Direction-of-arrival estimation for closely coupled arrays with impedance matching

Conventionally, the spacing between adjacent elements in an antenna array are set at half-a-wavelength in order to obtain maximum spatial resolution while avoiding spurious sidelobes. However, for antenna arrays that are implemented in a compact manner, the spacing between the adjacent elements can be significantly less than half-a- wavelength. The small spacing between the antennas results in strong mutual coupling between the antennas, which has been shown to affect the performance of array algorithms, through modifying the antenna pattern and reducing the antenna efficiency. Recently, impedance matching techniques have been proposed for multiple antenna systems for diversity and MIMO applications, in order to counteract the aforesaid coupling effects. In this paper, we study the impact of different impedance matching conditions on the Cramer-Rao bound performance of direction-of-arrival estimation using closely coupled arrays. We demonstrate that an appropriate matching network can drastically improve the CRB performance with respect to 50 ohm termination or self impedance termination with coupling, and the case of ideal antenna arrays with no coupling. For example, a tenfold reduction in RMSE over the simple matching terminations was obtained with the multiport conjugate match for a two signal scenario and an antenna separation of 0.1 wavelength.