In this paper, an eight-element MIMO array for 5G smartphone applications in the 3. GHz) is presented. The array consists of two types of four-antenna arrays (U-shaped and L-shaped coupled-fed loop elements), which are symmetrically distributed in the inner of the smartphone frame. The dimension of the system circuit board is 124 mm × 74 mm and the size of two elements is 4.8 mm × 9.8 mm (0.055λ × 0.11λ, λ represents the free-space wavelength at 3.45 GHz) and 4.9 mm × 12.5 mm (0.056λ × 0.14λ), respectively. The proposed MIMO array is simulated, and a prototype is fabricated and tested. The results show that all the elements can cover the desired band of 3.3-3.6 GHz under the condition of −6-dB impendence bandwidth. The isolations are enhanced to 15 dB by combining the inverted-I ground slots with neutralization line (NL) structure. In addition, the envelope correlation coefficient (ECC) via any two elements is below 0.15 that shows good independence in far-field radiation characteristic. The measured efficiencies of the elements in the operating band are higher than 40%. Moreover, the array ergodic channel capacity is also calculated based on the correlation matrix method to be about 35 bps/Hz with a 20-dB signal-to-noise ratio. In addition, the effects of the user's hand and the head has been analyzed as well. Based on the above, the proposed eight-element MIMO array is a prospective candidate for future 5G smartphone applications.INDEX TERMS Channel capacity, eight-element array, high isolation, MIMO antenna.

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In this paper, a compact dual-band MIMO antenna array for 5G smartphone applications is proposed. The MIMO antenna consists of eight folded monopole antennas operating at 3.45-GHz band (3300∼3600MHz). And the antenna elements with a size of 6.8 × 6.6 × 4 mm 3 (about 0.078λ× 0.075λ× 0.046λ at 3.45 GHz), are disposed along each edge of the system circuit board. By introducing the decoupling structures, the isolation between inner antenna elements is improved from 10dB to 15.1dB in 3.45-GHz band (3300∼3600MHz). Meanwhile the inner antenna units can generate an additional operating band covering 2400∼2700MHz due to the coupling effect between the antenna elements and the proposed decoupling structures, which is promising for solving the problem of terminal space shortage. To verify the design principle of the proposed MIMO antenna array, the antenna is fabricated and measured. The measured results of efficiency and ECC are analyzed as well to demonstrate the performance of the proposed MIMO array. Channel capacity, user proximity and SAR analysis and are also given in this paper.INDEX TERMS Decoupling structure, eight-element array, high isolation, MIMO antenna.