Spatial Diversity

Sir,
Spatial diversity is okay for single polarity multi-path in 2x2 MIMO devices, but you won't get the SNR per chain to get multiple data streams via each chain like you can often get in polarity diversity MIMO (H-pol/V-pol or X-pol). Also there are a many cases near metal and glass buildings where microwaves can reflect at angles that bring them to the receive antennas at a 90 degree polarity than it's origin. In these cases polarity diversity wins out over spatial polarity. 
The biggest issue with spatial diversity is that you can fit a 2x2 MIMO polarity diversity array on a single feed horn of a parabolic dish. For a super high gain point to point system using parabolic dishes using spatial diversity you would need to use 2 separate dishes instead of one. So polarity diversity is usually the best practice and most common practice for telecomms and for most AREDN networks.

-Damon K9CQB  

Best to review the 802.11n spec MCS rates used by AREDN and all part 15 devices.  AREDN devices, if a 2 antenna device, uses the MCS modulation and diversity options in real time to maximize the best data thoughput.   If the environment (inversion layer, etc. ) changes from moment to moment, the radio changes the MCS scheme to find the highest data thoughput option conditions allow.    Example of spatial diversity in use (this is 802.11n not just AREDN):

A) Ubiquiti Bullet single antenna device transmitting and received by a 2 antenna device.   This is 'receive diversity'.  The 2 antenna MIMO device receives the same signal on both antennas, combines them in a way to maximize the SNR and then decodes the signal to binary data.  This is MCS0 to MCS7 rates.

B) 2 antenna device transmitting same signal on both antennas and received on another 2 antenna device.  This is MCS0 to MCS7 modulation and coding schemes.   Same as A, the receive device combines the signals received to maximize the SNR and then decodes the signal to binary data.

C) 2 antenna device transmitting different data/signal on each antenna and received on another 2 antenna device.  This is MCS8 to MCS15 Modulation and Coding Schemes.   Data on each antenna is separately decoded to binary data, then combined into the data stream.  The data is split on each antenna to maximize the ability to do error correction at the receiver (and to minimize inter symbol interference and other bad things).

Joe AE6XE