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Abstract
Circumventing the attenuation of microwaves during the propagation is of prime importance to wireless communication towards higher carrier frequencies. Here, we propose a scheme of wireless communications via a functionalized meta-window constructed by an optically-transparent metasurface (OTM) consisting of indium tin oxide (ITO) patterns. When the signal is weak, the OTM can significantly strengthen the signal by focusing the incoming waves towards the windowsill, thus substantially enhancing the network speed. The intensity enhancement of microwaves at 5 GHz via an OTM is verified by both numerical simulations and experiments. Furthermore, the ability to increase the data transfer rate in a 5-GHz-WiFi environment is directly demonstrated. Our work demonstrates the feasibility of applying an optically-transparent meta-window for enhancing wireless communications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Wireless communication is ubiquitous in daily life [1,2]. Stimulated by the ever-increasing demand for wireless end-user devices and high-capacity wireless communications, the shift towards higher carrier frequencies has become an essential trend with the roll-out of 5 G cellular networks [3–6]. Despite the shorter wavelengths have prominent superiorities such as increased bandwidth and higher rates of data transfer over longer wavelengths [7–9], the reduced coverage area [10,11] and the increased propagation attenuation [12–14] preclude excellent signal reception for indoor applications. Conventional approaches to solve this issue, such as increasing the power of radiation, suffer from the drawbacks of increased power consumption [15], cost inefficiency, and could also be hazardous to health [16].
Recently, several metasurfaces-based approaches for signal enhancement in wireless communications have been demonstrated [17–20]. Metasurfaces, consisting of subwavelength-spaced unit elements with rationally designed, have been leveraged to unprecedentedly control over many degrees of freedom in the incident waves (e.g., phase, amplitude, polarization) [21–25] and advance new functionalities or applications in many fields such as holograms [26–28], invisibility cloaks [29–32], color filters [33], on-chip quantum gates [34], vortex beam generation [35–37], aberration-free meta-mirrors [38], and achromatic/aberration-free metalenses [39–43], etc. By utilizing the focusing function of metasurfaces, the wireless signals can be enlarged around the focal point so as to efficiently compensate for the propagation loss, such as attenuation during passing through a building’s walls [19]. Furthermore, the so-called reconfigurable intelligent surfaces [44–48] and machine learning-assisted designing techniques [49,50] have extended the functionality and potential of metasurfaces in wireless communication. However, most of these metasurface-enabled schemes are not compatible with windows, where transparency is required. Very recently, some efforts have been devoted to developing optically-transparent metasurface (OTM) lenses for wireless communications by adopting meshed metal patterns with a linewidth of 20 $\mu m$, which are almost invisible in the whole visible spectrum [20]. Nevertheless, the fabrication of such thin metal lines on macro-sized windows is challenging and cost-intensive [34,51]. Therefore, it is important to develop a cost-effective and feasible solution to the OTMs for wireless communication.
Indium tin oxide (ITO) as an ultrathin conductive material with simultaneously excellent solar transmission performance and good electric conductivity has been adopted to realize many optically transparent meta-devices at the microwave regime. Due to the conductivity of the ITO, meta-atoms composed of ITO can act as strong electromagnetic resonators similar to those composed of metals, such as copper and zinc, which can be regarded as perfect electric conductors in the microwave regime, the difference is the finite resistance of ITO would lead to intrinsic ohmic loss. By enhancing the absorption caused by ITO, broadband absorbers for both propagating waves [52–55] and evanescent waves [56] have been reported. On the contrary, by carefully optimizing the geometric configuration to minimize the surface currents flowing on the ITO to reduce the ohmic loss, high-efficiency coding metasurfaces for broadband scattering reduction have also been demonstrated [57]. Inspired by these researches, we find ITO a very promising candidate for the realization of OTMs in the proposed scheme of wireless communications.
In this work, we propose a strategy to enhance the wireless signals by using a meta-window consisting of an OTM composed of indium tin oxide (ITO) patterns on a transparent substrate, such as glass and acrylic. The OTM is designed based on the Fresnel-zone-plate (FZP) theory and consists of two types of meta-atoms with low and high transmittances separately. The incoming wireless signals passing through the OTM are focused on the other side such that the signal intensity is enlarged around the focal points, where a mobile unit or a signal extender can be installed. The focusing effect and the signal enhancement have been verified by both numerical simulations and near-field measurements. Finally, we perform a field trial for a 5-GHz Wi-Fi communication system to directly demonstrate the function of the meta-window in wireless communication. It is clearly observed that the network quality can be markedly improved, manifesting the signal boost from the OTM and hence the better coverage of the wireless signals. Our scheme of ITO-based OTMs for wireless communication is simple and cost-effective, and could have a practical impact on 5 G and 6 G wireless communications.
