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MIMO (Multiple Input Multiple Output) radar extends the traditional configuration of a single transmitter and receiver with the introduction of additional antennas. The motivation for introducing the multiple antennas is the ability to resolve the angular location of targets as well as making a system more robust to channel fading or other unfavourable operating conditions.
MIMO radar differs from Phased Array Radar, which similarly utilizes multiple receive and transmit antennas, in that a MIMO signal transmits orthogonal waveforms from each individual transmit antennas, giving channel diversity across each of the individual transmitter to receiver path ("virtual channel").
To achieve orthogonality, the transmitters must emit waveforms that are orthogonal in time, frequency or code, as illustrated below.
The collection of resulting virtual channels between each transmitter and receiver form the virtual array whose dimensions are determined by spacing of antennas in both transmitter and receiver arrays.
The result of constructing this virtual array gives rise to ability to resolve angular information from the received signals through estimating the phase differences between receive antennas. The images below illustrate how objects at particular angles from an antenna array gives rise to phase differences experienced by the individual antennas.