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Old 12-08-2012, 02:03 PM   #31
lrhorer
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Quote:
Originally Posted by Dan203 View Post
If MoCa uses MIMO like Wifi N then the ghost would actually improve the signal. Not sure if it does though, haven't read up on how exactly the MoCa standard works.
That is specious. First of all, by definition MIMO requires radiators that are spatially diverse, which would require dual ports on the MoCA adapters. MoCA does indeed employ spread spectrum modulation which makes interference from reflections less impactful to signal levels in a certain sense, but it does not mitigate the fact that the reflected signal arrives later in time than the primary signal. If the reflection is close to the radiator, then it can help, yes, although in a coaxial system usually not. The fact the carrier is spread out in frequency and not entirely dependent upon any single small section of the signal means the awful spectral response generated by the reflection is not an issue for the receiver, and the sections that are received have a higher S/N, giving better throughput and fewer errors. If the space-time displacement of the reflected signal results in a phase angle in the payload of greater than 90 degrees, however, it will tend to completely destroy the information in the bit stream. Only if the level of the demodulated ghost is typically at least 6dB lower than the demodulated primary can the ghost ordinarily be electronically rejected for phase angles larger than 90 degrees.

To put it a different way, again assuming a 100 Mbps payload, if the reflection is less than about .75 meters from one of the nodes, then the signal presented to that node will have up to a 3 dB higher signal level than if the line is properly terminated. Any node further than that away, however, will receive a horribly degraded bitstream from any node other than the one right by the reflection. Indeed, if the node is 1.5 meters from the reflection, then it is likely no other node on the system will be able to receive anything at all from that node for a 100Mbps payload. If the nodes drop the bit rate to 50 Mbps, they should all be able to talk once again. Note even the node that is half a meter or so from the reflection won't benefit much, however, since while the signal level may be higher, the S/N won't usually be.

The bottom line is the last thing anyone wants in any coaxial transmission system and most especially in a MoCA system is reflections.

Last edited by lrhorer : 12-08-2012 at 02:58 PM.
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Old 12-08-2012, 02:48 PM   #32
lrhorer
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Originally Posted by Kerwin51580 View Post
I just want to make sure I install this filter the right way. I figure I should power down both TiVos, both TAs, the cable modem and router and THEN install the filter. Then, power everything up in the reverse order? Does that sound right?
I know the horse has already left the barn on this one, but for future reference, there is no need to power anything down. Disconnecting the inoput cable won't send anything into a permanent tizzy. Of course, while it is disconnected, you won't get any cable service for the duration, but as soon as it is reconnected, everything will come right back up.
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