In many cases, MIMO technology can exploit both spatial and polarization diversity. Polarization describes the direction of an electric field emanating from an antenna element as it propagates through the atmosphere. Linear polarizations will have an electric field in either the vertical (90º) or horizontal (0º) planes whereas slant polarizations propagate -45º or +45º from the horizontal reference plane. Some polarization diversity systems employ circular polarization where the signal rotates forward in three dimensional space in either a clockwise (right hand) or counter-clockwise (left hand) fashion. Antennas with linear polarizations are generally omnidirectional monopoles or dipoles, while patch or helical antennas can exhibit circular polarization. Generally speaking, orthogonality between antennas is highly desirable to provide high isolation between the antennas across a specific frequency band.
Regardless of an indoor or outdoor environment, two parallel channels can be achieved through polarization diversity creating another level of freedom in a given environment. This is particularly useful for backhaul infrastructures that must consider the tradeoff of link distance and throughput as well as solutions operating in the unlicensed ISM bands with tight EIRP restrictions. The physical separation between antennas for adequate spatial diversity presents a space efficiency problem that can be overcome with two polarizations for equivalent, and in some cases better, system throughput . Experiments have shown that the use of two polarizations in environments with typical scattering can yield capacity gains from 10% to 20% over spatially separated antenna elements . Figure 3 shows a number of antenna types with their respective radiation patterns that utilize dual polarization diversity. Some also leverage spatial diversity for indoor/outdoor wireless LAN systems.