Designs of 8 × 8 MIMO Mobile Phone Antennas for 5G Mobile Communication System

The size of the mobile phone is $158 \mathrm{~mm} \times 80$ $\mathrm{mm} \times 8 \mathrm{~mm}$. The first antenna is designed in the frequency band of the fourth generation (4G) mobile communication system. It uses inverted-F antennas, monopole antennas, slit antennas, T-circuits, and parallel circuits. The frequency band is LTE 700, GSM 850 MHz at the low frequency band; GSM $1800 / 1900 / 2100 \mathrm{MHz}$, LTE $2300 / 2500 \mathrm{MHz}$ in the intermediate frequency band; 5G 3500/4900 MHz in the high frequency band. The antenna efficiency and gain are 76%/ $47 \% / 55 \%$ and $1.75 \mathrm{dBi} / 1.9 \mathrm{dBi} / 2.69 \mathrm{dBi}$, respectively. The second antenna is designed in the frequency band of the fifth generation (5G) mobile communication system. It is designed to be $4 \times 4$ MIMO on both sides of the long frame of the mobile phone. The frequency bands are 3500 and 4900 MHz. The antenna efficiency and gain are $73 \% / 91 \%$ and 4.4 $\mathrm{dBi} / 5.9 \mathrm{dBi}$, respectively. The designed $8 \times 8 \mathrm{MIMO}$ antenna (integration of 4 G and 5 G antenna) has a measured return loss above 6 dB and an isolation below -10 dB.