Impact of Antenna Design on MIMO Performance for Compact Terminals With Adaptive Impedance Matching

Using the metrics of channel capacity and multiplexing efficiency, the adaptive impedance matching (AIM) performances of two multiple-input multiple-output (MIMO) terminals with different antenna designs were evaluated and compared. The evaluation was performed in LTE Band 18 Downlink (860–875 MHz) under realistic usage conditions of two measured user handgrips and simulated propagation channels with different angular spreads (ASs). The results provide potential performance gains from AIM based on realistic MIMO terminal prototypes, and the underlying mechanisms by which the gains were achieved, which can serve as antenna and AIM circuit design guidelines. In particular, the evaluation revealed that ideal uncoupled AIM networks can increase the capacity by up to 52% relative to $50\text{-}\Omega$ terminations. However, the observed gains depend heavily on the antenna design, the user scenario, and the channel’s AS. For example, the wideband design in different user cases experienced capacity gain of 4%–9% from AIM in uniform 3-D channels, in contrast to the 1.3%–44% gain seen in a conventional narrowband design. In nonuniform channels with small ASs, the AIM gain for different mean incident angles depends on the absolute mean effective gain (MEG) and the change in correlation due to AIM; in cases where AIM has little impact on correlation, the mean incident angles with high AIM gains were close to those with high MEGs.