Resolving Doppler Ambiguity for Fast-Moving Targets With FDA-MIMO Radar

Although the problem of range ambiguity is solved with the frequency diverse array (FDA)-multiple-input multiple-output (MIMO) radar by utilizing the degree-of-freedom in the range domain, the phenomenon of Doppler ambiguity occurs for the fast-moving target. In this article, an offset frequency alignment method is proposed to solve the problem of Doppler ambiguity in FDA-MIMO radar. At the design stage, the Doppler frequency offsets, corresponding to different transmit pulses, are aligned by eliminating the residual phases. In this regard, the point targets can be focused in the transmit spatial frequency domain. Furthermore, the Doppler ambiguity index is estimated after principal velocity compensation resorting to a maximum likelihood criterion, where both the cases of known and unknown range ambiguity indexes are investigated, respectively. Moreover, in order to ensure the estimation performance, the frequency offset across the transmit array elements is designed by enlarging the term related to the Doppler frequency offset. At the analysis stage, the performance of Doppler ambiguity index estimation in terms of the root-mean-square error is assessed in comparison with the Cramer-Rao bound. Numerical results are provided to verify the effectiveness of the proposed method to resolve the Doppler ambiguity.