Nested inter-antenna spacing constrained sparse arrays for direction finding

In the design of a sparse array with large uniform degree of freedom (uDOF) for direction finding, the minimum inter-antenna spacing is usually constrained to be no more than half-wavelength. This is not attractive for reducing antenna mutual coupling especially when the antennas are dense. To alleviate antenna mutual coupling, we herein propose a new sparse array structure whose inter-antenna spacing is larger than half-wavelength. This three-level nested inter-antenna spacing constrained (termed NISC) array can be obtained by implicitly imposing the inter-antenna spacing constraint on three sparse uniform linear arrays (ULAs). The NISC difference co-array has holes due to the one-wavelength inter-antenna spacing constraint; however, these holes can be easily filled by adding just one antenna into NISC (termed NISC-A). Both the spatial smoothing subspace schemes and the compressive sensing schemes can be directly applied to the proposed NISC and NISC-A for direction finding. Simulation results are provided to illustrate the behaviors of NISC and NISC-A, in terms of antenna mutual coupling reduction, uDOF increase, and direction finding accuracy.