The core of the research is the design of multi-port antennas, e.g., several antennas positioned close to each other, is a challenge for modern wireless communications systems. The challenges associated with the design of multi-port antennas can be linked to the coupling between different elements. The coupling is described as the amount of power coupled to the neighboring radiation structures. The adverse effects of coupling can be viewed from two standpoints.
Firstly, to collect the maximum possible power from wireless channels, the radiation efficiencies of different antennas must be ideal. Coupling causes part of the radiated power to be dissipated at the termination of the nearby elements, and in this way basically deteriorates the radiation efficiencies.
Furthermore, the nature of MIMO wireless systems design inherently requires the received signals at different ports of the antennas to be independent. To our disadvantage, coupling contributes in creating spatial correlation between the received signals at different ports. The correlation between different received signals, in turn, by all means reduces the overall performance of these communication systems.
To deal with the coupling between nearby antenna structures different techniques are used. In the first step, we might be able to design our antennas in a way to make them radiate in different directions in space causing no mutual effects on each other. This approach can be more relevant for cases where the size of the antennas are not a limiting issue. A further technique to combat coupling is to use a proper cascaded matching network at the input of the antennas. In this technique, there is no strict limit on the positions of the antennas on the system and that how close they might be.
Figure: A sample of beam-forming for a four-port antenna in a rich uniform multipath environment.