Multiple-input multiple-output (MIMO) technology is a technique that exploits multiple antennas to increase channel capacity without requiring additional spectrum or transmit power. With multiple antennas at the transmitter and receiver, capacity gains can be achieved by utilizing spatial and polarization diversity. In practical applications, due to the constraint of the spacing of the antenna elements, polarization diversity is preferred since the antennas can be co-located. At least one study has concluded that, with three orthogonal components of the electric field and three of the magnetic field, it is possible to obtain 6 independent channels at a single point. However there are few published MIMO antenna designs, and even fewer exploiting tri-polarization.
A MIMO wireless communication system is one that includes typically a plurality of antennas at a transmitter and one or more antennas at a receiver. The antennas are employed in a multi-path rich environment such that due to the presence of various scattering objects (buildings, cars, hills, etc.) in the environment, each signal experiences multipath propagation. User data is transmitted from the transmit antennas using a space-time coding (STC) transmission method as is known in the art. The receive antennas capture the transmitted signals and a signal processing technique is then applied as is known in the art, to separate the transmitted signals and recover the user data.
MIMO wireless communication systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved compared with previous systems because higher data rates can be obtained with MIMO. The multipath rich environment enables multiple orthogonal channels to be generated between the transmitter and receiver. Data for a single user can then be transmitted over the air in parallel over those channels, simultaneously and using the same bandwidth. Consequently, higher spectral efficiencies are achieved than with non-MIMO systems.
One problem with existing MIMO systems concerns the large size of the transmit and receive antenna arrays. Typically, MIMO transmit and receive antenna arrays have used spatially diverse antenna arrays. That is, the spacing between the individual antenna elements is arranged to be large enough such that decorrelated spatial fading is obtained. This is desired in order to reduce a need for the number of orthogonal channels from being reduced. That is, if the fading characteristics between antenna elements are similar (correlated) then the number of orthogonal channels that can be realized is reduced. For example, for rooftop installations, or antennas on towers, separations of up to 20 wavelengths can be required to achieve decorrelated fading due to the low angle spread of the multipath propagation.
It is desirable to both provide more axes such as exploiting tri-polarization with a three axis antenna and smaller size such as can be provided with a compact 3-port orthogonally polarized MIMO antenna.