Wideband Gain Enhancement of MIMO Antenna and Its Application in FMCW Radar Sensor Integrated With CMOS-Based Transceiver Chip for Human Respiratory Monitoring

A novel compact multiple-input multiple-output (MIMO) antenna is proposed with wideband gain enhancement. It consists of two identical antenna elements spacing about half wavelength. First, a 1-to-2 Y-shaped power divider with wideband anti-phase outputs is designed to feed two modified Vivaldi radiators. The currents on the two radiating surfaces keep the same direction in a wideband to increase radiation without being canceled. Second, the square-ring unit cell is analyzed to form metasurface, placed in the front of the radiator. It can guide the forward electromagnetic (EM) radiation while reducing the backward radiation. Third, a U-shaped slot is etched between the two modified Vivaldi radiators. It is approximated as an effective electrically small dipole radiator. Such arrangement further effectively enhances the radiation performance in the direction of the main lobe and destructively interferes with the sidelobe radiation. These features make the antenna far-field radiation pattern reshaped. Thereby, the gain is improved in a wide bandwidth. Meanwhile, the MIMO antenna diversity performance is analyzed with low mutual coupling. Wideband gain enhancement contributes to improve the detection capability of the frequency modulated continuous wave (FMCW) radar sensor. As a proof of concept, the proposed MIMO antenna prototype is fabricated. The measured impedance bandwidth ranges from 11.5 to 21.3 GHz [59.76% fractional bandwidth (FBW)] with isolation of <inline-formula> <tex-math notation="LaTeX">$\ge 24.92$ </tex-math></inline-formula> dB. The gain is up to 10.6 dBi, the radiation efficiency is 88.01%–90.02%, and the envelope correlation coefficient (ECC) is <inline-formula> <tex-math notation="LaTeX">$\le 0.00122$ </tex-math></inline-formula>. Integrated with the transceiver chip fabricated on the 65 nm CMOS process, the proposed MIMO antenna is applied in the FMCW radar sensor. The test system is built up, and the experiments on different breathing conditions are conducted for different human subjects. The proposed MIMO antenna could reduce measurement error, thereby improving measurement accuracy for human respiratory monitoring.