  This study presents the design of four $178\times178$ $(mm)^{2}$ wideband,
high gain, highly efficient metasurface-based 4T4R MIMO (Multiple-Input
Multiple-Output) antennas with highly isolated ports, covering the middle and a
portion of the upper bands of the sub 6 GHz 5G frequency spectrum for 5G-based
systems, such as IoT (Internet of Things) applications, vehicular
communications (e.g., rooftop antennas of cars or trains), smart industries
(e.g., farms and factories). The radiating elements of these antennas use the
aperture-coupled feeding technique with a dumbbell-shaped slot, a truncated
square patch with two U-shaped slots, and a metasurface layer. The proposed
MIMO structures place four identical radiating elements like a $2\times2$
matrix with $90^\circ$ successive rotations to produce orthogonal
electromagnetic waves, improving the isolation between ports. Six-millimeter
spaces are added between these elements, and two vertical and horizontal strip
slots are carved on the ground as the decoupling structure to decrease the
mutual coupling. Simulation results show that Antenna\_{1}, Antenna\_{2}, and
Antenna\_{3} achieve gain values of 6.2 to 9.4 dBi, 8.2 to 11.6 dBi, 6.2 to 9.5
dBi, below -35, -25, and -33 isolation and almost 10 dB diversity gain from 2.8
to 4.7 GHz, 2.8 to 4.5 GHz, and 2.7 to 4.9 GHz, respectively. As a prototype,
Antenna\_{4} is manufactured, and measurements are performed. It achieves 6.28
to 10.45 dBi gain values, below -23 dB isolation, and 0.001 envelope
correlation coefficient over 2.7 to 4.3 GHz. The results confirm that the
proposed MIMO antennas are compatible with the 5G essential requisites.
