On the properties of small arrays with closely spaced antenna elements

In a multipath-rich environment, increased capacity can be gained by replacing single-input single-output (SISO) with multiple-input multiple-output (MIMO) communication systems. However, at mobile terminals severe limits on the implementation of multiple antenna elements are posed by,the size of the available platform (e.g. PC-card). In this situation, a reduction of element spacing significantly below lambda/2 becomes of high interest, but the associated system degradations must be traded against the benefit of a higher number of antenna elements. In a previous paper by the authors [1], pattern distortion and SNR degradations caused by mutual coupling (MC) in dense arrays were evaluated, utilising a novel eigenmode representation of the antenna array. In this letter the same approach is used to provide new results for the effect of MC onto the correlation between the complex valued radiation patterns provided by the different antenna ports of a dense array. It was shown in recent papers [2, 3], that MC can (in contrast to common belief) help to reduce the correlation between signals received at the different ports. This letter aims to provide a rigorous model for a quantitative analysis of this effect. Additionally, the reduction of signal power caused by MC is considered and the utilisation of a previously proposed decoupling network [1, 4] is briefly discussed. To allow for a compact presentation of the basic ideas, we restrict ourselves to the consideration of the most simple case, namely a symmetric 2-element structure and neglect dissipative losses in the antenna. However, a generalization to arrays with a higher number of antenna elements is straightforward.