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Optimal Design Method of MIMO Antenna Directivities and Corresponding Current Distributions by Using Spherical Mode Expansion
Maki ARAI Masashi IWABUCHI Kei SAKAGUCHI Kiyomichi ARAKI
Publication
IEICE TRANSACTIONS on Communications
Vol.E100B
No.10
pp.18911903 Publication Date: 2017/10/01
Online ISSN: 17451345 Type of Manuscript: PAPER Category: Antennas and Propagation Keyword: MIMO, antenna directivity, antenna structure, current distribution, spherical mode expansion, capacity maximization,
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Summary:
This paper proposes a new methodology to design optimal antennas for MIMO (MultiInput MultiOutput) 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 farnear 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.

