  This paper proposes a new methodology to design optimal antennas for MIMO
(Multi-Input Multi-Output) communication systems by using spherical mode
expansion. Given spatial channel properties of a MIMO channel, such as the
angular profile at both sides, the optimal MIMO antennas should provide the
largest channel capacity with a constraint of the limited implementation space
(volume). In designing a conventional MIMO antenna, first the antenna structure
(current distribution) is determined, second antenna directivity is calculated
based on the current distribution, and thirdly MIMO channel capacity is
calculated by using given angular profiles and obtained antenna directivity.
This process is repeated by adjusting the antenna structure until the
performance satisfies a predefined threshold. To the contrary, this paper
solves the optimization problem analytically and finally gives near optimal
antenna structure (current distribution) without any greedy search. In the
proposed process, first the optimal directivity of MIMO antennas is derived by
applying spherical mode expansion to the angular profiles, and second a
far-near field conversion is applied on the derived optimal directivity to
achieve near optimal current distributions on a limited surface. The
effectiveness of the proposed design methodology is validated via numerical
calculation of MIMO channel capacity as in the conventional design method while
giving near optimal current distribution with constraint of an antenna
structure derived from proposed methodology.
