Coarray Polarization Smoothing for DOA Estimation with Coprime Vector Sensor Arrays

In this paper, a new scheme is proposed to estimate the directions-of-arrival (DOAs) of multiple sources using a coprime electromagnetic vector sensor (EMVS) array. Each EMVS consists of three mutually orthogonal electric dipoles and three mutually orthogonal magnetic loops, all collocated at a single point in space. By exploiting the polarization diversities embedded in the vector sensors, the COarray Polarization Smoothing (COPS) is presented to increase the degrees of freedom (DOFs) from the polarization coarray domain. In contrast to the widely used spatial smoothing technique, the COPS offers the following new insights: 1) it is source polarization-dependent and is proved to offer $20$ DOFs at most, 2) it does not reduce the efficient spatial coarray aperture of the coprime array so that it enables the use of all virtual coarray sensors without resorting to the nontrivial sparse recovery or array interpolation operations, and 3) it can be synthesized with the spatial smoothing to get the DOFs multiplied. Finally, the efficacy of the COPS scheme is verified by numerical examples.