How to make smart antennas in handsets feasible

Smart antennas are clusters of several antenna elements. By their directivity, they are offering fascinating possibilities for celluar communication networks. In some base stations they are already in use. However, their integration into mobiles is much more difficult and is not realized yet. Obviously, the limited space in handhelds is a major challenge. The narrow space in a mobile forces to offend against the golden rule, that the spacing between the array elements should not be smaller than half of the used wavelength of the transmitted or received radiation. With a wide spacing a better directivity can be obtained.