f-MISC: Flexible Sparse Antenna Array Based on Maximum Inter-Element Spacing Principle

For a robust direction of arrival (DOA) estimation in a mutual coupling environment, a preferable sparse antenna array should have three main features: a hole-free difference co-array (DCA), more uniform degrees of freedom (uDOFs), and fewer sensor pairs with smaller separations. This letter proposes a flexible sparse antenna array based on the maximum inter-element spacing constraint (f-MISC). The proposed f-MISC antenna array comprises five uniform linear arrays (ULAs) and two separate elements, which are configured by an inter-element spacing (IES) set. The maximum IES and the total number of elements govern the IES set. Compared with the original MISC arrays and its improved variants, the f-MISC reduces the mutual coupling effects and enhances the uDOFs. In addition, the maximum IES of the proposed f-MISC can be flexibly adjusted to control the mutual coupling as required. Advantageously, the f-MISC possesses a hole-free DCA. Closed-form expressions for antenna placement, uDOFs, and weight functions are derived. Simulation results confirm the superiority of f-MISC arrays.