Antenna separation is critical with MIMO systems as MIMO performance requires that the multiple paths between each of the transmit and receive antennas are highly de-correlated. The de-correlation of the antenna paths in turn has a heavy dependence on the angular separation of the paths when viewed from the perspective of the receiving antenna. One way to achieve path de-correlation is to separate the antennas in space. Angular separation improves as the distance increases between the antennas in the system. The minimum spacing between antennas specified in industry literature ranges widely, from 3 to 7 wavelengths. In theory, one wavelength of separation should be sufficient, but it depends on the environment and the amount of signal scattering.
Current LTE operator guidelines for spatial separation are on the order of 2 to 3 wavelengths, or 2’7” to 3’11” at 750MHz. For basic, linear antenna systems, TE Connectivity recommends 4 to 6 wavelengths of separation where possible (roughly 5’ to 8’ spacing), and an absolute minimum of 2 to 3 wavelengths when tight spaces dictate.
As an alternative to spatially separated antennas, dual-pole antennas are increasingly being used in MIMO deployments, as the measured performance differences between the two antenna configurations are negligible in real world applications. Dual-pole antennas are a great solution in situations where only a single antenna housing can be installed.
You can read the rest of this article in the September issue of Electronic Specifier Design by clicking here.