Fundamental limitations on small multi-beam antennas for MIMO systems

The paper finds theoretical limitations with MIMO technology of higher order when space is limited. The results show that it is possible to achieve up to six independent identically distributed (i.i.d.) channels in rich isotropic multipath (RIMP) by combining different incremental sources to an incrementally small 6-port antenna. This means that we with incrementally small antennas can generate MIMO performance with maximum six data streams in RIMP (rank 6). We also show that it is possible to achieve completely isotropic full-spherical angular coverage by using an incrementally small 3-port antenna consisting of three orthogonal electric or magnetic dipoles. However, it is possible to get isotropic spatial coverage with 3 dB higher directivity in all directions by using a 6-port antenna consisting of both three orthogonal electric dipoles and three orthogonal magnetic dipoles, or alternatively a 6-port Huygens antenna consisting of 6 uncoupled Huygens sources. In random-LOS (having only a single angle of arrival) there will be a maximum of two data streams (rank 2) available, associated with two orthogonal polarizations. We show that both streams can be received with the 3-port incremental dipole antenna, and with 3 dB higher power with either of the 6-port antennas. The randomness of the angle of arrival in LOS is caused by the arbitrary orientation and location of the user-terminal.