RF beamforming with closely spaced antennas

This paper investigates the performance enhancements using radio-frequency (RF) beamforming on closely spaced antennas in multiple-input multiple-output (MIMO) communication system. Of specific interest is the case of a array of extremely closely spaced antennas with complexity constraints, here implemented being restricted to a single RF chain. Our system model is developed in context of closely spaced metamaterial antennas that have low mutual coupling. Even without mutual coupling, the physical location of the antennas still contributes to strong spatial correlation. We also re-derive the mutual information equation for both RF and digital beamforming techniques under the assumption that the transmitter has no information regards to the channel. Further, by comparing the capacity improvement between different receiver combining techniques, this paper suggests that in a closely spaced antenna system with only one RF-chain and transmitter, selection across beams provides higher capacity than using the traditional approach of element-selection. Furthermore, beamforming based MIMO processing is more robust to different scattering environments.