Improve Degrees of Freedom in MIMO Antennas Based on Co-located Electric and Magnetic Dipoles (Invited)

In multi-input multi-output (MIMO) systems, a high channel capacity can be realized by increasing the number of antennas, or the degree of freedom (DOF) more precisely. In the traditional MIMO systems, designing antenna arrays by separating each radiated element in physical space were the common methods to increase the number of antennas. However, it took up much space because the distance between the adjacent element had to be further than a certain wavelength for high DOFs. For a small size, polarization diversity can be used to increase the DOFs, and it can be also a suitable solution for multi-path effect. The tri-polarized antennas were studied which fully used three dimensions of the physical space for 3 DOFs.However, 3 DOFs are not the limitation of the polarization-diversity antennas even if the three dimensions are fully used. The co-located antennas with Electric dipoles and Magnetic dipoles were researched for higher DOFs. It has been proved that 1 E-dipole and 1 M-dipole can get 2 DOFs, and 3 orthogonal E-dipoles and 3 orthogonal M-dipoles collocated at the same centers could realize 6 DOFs in theoretical aspect. In addition, an E-dipole and a M-dipole pointing in orthogonal directions with the same centers have a same polarization in the far field, so the isolation enhancement is the challenge of the collocated antennas with E-dipoles and M-dipoles.There are some methods to increase the isolation between the E-dipoles and M-dipoles: i) Based on the collocated antenna theory, the phase centers of E dipoles and M dipoles need to be closed; ii) Artificial magnetic conductors (AMC) can reduce the distance between the E dipole and M dipoles, which is mainly designed for E dipoles; iii) E dipoles or M dipoles can be rotated 45° for 3 dB isolation increase. With simulation and experimental verifications, 4-DOFs antennas for directional radiations and 6-DOFs antennas for full space radiations are researched.