Adaptive reactance-controlled antenna systems for multi-input multi-output applications

The study addresses the limited performance of multi-input multi-output (MIMO) systems under the real-life effects of spatial correlation and antenna mutual coupling. For this reason, an adaptive antenna system (AAS) capable of changing its transmission parameters via passive radiators attached to tunable loads, according to the structure of the RF propagation channel is described. The multi-active multi-passive array structure is tractably analysed using the active element response vector (instead of the classical steering vector) and the active element current vector (all being functions of the variable loading). The AAS targets at maximising tight MIMO ergodic and outage rate bounds, relying on partial channel knowledge when tuning to a different loading state for optimising the rate of communication. The proposed AAS can be limited to practical dimensions whereas the passive antennas require no extra RF hardware, thus meeting the cost, space and power constrains of the users' mobile terminals. The simulation results show that the AAS, thanks to its 'adaptivity', is able to achieve satisfactory performance even in poor scattering environments whereas a significant part of the mutual information that is lost owing to the spatial correlation and the electromagnetic coupling is successfully retrieved.