In this paper, we investigate the thermal noise behavior of the multi-antenna communication systems, when antenna elements are closely spaced. We analyze the mutual coupling effect on thermal noise. We apply the Nyquist's thermal noise theorem to determine thermal noise power in the multi-antenna system and to confirm the partial correlation of thermal noise for antenna spacing lower then one wavelength. Simulation results confirm the decrease of thermal noise power level when antenna spacings drop below a half wavelength.
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