The concept is schematically shown in Fig. 1. High-frequency wireless signals arriving outside the wall from a base station might possess low intensity due to its poor coverage caused by the short wavelengths as shown in Fig. 1(a). When it is too weak, it would be difficult to build a network connection for mobile devices or signal extenders. Our solution to this issue based on a meta-window is visually shown in Fig. 1(b). A meta-window is capable of focusing the incoming wireless signal to the windowsill on the other side of the window. Due to the focusing effect, the signal intensity near the focal point is significantly enhanced. Thus, a mobile device or a signal extender near the focal point on the windowsill can connect to the wireless network, further extending the signal to the indoor environment. Such a noninvasive strategy based on a meta-window can thus solve the issue of weak indoor signals for wireless communications with high carrier frequency. Such a scheme utilizing a functionalized meta-window with OTM can also be extended to the reflection-type application, where the source and receiver are on the same side of the meta-window. A reflection-type OTM compatible with windows might be an important addition to the reconfigurable intelligent surfaces, which are usually opaque and hence only applicable to walls of buildings.
Fig. 1. Schematic diagrams of the microwaves passing through a window and a meta-window. (a) When the weak wireless signal transmits through a window, the signal strength is too weak to establish a wireless communication channel. (b) When a meta-window is applied, the signal at the focal point is significantly enhanced. The focal point of the meta-window is designed to be above the windowsill, making it convenient to place the Wi-Fi extender or a mobile unit at the focal point.
2. Design and numerical analysis of the OTM
In order to design a cost-effective OTM with high optical transparency that can manipulate microwaves, we adopted a patterned ITO film that can interact with microwaves [57] and is simultaneously optically transparent in the visible spectrum. The focusing effect of the OTM is designed based on the FZP theory. An FZP consisting of concentric ring-shaped zones with alternating high and low transmittance is designed to cause diffraction of incoming waves, leading to constructive interference at a specific focal point. Therefore, we designed two meta-atoms with separately high and low transmittances, which are referred to as meta-atom 1 and meta-atom 2, by using an optically-transparent acrylic substrate with a relative permittivity of 2.58. The period of the two meta-atoms is p = 21 mm and the thickness of the optically transparent acrylic substrate is 2.5 mm. Meta-atom 1 is simply the bare acrylic substrate without an ITO pattern. Meta-atom 2 has an ITO cross with a length of l = 19.7 mm and a width of w = 2.6 mm as shown in the inset in Fig. 2(a). Since the transparency of ITO film has a negative correlation with its conductivity, the balance between the transparency and conductivity should be considered, though a higher conductivity could lead to stronger electromagnetic resonance and hence better obstruction of the electromagnetic waves. Therefore, the ITO film with a sheet resistance of 5 Ω/sq and an average transmittance of 78% in the visible spectrum is chosen. The calculated transmission amplitudes of the meta-atom 1 and meta-atom 2 by using the finite-element method are shown in Fig. 2(a), it is found that at the operating frequency of 5 GHz, the transmission amplitude of meta-atom 1 is as high as 0.98, and the transmission amplitude of meta-atom 2 is only 0.26. While at lower frequencies, the transmission amplitudes of the two meta-atoms are more comparable, e.g., 0.84 and 0.99 at 2.4 GHz. The arrangement of the two meta-atoms to construct an OTM can then be obtained through the FZP theory. The outer radius of the k-order wave zone can be expressed as follows:
where k is a positive integer, and f and λ are the focal length and operating wavelength, respectively. In our design, a focal point at (0, 0, 300) with a focal length of 300 mm is assumed. The unit of the coordinate is a millimeter. Considering the fixed period of the meta-atoms, the ring-shaped zones of the FZP should be discretized. The designed OTM that operates at 5 GHz, which consists of 28 × 28 meta-atoms, is shown in Fig. 2(b).Fig. 2. Design of the OTM and the numerical demonstration of its focusing effect. (a) Simulated transmission amplitude of the two meta-atoms. (inset) Schematic of the meta-atom 2 composed of optically transparent ITO pattern deposited on an acrylic substrate. (b) Schematic view of the designed OTM consisting of $\mathrm{28\ \times 28}$ meta-atoms. (c) Simulated field-intensity distributions behind the meta-window at 5 GHz in the x-z plane, x-y plane, and y-z plane, respectively. (d) Simulated field-intensity distributions when the meta-window is replaced by an acrylic plate with the same thickness. The unit of the coordinate in (c) and (d) is a millimeter. (e) The simulated field-intensity profiles in x-direction across the focal point at different frequencies.
