On user effect compensation of MIMO terminals with adaptive impedance matching

Proximity of user has been established as a major cause of performance degradation in multiple-input multiple-output (MIMO) terminals. Here, we investigate the potential of adaptive impedance matching (AIM) to improve the performance of two MIMO terminals in a free space (FS) and a two-hand (TH) user scenario, assuming uniform 3D angular power spectrum for the incoming signal. The results show that lossless uncoupled AIM circuits can enhance the MIMO capacity of the terminals by up to 43% relative to standard 50 ohm terminations. In particular, the largest capacity gain is obtained in the terminal with good matching but narrower antenna bandwidth and worse isolation performance, for the TH scenario. On the other hand, the terminal with poorer matching but higher isolation and significantly larger bandwidth has lower average capacity in FS. However, it is more robust to user proximity, and hence it achieves only 8% capacity gain with AIM in the TH scenario. Therefore, antenna design parameters can significantly influence potential benefits of AIM in practice.