02-03-2014 10:47 AM
I'm trying to understand the realistic difference between 2x2 and 3x3 MIMO in a wireless bridge situation - purely as a point to point solution. Assuming that all values are the same - AP power, antenna direction/gain/frequency, with the only difference being that one solution is 2x2 and the other is 3x3, will the difference plainly be a PHY max rate of 300M vs 450M?
I've had mixed answers when it comes to the advantages of going 2x2 vs 3x3 as some people I've spoken to say it will far better to go 3x3 and others say that 3x3 has very little increase in actual performance when compared to 2x2.
02-03-2014 12:36 PM
There is a lengthy discussion on this in the Aruba Outdoor MIMO Wireless Networks VRD on our web site, pages 63-66.
In general, it is difficult if not impossible to achieve 3SS rates outdoors. This is because there are only two usable antenna polarizations (H and V). All other polarizations, including +/-45 and circular are ultimately variations on H and V.
The reason polarization matters is that we do not generally have very much multipath outdoors, especially as compared with reflection-rich indoor environments. So to achieve the 2nd spatial stream we have to "create" multipath which we do with dual-pol antennas.
However, since there is no 3rd or higher-order disrete polarization, this same technique cannot be extended past 2SS.
Now, there is evidence to suggest that using a 3SS AP with a triple-pol antenna (H, V, +/-45) can produce a slight range benefit for a given rate. This is due to processing gain in the baseband on both sides of the link. But in practice you will only see 3SS rates for 20-50 meters past the AP in an outdoor setting before they fall back to 2SS.