We then perform finite-difference-time-domain simulations to study a meta-window constructed by the designed OTM under the illumination of a normally incident plane wave with transverse electric polarization, i.e., electric field along the y direction, from + z direction. Figure 2(c) shows the simulated field-intensity distributions behind the meta-window in the x-z, x-y, and y-z planes across the distinct focal point of (0, 9, -290), which deviates slightly from the theoretical setting of (0, 0, 300) due to the discretization of the FZP of the OTM. At the focal point, the field intensity is enhanced by 9.5 times. For comparison, we simulate the field-intensity distributions when the meta-window is replaced by an acrylic plate with the same thickness of 2.5 mm and show the simulated results in Fig. 2(d). From Fig. 2(d), it is seen that the field intensity is much weaker in the whole region. We note that the meta-window also applies to the incidence with transverse magnetic polarization, i.e., electric field along the x direction, due to the C4v symmetry of the meta-atoms. We also study the field-intensity distributions behind the meta-window at other frequencies. Figure 2(e) shows the simulated field-intensity distributions along x-direction across the focal point, which is depicted by the white dashed line in Fig. 2(c). One can find that at frequencies ranging from 4.4 GHz to 5.6 GHz, a field intensity enhancement larger than 6 is obtained, demonstrating the considerable bandwidth of the focusing effect of the designed meta-window. Especially, the field-intensity enhancement at 5.6 GHz approaches 11. We note that a higher field-intensity enhancement can be achieved by using an OTM based on the phase-type FZP because more transmitted waves are allowed. And such enhancement can be further increased by adopting gradient phase-shift distributions of meta-lenses, where a series of distinct meta-atoms are required, to improve the focusing efficiency. In addition, it is worth noting that at lower frequencies such as 2.4 GHz, the focusing effect of the meta-window becomes invalid, which is shown as the dashed line in Fig. 2(e), due to the comparable transmittance of the two meta-atoms. Therefore, the meta-window barely affects the 2.4 GHz wireless channel, which possesses better coverage owing to the longer wavelength. More details of the simulated field-intensity distributions at 2.4 GHz are shown in Fig. S1 in Supplement 1. Besides, we emphasize that apart from fulfilling the focusing functionality, the meta-window can simultaneously maintain excellent optical transparency on account of the transparent meta-atoms.
The full-wave simulations are performed using finite-difference time-domain (FDTD) method. For simulations of the meta-atoms, unit-cell boundary conditions are applied in the x and y directions, while the wave ports are applied in the z direction. Two Floquet ports are set in z direction to calculate S-parameters. The port mode with minimum order is selected to excite an incident plane wave. For simulations of the whole metasurface lens, the open-boundary condition is applied to all boundaries, and a plane wave incidence is adopted.
3. Experimental demonstration of the signal enhancement via the meta-window
First, we experimentally verify the focusing functionality of our meta-window. The ITO pattern in the designed meta-window in Fig. 2 is fabricated by laser etching and the fabricated meta-window is shown in Fig. 3(a), where the meta-window appears almost transparent to the visible light. The experimental setup for the microwave measurement is shown in Fig. 3(b). The meta-window is erected and supported by four foam blocks with a relative permittivity near 1. An emitting horn antenna is used to emit the incident waves and a receiving antenna is fixed on the moving stage to measure the near field. The emitting horn antenna and the receiving antenna are connected to the Vector Network Analyser (KEYSIGHT N5224B) to acquire the magnitude and phase of the transmitted waves. Absorbers around are used to minimize the microwaves reflected by the surrounding environments. The measured field-intensity distributions at 5 GHz of the electric fields behind the meta-window in the x-z plane, x-y plane, and y-z plane across the focal point (4, −90, 392) are plotted in Fig. 3(c) respectively. For comparison, Fig. 3(d) shows the measured field-intensity distributions when the meta-window is replaced by an acrylic plate with the same thickness of 2.5 mm. From Fig. 3(c), a distinct focal point with a field intensity enhancement of 8.3 is observed, which is in line with the numerical simulations, verifying the focusing performance of the meta-window. The position of the focal point deviates slightly from that in the simulation due to the curvature and tilt of the meta-window in experiments. On the contrary, when the meta-window is replaced with a bare acrylic plate, the measured field intensity is much smaller in the whole region and no focusing effect can be observed (Fig. 3(d)). The measurement results confirm that the designed meta-window can efficiently focus the 5-GHz incident waves to the focal point, where field intensity is enhanced by 8.3 times. In addition, the measured field-intensity results also reveal that electromagnetic waves at 2.4 GHz can transmit through the meta-window almost unaffected, similar to that transmit through the acrylic plate (see details in Fig. S2 in Supplement 1).
