Nowadays, sparse multiple input and multiple output (MIMO) radar design considers the difference coarray of the sum coarray (DCSC) concept and the uniform degrees of freedom is improved significantly. Current sparse MIMO arrays are designed based on transmit and receive arrays with hole-free difference coarrays (DCAs). Recently, the uniform linear array (ULA) fitting (UF) principle is proposed which aims to design sparse arrays (SAs) with desired capacity using cascaded ULAs. However, SAs designed via UF can not guarantee hole-free DCAs. In this letter, we aim to use SAs designed via UF to devise sparse MIMO arrays with low mutual coupling. Strategies of using SAs with non hole-free DCA to design sparse MIMO arrays is introduced as well. Numerical simulations verifies the good performance of the proposed sparse MIMO array in high coupling scenarios.

A novel sparse array (SA) structure is proposed based on the maximum inter-element spacing (IES) constraint (MISC) criterion. Compared with the traditional MISC array, the proposed SA configurations, termed as improved MISC (IMISC) has significantly increased uniform degrees of freedom (uDOF) and reduced mutual coupling. In particular, the IMISC arrays are composed of six uniform linear arrays (ULAs), which can be determined by an IES set. The IES set is constrained by two parameters, namely the maximum IES and the number of sensors. The uDOF of the IMISC arrays is derived and the weight function of the IMISC arrays is analyzed as well. The proposed IMISC arrays have a great advantage in terms of uDOF against the existing SAs, while their mutual coupling remains at a low level. Simulations are carried out to demonstrate the advantages of the IMISC arrays.

Recently, uniform linear array (ULA) fitting (UF) principle is proposed for sparse array (SA) design using pseudo-polynomial equations. Typically, it is verified that the designed SAs via UF enjoy a lower bound on uniform degrees of freedom (uDOF), which is approximately $0.5N^2$ with $N$ sensors. Herein, an improved UF (IUF) scheme is proposed to significantly increase the lower bound on uDOF, which is realized by introducing two sub-ULAs, with name of adjoint transfer arrays (ATAs), on both sides of a transfer sub-ULA. The ATAs together with the transfer sub-ULA serve as a new layer to construct an adjoint transfer layer (ATL) that has improved aperture in comparison with traditional transfer layer to improve the lower bound on uDOF, which is derived in detail. Furthermore, two novel SAs are developed based on the ATL to verify the effectiveness of the proposed IUF scheme to increase the uDOF. Numerical simulations verify the superiority of devised SAs for direction-of-arrival (DOA) estimations.

Sparse array (SA) geometries are proposed to obtain low mutual coupling (MC) and improved degrees-of-freedom (DOF), which are composed of concatenations of multiple uniform linear arrays (ULAs) with certain closed-form expressions and are able to provide long virtual difference coarrays (DCAs) than many existing SAs. Meanwhile, the new proposed SAs have low coupling leakage, hence the proposed SAs have good trade-off between DOF and MC. Numerical experiments are created to give a verify the SA superiority in the presence of heavy MC.