Radar sensors are used in motor vehicles, for example, in order to measure the distances, relative velocities, and azimuth angles of vehicles or other objects located in the area in front of the own vehicle. The individual antenna elements are then disposed, for example, at a distance from one another on a horizontal line, so that different azimuth angles of the located objects result in differences in the path lengths that the radar signals must travel from the object to the respective antenna element. These path length differences result in corresponding differences in the phase of the signals that are received by the antenna elements and evaluated in the associated evaluation channels. By equalizing the (complex) amplitudes received in the different channels with corresponding amplitudes in an antenna diagram, the angle of incidence of the radar signal, and thus the azimuth angle of the located object, can be identified.
To allow a high angular resolution to be achieved, the aperture of the antenna should be as large as possible. If the distances between the adjacent antenna elements are too large, however, ambiguities can occur in the angle measurement, since the same phase relationships between the received signals are obtained for path length differences that differ by an integral multiple of the wavelength λ. An unambiguous angle measurement can be achieved, for example, with a uniform linear array (ULA) structure in which the antenna elements are disposed at distances of λ/2. In this case, however, with increasing aperture there is also an increase in the number of antenna elements and thus also in the number of evaluation channels, so that correspondingly high hardware costs ensue.
In a multiple input/multiple output (MIMO) radar, a greater angular resolution capability is achieved by working not only with several receiving antenna elements but also with several transmitting antenna elements, different combinations of transmitting and receiving antenna elements being evaluated, for example, on a time-multiplexed or optionally also frequency-multiplexed or code-multiplexed basis. The varying positions of the transmitting antenna elements then result in additional phase differences and thus in signals that are equivalent to signals that would be obtained with a configuration having a single transmitting antenna element and additional (virtual) receiving antenna elements. The aperture is thereby virtually enlarged, and the angular resolution thus improved.
In the interest of maximum angular resolution, it is advantageous if the virtual antenna array is thinned out in such a way that the individual antenna elements are at relatively large distances from one another. In these circumstances, however, the unambiguity condition is no longer met, so that especially with noisy radar echoes, ambiguities and thus “jumpy” angle measurements occur, i.e. abrupt changes in the measured azimuth angle occasionally occur when a radar target is tracked over a longer period of time.