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.

In wireless access system, we need to use a limited frequency and electric power efficiently. And so we propose the fixed wireless access network using 5 GHz frequency which band has a good propagation performance in line of sight (LOS). In the proposed network, the several multi-level modulation methods are combined and identical frequency is reused by considering on the antenna directivity. As constructing this network, we can efficiently use frequency in 5 GHz band and enlarge system capacity. In this paper, it is assumed that user terminals are distributed nonuniformly over the service area. We analyze accommodation number of user terminals and the optimum combination of modulation methods. Numerical results show that most effective method is the combination of 16QAM and 256QAM, which can accommodate up to about 1.4 times as many users as only QPSK modulation method.