  The performance of holographic multiple-input multiple-output (MIMO)
communications, employing two-dimensional (2-D) planar antenna arrays, is
typically compromised by finite degrees-of-freedom (DOF) stemming from limited
array size. The DOF constraint becomes significant when the element spacing
approaches approximately half a wavelength, thereby restricting the overall
performance of MIMO systems. To break this inherent limitation, we propose a
novel three-dimensional (3-D) antenna array that strategically explores the
untapped vertical dimension. We investigate the performance of MIMO systems
utilizing 3-D arrays across different multi-path scenarios, encompassing
Rayleigh channels with varying angular spreads and the 3rd generation
partnership project (3GPP) channels. We subsequently showcase the advantages of
these 3-D arrays over their 2-D counterparts with the same aperture sizes. As a
proof of concept, a practical dipole-based 3-D array, facilitated by an
electromagnetic band-gap (EBG) reflecting surface, is conceived, constructed,
and evaluated. The experimental results align closely with full-wave
simulations, and channel simulations substantiate that the DOF and capacity
constraints of traditional holographic MIMO systems can be surpassed by
adopting such a 3-D array configuration.
