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Here is where you talk about issues you see in your RF plant.
6 posts • Page 1 of 1
We're currently running 3 x 6.4Mhz @QAM64 per node. We would like to increase our us capacity. Switching to 4 channels should be simple, but we're considering switching to 6 channels instead to get even more capacity.
Does anyone have experince with this amount of channels?
Our plant seems pretty stable from 22MHz and all the way up to 65MHz. With 200KHz spacing between the channels (is that enough?), I arrive at the following channel plan.
I talked to one of our rf technichians today, and he said we're using FP returnpath transmitters, not DFB. Maybe we need to change to DFB? According to the RF guy FP should be okay since we dont have any really long fiber streches. I think we're at 5-6KM max.
Any input appreciated
I'm no expert but these are some snippets I have in my notes :
(sorry dont know where this quote came from)
"If you plan to deploy more than one or two 64QAM signals, forget Fabry Perot (FP) lasers, Distributed feedback (DFB) or digital return lasers are a must."
"You can have multiple 64-QAM channels, each of which is now supporting a 6.4 MHz-wide carrier. With upstream four-channel bonding, all four modem reverse transmitters will fire at the same time. The lasers may need to be turned down to accommodate the higher optical modulation index. This may result in more energy transferred to the laser, so the backoff needs to be set accordingly. When the backoff to the laser is established, it might mean old Fabry-Perot (F-P) lasers are not adequate and that you may have to buy new reverse distributed feedback (DFB) transmitters to get the performance you need to maintain operating headroom."
http://www.cable360.net/ct/deployment/b ... 35887.html
"Significant consideration must be given to the total RF power loading that will now be realized with upstream channel bonding in DOCSIS 3.0 modems. In previous DOCSIS specifications, only one upstream channel was present. For DOCSIS 3.0, at least four upstream DOCSIS channels will be transmitting at the same time, possibly with a 6.4 MHz bandwidth each, resulting in nearly 26 MHz of contiguous upstream channel loading. This is a lot of power hitting the return path fiber-optic transmitter.
The probability of laser clipping is increased, especially if one has legacy Fabry-Perot (F-P) lasers in the return path fiber nodes. It is a good idea to upgrade to distributed feedback (DFB) lasers, which have significantly more dynamic range. As well, one should have a comprehensive plan to monitor laser clipping in the return path."
and I have an arris Doc here called "Upstream BER/NPR Testing: DOCSIS 3.0 Assessment" where they say stuff like "For the system with one or two DOCSIS channels, FP transmitters are good enough to carry 16-QAM" and "A fully loaded DFB Tx has enough dynamic range to cover temperature variations and ingress for 64-QAM", and "12dB operational margin (noise and clipping) for ingress and temperature variations was recommended in a cable-tec expo 2004 workshop. This leaves on 2 dB of usable dynamic range in the FP link, but 19 dB in the DFB laser".
Thank you for the tips
I think we will try to get a statement from the vendor specific to our case regarding FP vs DFB performance.
For reference, the cisco cmts config guide mentions that US channels should be atleast 20 Khz spaced (http://www.cisco.com/en/US/docs/ios/cab ... m_mgt.html), so i guess we could even squeeze it a bit tighter than the 200 KHz i proposed.
6 posts • Page 1 of 1
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