Why in MIMO Synthetic Aperture Radar must the transmitted signals from each antenna be orthogonal?
You have to be able to tell them apart from each other, so that you can assign a particular received signal to the proper transmitter. They can be separated in time, frequency, code, etc.
so the reason behind that is in order to separate them for the processing at the receiver. is there any reference or paper that elaborates on this issue that can help me
yes... it is so they can be separated as part of the processing in the receiver...
Not only separating them in processing, but also to decrease the interference. You can find information about orthogonality in many systems like OFDM, CDMA, and radar systems. So, don't limit your search to SAR systems.
Any communications text will show that signals are only separable by their differences. For equal energy signals, this means orthogonal aspects of the signals. For radar, received energy cannot generally be assumed to be different from different transmitters. Consequently, the useful aspects of received signals in MIMO systems are their orthogonal components. Radar systems and communication systems differ mainly in where the interesting information modulates the signal.
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