High-Isolation Eight-Element MIMO Array for 5G Smartphone Applications

In this paper, an eight-element MIMO array for 5G smartphone applications in the 3.45-GHz band (3.3-3.6 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 x 74 mm and the size of two elements is 4.8 mm x 9.8 mm (0.055 lambda x 0.11 lambda, lambda represents the free-space wavelength at 3.45 GHz) and 4.9 mm x 12.5 mm (0.056 lambda x 0.14 lambda), 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.