| MIMO(Multiple-input multiple-output)wireless systems, in which multiple antennas are used at both transmitter and receiver, have demonstrated the great potential for increased spectrum efficiency and channel capacity.The major issue of MIMO systems is that capacity gain is highly dependent on the spatial correlation,which is a function of the antenna array and channel characteristics.In order to achieve a compact array,mutual coupling becomes particularly significant as the interelement spacing is decreased.Therefore,it is of great value to analyze the impacts of mutual coupling on the properties of MIMO channel.In this paper,the relation between different energy distributions and spatial correlation and the effect of mutual coupling on the performance of MIMO systems are studied.The main work of this paper detailed as follows.Firstly, we derive spatial correlation functions of a uniform circular antenna arrays for three type of angle-arrival(AOA) distributions and their bi-modal distributions.Three different AOA distributions are considered in this papena uniform angular distributions Gaussian angle distribution,and a Laplacian angle distribution.The generalized spatial correlation functions are investigationed carefully by exact and approximate analyses.Results show that the oscillation of correlation function depends on the difference between the AOA’s of the two modes.Secondly, the impacts of mutual coupling on the performance of MIMO wireless channel are analyzed. From the analysis of the antenna model by network equivalent analysis, the impedance matrix and the mutual coupling matrices of transmitter and receiver are obtained. A general expression of the spatial correlation between a pair of mutually coupled antennas is explicitly derived, and the conditions under which mutual coupling has no effect on the spatial correlation are also identified. The effects of mutual coupling on the parallel antenna array and the collinear antenna array are investigated. Simulation results show that the impacts of mutual coupling on parallel antenna array and collinear antenna array are quite different in the performance of MIMO channel, and the performance of MIMO system affected by mutual coupling can be improved effectively by properly selecting antenna array.Thirdly, considering the effect of mutual coupling, the channel capacity of a uniform linear array (ULA) will decrease rapidly with the decreasing of distance between antenna array elements, this paper provides a uniform circular array (UCA) with symmetric structure. A general expression of the spatial correlation between a pair of mutually coupled antennas is explicitly derived base on the established equivalent model, then the stability of the channel capacity of ULA and UCA are also compared in the case of truncated Guassian distribution of power azimuth spectrum. Simulation results show that this symmetric UCA can provide more stable channel capacity than ULA due to its complementary properties among the antenna array elements. The effect of mutual coupling on the performance of MIMO system can be improved effectively by selecting UCA.Fourthly, a three-dimensional antenna array channel model for frequency nonselective Rayleigh fading channel is set up and the channel model setting is extended to space. This paper analyzes mutual coupling between antenna arrays and obtains the equivalent network model of the multiple-antenna. The general spatial correlation expression of channel model with the impact of mutual coupling is derived. When the receivers are different antenna arrays, the effects of the angular parameters associated with the mean azimuth of arrival(MAOA),azimuth spread(AS),mean elevation of arrival(MEOA)and elevation spread(ES) on the system capacity is investigated. Numerical results verify the different angle spread impact on spatial correlation. The simulation results indicate that the system capacity with the mutual coupling is more dependent on the MAOA and MEOA than the AS and ES. |
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