DOCSIS 3.1: What is OFDM?

[@ZCorum]

There is an error in the above video when we show the spacing between the OFDM subcarriers. We incorrectly list that as 25 MHz to 50 MHz. That should be 25 kHz to 50 kHz.

[@studentloanforgivenessfore7768]

Good job, great information and easy to understand delivery

[@joejoseph4425]

Great tech talk guys and terrific OFDM overview! Can you give us your perspective on what scenario or condition would dictate a provider selecting 25MHz or 50MHz subcarrier spacing? I looked over our CMTS configs, and noticed we are 25MHz across the board. Thanks in advance!

[@ZCorum]

Joe, thanks for your question. First, I noticed we have an error in our video. The subcarrier spacing should be shown as kHz and not MHz. As far as your question, 25 kHz subcarrier spacing is more efficient than 50 kHz subcarrier. Over a 192 MHz OFDM channel you can get up to up to 3800 active subcarriers at 50 kHz spacing or up to 7600 active subcarriers if spaced at 25 kHz. However, 50 kHZ spacing is supposed to be less prone to interference from high-power impulse noise. You will need a cleaner plant to run at 25 kHz spacing.

[@dmac232000]

What is the throughput or speed capacity of an OFDM carrier depending on how wide the carrier is also by different profile modulations?

[@ZCorum]

This may provide some answers to that question if you are good with math: volpefirm.com/ofdm-capacity-in-docsis-3-1-and-4-0/

[@chrisraz8046]

I was wondering if you could explain Standard Deviation in OFDM. Do you want a higher or lower value? Thanks

[@ZCorum]

The standard deviation of the MER (Modulation Error Ratio) is measured across the OFDM channel. You want a lower standard deviation (preferably less than 2 dB).

[@billburns720]

orthogonal - /ôrˈTHäɡənl/ - I kept hearing them pronounce it or-thag-o-nal...