Since DOCSIS 3.1 cable modems are out in the market for few months and ISPs such as Comcast have enabled OFDM in many of their regions, a question that is getting commonly asked what do “active sub-carrier number range” mean and what is the total OFDM spectrum size.
Here we take take example from our best CM1000 DOCSIS 3.1 Cable modem GUI interface and explain some of the DOCSIS 3.1 details:
Here is the common screenshots you will see published in the forums: If you look at Downstream OFDM channels, it shows Active Subcarrier Number range for channel 1 is 1108 ~ 2987.
Calculations
- Subcarrier Range = 1108 ~ 2987
- Total number of active subcarriers = 2987-1108+1 = 1880 Active subcarriers
- DOCSIS 3.1 Subcarrier spacing options are either 25 KHz or 50 KHz. In this case, CMTS is provisioned for 50 KHz subcarrier spacing
- So, 1880 carriers * 50 KHz sub-carrier spacing = 94 MHz OFDM spectrum
- But there are additional 1 MHz of exclusion bands at upper and lower edge of the OFDM spectrum.
- So total OFDM spectrum block in this case = 96 MHz OFDM spectrum
Here is two part youtube video series which explains basic OFDM and OFDMA concepts. It is not entirely related to DOCSIS 3.1 but basic OFDM information is still relevant for DOCSIS 3.1 networks.