  Reconfigurable intelligent surface (RIS) has been widely considered as a key
technique to improve spectral efficiency for 6G communications. Compared with
most existing research that only focuses on the reflective RIS, the design and
prototyping of a novel transmissive RIS are presented in this paper, and its
enhancement to the RIS-aided communication system is experimentally
demonstrated. The 2-bit transmissive RIS element utilizes the penetration
structure, which combines a 1-bit current reversible dipole and a 90{\deg}
digital phase shifter based on a quadrature hybrid coupler. A transmissive RIS
prototype with 16$\times$16 elements is designed, fabricated, and measured to
verify the proposed design. The measured phase shift and insertion loss of the
RIS element validate the 2-bit phase modulation capability. Being illuminated
by a horn feed, the prototype achieves a maximum broadside gain of 22.0 dBi at
27 GHz, and the two-dimensional beamforming capability with scan angles up to
$\pm$60{\deg} is validated. The experimental results of the RIS-aided
communication system verify that by introducing the extra gain and beam
steering capability, the transmissive RIS is able to achieve a higher data
rate, reduce the transmit power, improve the transmission capability through
obstacles, and dynamically adapt to the signal propagation direction.
