I don't think chain separation in respect to the phase is so important. There are manufacturers that successfully use multi antenna system, up to 27 antennas/chains. Most of them share the same polarization (Ruckus) and polarity separation doesn't seem to bother them...
In small soho 3x3 routers high speed can be obtained by using all three chains between (3x3) devices. Coming and going to similar polarized antennas. How come polarization is not an issue here?
I think the bigger issue in creating long range backhauls with 3x3 systems is the fact that a bigger part of the data stream increase has to come from multipath receipt.
With an indoor 3x3 system their is plenty multipath signal so plenty of phase shift between streams that the mimo technology can use to build reliable high date rate.
On long haul links that need to be obtained by high gain antennas the multipath receipt is supressed a lot by the use of these antennas. Any beam that is not directly coming from the main antenna beam (and that therefore is a multipath signal) hits the receiver antenna under a angle out of its main lobe. The higher gain the antenna (to reach bigger distance/higher signal level) the worse the multipath reception..
Hence a multi antenna/multi stream (4x4, 5x5 up ??x??) on short range that use antenna with sensitivity in wide azimuth range do fine. The chipset is presented with many different streams all in relative same signal level and can build a good data rate on it.
A high directional antenna system will on the other end probably only receive the main radio beam (because any multipath beam just won't reach the other end any more, or to be weak to be usable) and when 3 chains are used the only separation is the polarization. Since each stream also hits the other stream's receiver antenna yes, cross polarity separation is the only thing left in basic wifi systems.
A solution could be indeed to separate each stream's antenna a couple of meters and/or differently point them slightly away from the other end towards a reflective surface. Now we create more time delay and thus phase shift the receiver can work with.
One trade off though; signal strength. Since we need to obtain high s/n level, thus high signal to get to the high mcs rates, we now need even higher gain antennas on both end, that again have higher compressed sidelobs.
Imho the trade off is such that there is a arbitral distance where a duo stream system ('ac' or any other coding system) will benefit more from a high signal than a multi stream system would be from phase shifting due multipath.
So maybe my 350meter link running over many more or less reflective house roofs might benefit from triple chain even if the max pol. separation will be 45 degrees, where my 680 meter link already might proof to be on the edge of a benefit a triple chain will bring me.
In no doubt my 8 km link will benefit more from just a very high gain duo stream antenna setup with 'ac' radio compared to somewhat lower gain triple chain setup.
In using the very high gain (30 + 34dB pair antenna, both in 45º slant setup to avoid 3rd party interference as much as possible) setup the signals will be high enough to achieve vht mcs rate 8 or 9 and thus 360 to 400 Mbit/s. If I need even more I have to go to 80Mhz wide channel.
Its interesting to know that some manufacturers are using circulair polarization in a 4x4 setup and together with some other improvement claim to have much bigger date rates possible than we could think off in using basic stuff.
Probably if I can't achieve what I want with MT or Ubnt I'll probably go for such solution or go for licensed.
But now the trade off is money!
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Rudy R. Puister
WISP operator based on MT routerboard & ROS.