MIMO transmission diversity and spatial smoothing: A joint decorrelation effect study

To improve the performance of high-resolution bearing estimation algorithms in the presence of coherent targets, we investigate the joint decorrelation effect produced by the multiple-input-multiple-output (MIMO) based transmission diversity smoothing (TDS) and the spatial smoothing (SS). We utilize the degree of freedom (DOF) of smoothing (i.e., the number of covariance matrix averaged for smoothing) to evaluate the decorrelation effect quantitatively. Investigating the Hermitian square root of the jointly smoothed echo covariance matrix, we find the smoothing DOFs by the joint decorrelation processing is a product of that of SS and TDS. Due to the remarkably increased smoothing DOFs, the joint decorrelation processing provides great potentials to improve the decorrelation effect. Moreover, sparsely placing transmitters helps fully utilize the obtained smoothing DOFs to strengthen the non-singularity of the echo covariance matrix, and thus improves the joint decorrelation effect. Besides, introducing forward-backward spatial smoothing contributes to further improve the decorrelation effect. By balancing the subarray aperture and the smoothing DOFs, an upper limit of the subarray number and the maximum resolvable number of coherent targets are concluded for MIMO array design and performance evaluation. Numerical simulations validate the effectiveness of the joint decorrelation processing and its robustness against the transmit manifold errors.