Reconfigurable intelligent surface module

Abstract

A reconfigurable intelligent surface (RIS) module (10) includes a dielectric layer (40) including a main surface and a plurality of resonance cells (50) periodically arranged on the main surface. Each of the plurality of resonance cells have variable capacitance and a resonance frequency that varies with a value of the variable capacitance. RIS module includes a wiring pattern (60) formed on the main surface and configured to provide an electrical signal for controlling the variable capacitance to each of the plurality of resonance cells and a conductor pattern (70) formed on the main surface and located between adjacent resonance cells of the plurality of resonance cells and not in contact with the plurality of resonance cells or the wiring pattern.

Description

RECONFIGURABLE INTELLIGENT SURFACE MODULE

[0001] The present invention relates to a reconfigurable intelligent surface module.

[0002] In recent years, reconfigurable intelligent surfaces (RIS) have been extensively studied (Non-Patent Document 1). RIS include a plurality of periodically arranged resonance cells, a wiring pattern configured to provide each of the plurality of resonance cells with an electrical signal to control the variable capacitance of that resonance cell.

[0003] E. Basar, M. Di Renzo, J. De Rosny, M. Debbah, M. -S. Alouini and R. Zhang, "Wireless Communications Through Reconfigurable Intelligent Surfaces," in IEEE Access, vol. 7, pp. 116753-116773, 2019, doi: 10.1109 / ACCESS.2019.2935192.

[0004] Although the characteristics of RIS depend on the shape of each resonance cell, the shape of the resonance cell may be restricted by the wiring pattern, and as a result, the characteristics of RIS may be limited. For example, in the case of RIS that changes the direction of emission of an electromagnetic wave by changing the phase distribution of the electromagnetic wave to be controlled (e.g., carrier wave), the phase shift range may be limited by the restrictions on the shapes of resonance cells due to the wiring pattern.

[0005] An objective of the present invention is to provide a RIS module with reduced characteristics limitations due to a wiring pattern.

[0006] A reconfigurable intelligent surface module according to the present invention, comprising: a dielectric layer including a main surface; a plurality of resonance cells periodically arranged on the main surface, each of the plurality of resonance cells having variable capacitance and a resonance frequency that varies with a value of the variable capacitance; a wiring pattern formed on the main surface and configured to provide an electrical signal for controlling the variable capacitance to each of the plurality of resonance cells; and a conductor pattern formed on the main surface and located between adjacent resonance cells of the plurality of resonance cells and not in contact with the plurality of resonance cells or the wiring pattern.

[0007] According to the above configuration, the limitation of characteristic by the wiring pattern is reduced.

[0008] Fig. 1 is a plan view of a reconfigurable intelligent surface (RIS) module according to an embodiment of the present invention.Fig. 2 is an enlarged schematic plan view of the region A of Fig. 1.Fig. 3 is an end view illustrating the BB section of Fig. 2.Fig. 4 is a graph illustrating the simulation results for a resonance cell.Fig. 5 is a graph illustrating the simulation results for the resonance cell.Fig. 6 is a graph illustrating the simulation results for the resonance cell.Fig. 7 is a graph illustrating the simulation results for the resonance cell.Fig. 8 is a graph illustrating the simulation results for the resonance cell.Fig. 9 is a graph illustrating the simulation results for the resonance cell.Fig. 10 is a graph illustrating the simulation results for the resonance cell.Fig. 11 is a graph illustrating the simulation results for the resonance cell.Fig. 12 is a graph illustrating the simulation results for the resonance cell.Fig. 13 is a graph illustrating the simulation results for the resonance cell.Fig. 14 is a graph illustrating the simulation results for the resonance cell.Fig. 15 is a graph illustrating the simulation results for the resonance cell.Fig. 16 is a graph illustrating the simulation results for the resonance cell.Fig. 17 is a partial schematic perspective view of a RIS module for the variant.

[0009] An embodiment and variations thereof of the invention will be described with reference to the drawings. In each drawing, the same or similar elements are marked with the same reference sign. For multiple same elements, only some of them are signed. In drawings, some features are simplified and / or exaggerated for ease of understanding.

