Dual-Band End-Fire Four-Element MIMO Antenna Array Using Split-Ring Structure for mm-Wave 5G Applications

This paper presents a dual-band split-ring structure that provides end-fire radiation characteristics for multi-input multi-output (MIMO) mm-wave 5G applications. The proposed single antenna element consists of a main split-ring resonator (SRR) directly fed using a high impedance microstrip transmission line. A lamda/4 transformer is employed to match the high impedance transmission line with a 50-ohm line. Two symmetric parasitic SRRs are used as directing elements to give end-fire radiation properties for the introduced antenna at the first resonant frequency. The parasitic SRRs are also utilized to produce the second resonant frequency by establishing a capacitive link with the main SRR. The single element antenna radiates at 24.5 and 28.5 GHz. A cross-shaped MIMO structure composed of four elements of the proposed antenna is introduced to improve the isolation of the MIMO system due to its orthogonal polarization characteristics. The proposed MIMO demonstrates acceptable values of gain and radiation efficiency, reaching 9.8 dB and 93%, respectively, at the first resonant frequency, and 8.7 dB and 95%, respectively, at the second band. MIMO performance is also studied, and the proposed MIMO system provides a very low envelope correlation coefficient of lower than
$10^{-4}$
over the two operating frequency bands. A diversity gain is also calculated, it is around 10 dB over the two operating bands. The proposed high-isolation dual-band cross-shaped MIMO array is a promising candidate for millimeter-wave 5G applications.