Augmented co-prime array for enhanced DOA estimation with hole identification and filling strategies

The co-prime array can effectively reduce mutual coupling, but the holes in difference co-array greatly decrease the uniform degrees of freedom (uDOFs). Hole filling is a considerable strategy to increase uDOFs, and it is critical for direction-of-arrival (DOA) estimation, since the inappropriate hole filling strategy can result in severer mutual coupling. To simultaneously increase DOFs and reduce mutual coupling, a novel generalized co-prime array structure with a sparse hole-filling strategy is proposed in this paper. Firstly, a k-times extended coprime array with displaced subarrays (kCAD) is proposed, which can provide larger aperture and thus has the potential of increasing uDOFs. Then, in the kCAFD's difference co-array, the closed-form expressions of hole locations are derived. Finally, all the holes in the center of the difference co-array are filled by incorporating two extra subarrays, yielding a greatly increase in uDOFs. This structure is called as a supplementary co-prime array with displaced subarrays (SCAD) in this paper. Numerical simulations show that the proposed SCAD is superior to the traditional existing sparse arrays in terms of the uDOFs and mutual coupling.