Fig. 3. Experimental verification of the wave focusing and the signal enhancement by the meta-window. (a) The photograph of the fabricated meta-window. (b) The experimental setup for characterizing the meta-window. (c) Measured field-intensity distributions behind the meta-window at 5 GHz in the x-z plane, x-y plane and y-z plane, respectively. (d) Measured field-intensity distributions when the meta-window is replaced by an acrylic plate. The unit of the coordinate in (c) and (d) is a millimeter.
Next, we performed a direct wireless-communication experiment by using the meta-window. To mimic a low-energy radiated 5 GHz source, we put a commercial 5-GHz Wi-Fi router inside a box made of aluminum foil, which is filled with and surrounded by microwave absorbers as schematically shown in Fig. 4(a). The commercial 5-GHz Wi-Fi router is configured at the 36th channel with a center frequency of 5.18 GHz and a bandwidth of 40 MHz. An erected meta-window or acrylic plate, which serves as a contrast, is supported by four foam blocks with a relative permittivity near 1. Then we placed a smartphone at the focal point of meta-window to measure the download and upload speeds. Firstly, we measured the download and upload speeds at the same location when the bare acrylic plate was applied. The measured results shown in Figs. 4(b) and 4(c) as yellow rectangles depict an average download speed and an upload speed as low as 1.45 Mbps and 2.46 Mbps, respectively. In some measurements, the upload speed even approaches 0 due to the bad network connection. Surprisingly, when the meta-window was applied, owing to the focus functionality and the significant signal enhancement, the quality of the wireless signals is improved and the average down and upload speeds are increased to as high as 10.10 Mbps and 6.17 Mbps, respectively (blue rectangles in Figs. 4(b) and 4(c)), indicating that the quality of the wireless signals was markedly improved. Such a field trial of meta-window-enabled 5-GHz Wi-Fi communication proves that the proposed meta-window as a transparent window can realize the signal boost and hence extend the coverage of wireless communications with higher carrier frequency.
Fig. 4. Experimental demonstration of the enhanced wireless communications for 5-GHz Wi-Fi network by the meta-window. (a) The schematic diagram of the experimental setup for 5 GHz Wi-Fi wireless communications by meta-window. (b),(c) Measured download (b) and upload (c) speeds of 11 times repeated measurements with the meta-window (blue rectangles) and the acrylic plate (yellow rectangles). The dashed lines represent the average value of the download and upload speeds.
4. Conclusion
In conclusion, we hereby propose a noninvasive method compatible with windows to enhance wireless communication. We show that a meta-window consisting of ITO patterns on the acrylic substrate can efficiently focus the microwaves at 5 GHz so as to enhance the signal intensity behind and hence improve the wireless communications. The focusing functionality of the meta-window is proved by both numerical simulations and microwave experiments. At the focal point, a field-intensity enhancement of 8.3 is achieved in practice. We also perform a field-trial experiment of meta-window-enabled wireless communication of a commercial 5-GHz Wi-Fi router and realize a 7-fold increase in download speed. Our work puts forward and demonstrates a feasible route for improving wireless communications by optically transparent meta-devices, which could find practical applications in the next-generation wireless systems utilizing high-frequency wireless signals with intrinsic poor coverage.
Funding
National Key Research and Development Program of China (2022YFA1404303, 2020YFA0211300); National Natural Science Foundation of China (11974176, 12174188).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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