Accuracy Improvement for Direction of Arrival Estimation by the Use of a Mirror Element

In this paper, a method to increase the achievable angle accuracy of a linear digital beamforming array by introducing a reflector element acting as mirror for the electromagnetic wave is presented. The mirror element leads to an increase of the effective array aperture, as well as a quasi-doubling of the number of elements. The major difference compared to a real increase of the number of array elements is that the information from the mirror elements cannot directly be separated from the signals coming from the real antenna elements. Therefore, an alternative approach to classical beamforming algorithms becomes necessary to take advantage of the additional information. In this work, the method of least squares is used to develop an efficiently implementable algorithm. The performance improvement predicted by the Cramér-Rao lower bound is confirmed by measurements conducted with an eight-channel frequency-modulated continuous-wave radar prototype operating at 79 GHz. The beamforming performance improvement observed in the measurements reached up to 16 dB, which is in good agreement with the predicted level.