PNM).
This paper will explain how PNM and PMA work together to optimize the OFDM downstream and later the OFDMA upstream to maximize the data throughput in the presence of downstream and upstream impairments.