Orbital Angular Momentum Radio-Wave-Based 2-D Target Angle Estimation With Mutual Coupling

Vortex electromagnetic waves are conveniently produced by a uniform circular array (UCA). The mutual coupling matrix (MCM) of UCAs presents rotational symmetry, which can be employed to cut down the parameters in the circuit model. The impedance matrix also does not rely on a specific port. This work uses rotational symmetry to expand the MCM into the orbital angular momentum (OAM) mode domain. The OAM-domain phase sequence is derived by the discrete Fourier transform (DFT) of the first row of the MCM. The connection between the number of OAM modes and the excitation of elements is described. Furthermore, an improved propagator method (PM) for OAM-based decoupled 2-D angle estimation is proposed. This method employs the original steering vector of OAM waves to process the receiving signal, which avoids errors caused by the Bessel function approximation. The estimation error is also reduced by redefining the propagator matrix. In terms of computational complexity, the proposed method avoids spectral search-and-element level eigenvalue decomposition (EVD). The results demonstrate that the coupling is compensated effectively by the OAM-mode domain MCM. The error of the proposed method is less than 1° under the condition of SNR = 20 dB and 1000 snapshots. The computational complexity is reduced by 1-2 orders of magnitude compared to the origin PM. The method is also suitable for 1-D estimation scenarios.