We recently had these questions about spatial separation and cross polarization on our YouTube channel:

Question:

So, is larger spatial separation needed in addition to cross polarization, or does cross polarization alone provide adequate de-correlation? Also, what is the actual reason why 2 cross-polarized antennas work better? Does it enhance multipath rejection?

Answer:

Cross polarization gives close too perfect de-correlation and no spacial separation is required, it does, of course, only provide two MIMO channels (2×2 MIMO).

The spacing becomes a requirement under the following cases:

• 4×4 MIMO or more. Typically you will still use two xpol antennas spaced apart to benefit from cross polarised de-correlation.
• When you are using omni-directional antennas which are easiest to design using vertical polarisation the only option is spacial separation.

Outdoor vertical spacing is best as you rely on reflections to give decorrelated channels and sideways reflections from buildings are less likely (sideways/horizontal separation benefits most from this types of reflection) whereas mother earth will always provide at least one reflected path and then vertical separation gives the best decorrelation.