Beyond 20 Bits/s/Hz Channel Capacity Realization With Compact MIMO Antennas for Human-Centric IoT Devices

This article presents a highly compact multiple-input-multiple-output (MIMO) antenna system that maintains a high and stable channel capacity above 20 bits/s/Hz in dynamic channels for human-centric Internet of Things (IoT) devices. The MIMO array consists of four compact antenna elements and four compact control circuits operating at 2.45 GHz. This antenna system achieves a 64.5% reduction in dimension compared to the traditional microstrip patch. Each antenna is connected to a compact control circuit, boasting a wide power-tunable range exceeding 30.3 dB and a phase difference within $90^{\circ }~\pm ~15^{\circ }$ . The transmission coefficient between every two elements of the MIMO array is designed to be less than −20.9 dB. The well-integrated structure of the antenna and coupler realizes a wide range weight function that mitigates the polarization mismatch caused by the variations in the cross-polarization power ratio (XPR) of incoming waves and the antenna’s inclination angle during human arm-swing motion in human-centric IoT use scenarios. The radiation performance concerning polarization control was examined in an anechoic chamber. Furthermore, a $4\times 4$ MIMO channel capacity experiment was conducted using a 3-D spatial fading emulator with an arm-swinging human phantom. A reasonable agreement between the measured and simulated results indicates that the proposed antenna system can achieve a high and stable MIMO channel capacity of over 20 bits/s/Hz irrespective of the XPR and inclination angles.