[0010] Fig. 1 to 3 illustrate a reconfigurable intelligent surface (RIS) module 10 according to this embodiment. Here, one RIS is configured by one RIS module 10, but one RIS may be configured by a combination of multiple RIS modules. The RIS module 10 is configured as a reflective beamformer that controls the direction of reflection of electromagnetic waves to be controlled. The electromagnetic wave to be controlled is, for example, an electromagnetic wave of a specific frequency (carrier wave of wireless communication) from a source such as an antenna. The specified frequency may be a single frequency or a specified band of frequencies comprising multiple frequencies. The RIS module 10 includes a substrate 20, a reflector layer 30, a dielectric layer 40, a plurality of resonance cells 50, a wiring pattern 60, and a conductor pattern 70.

[0011] The substrate 20 is an insulating substrate that provides the necessary strength and rigidity to the RIS module 10.

[0012] The reflector layer 30 is formed on the entire main surface (upper surface) of the substrate 20 and is configured to reflect incident electromagnetic waves as described below. The reflector layer 30 is formed from any conductive material such as metal, high conductivity polymer, conductive oxide, and other high electric conductivity material, such as carbon nanotube and graphene.

[0013] The minimum thickness of the reflector layer 30 has to exceed the skin depth δ and can be obtained from the following equation 1. In the equation 1, “ρ” is the resistivity of the used material of the reflector layer 30, “μ0” is the permeability of free space (μ0= 4π*10-7), “μr” is the relative permeability of the used material (usually equals 1), and “f0” is the frequency of the electromagnetic (EM) wave to be controlled (operating frequency).

[0014] For example, for Au thin film of ρ = 2.24 μΩ ・ cm, μr= 1 at 300 GHz operating frequency, the skin depth is 0.138 μm. The reflector layer 30 has a thickness greater than this calculated value δ.

[0015] The dielectric layer 40 is formed on the entire main surface (upper surface) of the reflector layer 30. The dielectric layer 40 functions as a spacer that provides spacing between the reflector layer 30 and the plurality of resonance cells 50. The dielectric layer 40 consists of most of the volume of the RIS module 10. The dielectric layer 40 is formed from any type of non-conductive dielectric material such as polyimide (PI), benzocyclobutene (BCB), parylene, polyethylene (PE), and polytetrafluoroethylene (PTFE).

[0016] The plurality of resonance cells 50 are formed on the main surface (upper surface) of the dielectric layer 40 and are periodically arranged. The plurality of resonance cells 50 constitute an array (in this case, a matrix) due to the periodic arrangement. Each resonance cell 50 has variable capacitance and a resonance frequency that varies with a value of the variable capacitance. Here, each resonance cell 50 includes a combination of a resonator 51, such as a metamaterial cell, and a variable capacitance element 52, such as a variable capacitance diode or transistor.

[0017] The resonator 51 includes a first conductive film 51A, a second conductive film 51B, and gaps G separating the first conductive film 51A and the second conductive film 51B. The conductive films 51A and 51B are formed from any conductive material such as metal, high conductivity polymer, conductive oxide, and other high electric conductivity material, such as carbon nanotube and graphene.

[0018] The first end and the second end of the variable capacitance elements 52 are connected to an end of the first conductive film 51A and an end of the second conductive film 51B, respectively, the ends of the films 51A and 51B forming the gap G. The variable capacitance element 52 includes a variable capacitance CLthat varies in accordance with the voltage applied to the first and second ends. The number of variable capacitance elements 52 is arbitrary, and two or more variable capacitance elements 52 may be incorporated into a single resonance cell 50.

[0019] The resonance frequency fRof the resonance cell 50 can be generally calculated from equation 2 below. “L” is the equivalent inductance of the resonance cell 50 and “Ceq” is the equivalent capacitance of the resonance cell 50.

