A multiple-input multiple-output (MIMO) antenna system is proposed for fifth generation (5G) and fourth generation (4G) mobile communication. The design meets all of the requirements of both 5G and 4G antennas using only a single structure. Since 5G will work at millimeter-wave (mm-Wave), the proposed design serves triple bands at mm-Wave (28, 37 and 39 GHz) for 5G in addition to 2 GHz band (1.8-2.6) for 4G. Each MIMO element consists of a slot-based antenna, fed by two microstrip feeders for the 5G and 4G bands. The design works as a tapered slot antenna at mm-Wave offering end-fire radiation for 5G and works as an open-ended slot antenna for a 2 GHz band offering omni-direction radiation for 4G. The slot antenna type used in the proposed design produces wide bandwidths for the 5G and 4G. The overall volume of each MIMO antenna element is 0.21 × 0.10 × 0.003 λ 3 , where λ is the wavelength of the lowest operating frequency. As a proof of concept, a prototype is developed and tested. The measured results show a wide impedance bandwidth of |S 11 |< -10 dB covering the band 27.5-40 GHz for 5G, and impedance bandwidth of |S 11 | < -6 dB covering the band 1.8-2.6 GHz for 4G.INDEX TERMS 5G, 4G, MIMO antenna, mobile communication, slot antenna.

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The design of 5G 4-element antenna array system with the capability of switching its operation between right hand circular polarization (RHCP) and left hand circular polarization (LHCP) is proposed. The CP-reconfigurability is achieved by electronically switching two PIN-diode pairs in the feeding circuit, which consists of a branch line coupler and two feeding arms. The proposed configuration reduces the number of tuning elements and allows the use of imperfect PIN diodes at millimeter-wave frequency. The fabricated prototype exhibits a 3-dB axial ratio bandwidth from 27.2 to 28.35 GHz. A maximum measured gain of about 6 dBic and a radiation efficiency of around 51% over the bandwidth are achieved for both CP states. INDEX TERMS Branch-line coupler, circularly polarized, patch antenna array, polarization-reconfigurable, 5G.