Decorrelating Closely-Placed Antennas by Pattern Design in Uniform Scattering Environments

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MIMO system's performance depends greatly on the correlation of employed antennas. The more the correlation is, the worse the performance will be. We propose a solution to the problem of decorrelating closely spaced antennas assuming the antennas have identical radiation patterns with the same steering angles. In fact, we will show that by proper steering of the antenna patterns, the decorrelation distance is reduced to 0.25λ which is a great improvement comparing with the well-established 0.4λ result coming from the Bessel-shaped correlation expression. Also, we analyze this result in the regime of very closely placed antennas and by proposing the definition of Decorrelation Factor of an antenna pattern, provide the framework of comparing the correlation behavior of different antennas. Furthermore, we apply our results to two practical directional antennas, namely, the half-wavelength dipole and the vertical electric dipole antennas.