AWR2544LOPEVM: Far Field Calculation

Tool/software:

Hi,

As we all know, the far-field estimation is given by 2D^2/lambda, where D is the largest antenna dimension.

I would like to understand how to calculate D. 

Here are some possible options:

1. Distance from the first to the last virtual array element (in azimuth).
2. Distance between the first and the last transmitters.
3. Largest distance between a transmitter and a receiver.

Please let me know if any of the options above are correct and whether the size depends on MIMO or beamforming mode.

Although my question is about the AWR2544, I would like a general answer.

Thanks,

Shlomi