In 6G technologies, wireless communication based on millimeter-wave (mmWave) is promising for its higher speed and lower latency. But to solve the path loss problem derived from its short wavelength, ultra-massive multi-input multi-output (MIMO) architecture is currently adopted where large number of antennas are required. This RF system with increased number of antennas result in problems of large power consumption and complexity. Thus, mmWave beam controlling reconfigurable intelligent surface (RIS) with low power consumption may role as a key technology for the post-MIMO [1]. Various types of beam steering metasurface have been reported, but most of them are relying on its nanostructure, diode operation, etc. so that control in active and gradual way is unavailable once it is fabricated.

Electrowetting technology which controls the contact angle of liquid droplets on the substrate by applied voltage has received great attention in various fields such as display, lab on a chip by its advantages of low power consumption and structure simplicity. Moreover, electrowetting beam steering prism system where the meniscus formed by two immiscible liquids can be tilted by applying different voltage on each side is reported [2]. Therefore, we suggested a new device based on electrowetting for post-MIMO which enables the active control of beam propagation path.

In this work, we designed an electrowetting device to reflect the incident beam in controlled behavior which shows larger steering angle range compared to the transmissive prism device (Fig. 1(a)). Aluminum oxide, PTFE (polytetrafluoroethylene) are deposited as the dielectric layer and hydrophobic layer for the electrowetting system. Water which shows higher refractive index in GHz range and dodecane were selected as two immiscible liquids (Fig. 1(b)). Since mmWave beam steering device operation behavior is rarely reported in electrowetting fields, several calculations were made to verify the device operation.

Simulated with HFSS in 28GHz (wavelength λ=10.7㎜) condition, with the variation of size, it is proved that device window size should be over 2λ to ensure enough directivity. After that several models were simulated with the meniscus tilted in apex angle ranging from -25° to 25°. For a 40mm sized device, several lobes in radiation plot were observed from -32° to 34° reacting to the incoming wave with the incident angle of 5° (Fig. 2).

In summary, this electrowetting-based beam steering device not only showed comparable steering range but also extended the application of electrowetting in RIS for post-MIMO in 6G technology.

Acknowledgement

This work was supported by the Institute for Information & communication Technology Planning & evaluation (IITP) grant funded by the Korea government (MSIT) (No.2021-0-00486)

References

[1] Sun et al., “MIMO for Millimeter-Wave Wireless Communications: Beamforming, Spatial Multiplexing, or Both?”, IEEE Commun. Mag, 52(12), 110-121 (2014)

[2] Cheng et al., “Adaptive beam tracking and steering via electrowetting-controlled liquid prism”, Appl. Phys. Lett., 99, 191108 (2011)