Analyze UCA Based OAM Communication From Spatial Correlation

Orbital Angular Momentum(OAM) of Electromagnetic(EM) wave can be seen as a new degree of freedom(DoF) for wireless communication besides time and frequency. Uniform Circular Antenna Array(UCA) fed with signal of equal amplitude and equal phase difference on each antenna element is a good OAM generation method on radio frequency(RF) band. Previous researches have shown that UCA based OAM communication system is essentially a special case of Multiple-input Multiple-output(MIMO) system and elaborated the viewpoint from different perspectives. In this paper, we demonstrate the above viewpoint from a new perspective. Specifically, for UCA based OAM communication in ideal(line of sight(Los) and coaxial) channel, we systematically analyze the variations of radiation pattern and spatial correlation of UCA under different radius and distance settings. To extend our demonstration, we first simulate the channel capacity variation with the adjustment of UCA radius and distance, and then simulate the channel capacity of both UCA and uniform linear antenna array(ULA) based communication systems, and find that ULA can also achieve similar capacity performance as UCA when spatial distance scale is similar. As is known that MIMO communication system provides spatial DoF besides time and frequency, we can conclude that mode DoF of UCA based OAM essentially originates spatial DoF of UCA based on the analysis in the paper.