[0020] The equivalent inductance L can be calculated from equation 3 below. “μ0” is the permeability of free space, “a” and “b” are the dimensions of the resonance cell 50, and "t” is the thickness of the conductive films 51A and 51B.

[0021] The equivalent capacitance Ceqis a sum of the gap capacitance Cgof the gap G and the variable capacitance CLof the variable capacitance element 52, as illustrated in equation 4 below. The gap capacitance Cgcan be calculated from equation 5 below. “ε0” and “εc” are permittivity of free space and effective permittivity of the material between the gap G (here, the material of the dielectric layer 40), “w” is the width of the conducting films 51A and 51B, “g” is the length (width) of the gap G.

[0022] Since Ceqand L are proportional to the size of the resonance cell 50, the resonance frequency fRis inversely proportional to the size of the resonance cell 50. The size of the resonance cell 50 is less than half the wavelength of the electromagnetic wave at the operating frequency. In the resonance cell 50, L and Cgare constant, and CLis variable. The resonance frequency fRvaries in accordance with the variable capacitance CL. Therefore, in the resonance cell 50, the resonance frequency fRis controlled by controlling the variable capacitance CLby controlling the voltage applied to the variable capacitance element 52. This control of the resonance frequency fRcontrols the amount of phase shift of the reflected electromagnetic wave relative to the incident electromagnetic wave to be controlled.

[0023] The resonance cells 50 are oriented to couple to an incident external electric field (E) due to the electromagnetic wave in order to induce a current flow in the resonance cells 50. Here, the size, shape, and distance between cells are adjusted to achieve coupling in the millimeter wave frequency range.

[0024] The wiring pattern 60 is formed on the main surface of the dielectric layer 40. The wiring pattern 60 is formed integrally with the conductive films 51A and 51B of the resonator 51. The wiring pattern 60 may be formed separately from the conductive films 51A and 51B. The material of the wiring pattern 60 may be the same as or different from the material of the conductive films 51A and 51B. The wiring pattern 60 includes a plurality of input terminals 61, a plurality of control lines 62 connected to the plurality of input terminals respectively, a ground terminal 63, and a plurality of ground lines 64 connected to the ground terminal 63. The plurality of RIS cells 20 in each column are located on a pair of control lines 31 and ground lines 32. The control line 62 and ground line 64 are intermittent lines, with a plurality of first conductive films 21A in the same column each placed in the multiple interrupted portions of the control line 62, and a plurality of second conductive films 21B in the same column each placed in the multiple interrupted portions of the ground line 64.

[0025] The plurality of input terminals 61 and ground terminal 63 are configured to be connected via wirings W to a controller C that controls the RIS module 10. The controller C inputs a control signal, which is an electrical signal (in this case, a voltage signal), to each input terminal 61. The control signal input to the input terminal 61 is commonly supplied to each of the first conductive films 51A of the resonance cells 50 belonging to the column corresponding to that input terminal 61 via the control line 62 connected to that input terminal 61. The ground terminal 63 is grounded via controller C. This grounds the second conducting film 51B of each resonance cell 50 connected to the ground terminal 63 via ground line 64. With these configurations, the voltage according to the control signal for each column is applied to each variable capacitance diode 22, and the resonance frequency and the amount of phase shift of each resonance cell 50 is controlled on a column-by-column basis. When the variable capacitance element 52 is a transistor, the wiring pattern 60 includes signal lines that control the gate as appropriate.

[0026] The conductor pattern 70 is formed on the main surface of the dielectric layer 40. The conductor pattern 70 may be formed in the same material and within the same process as the conductive films 51A and 51B, etc. The conductor pattern 70 is located between adjacent resonance cells 50, is not in contact with the resonance cells 50, and is not in contact with the wiring pattern 60.

[0027] The conductor pattern 70 includes a plurality of conductor patches 71 located between each of the resonance cells 50 that are adjacent to each other in the row direction (transverse direction in Fig. 1). Each conductor patch 71 is formed in a rectangular shape that is long in the column direction so as to separate the entire adjacent column of the columns of resonance cells 50. Each conductor patch 71 is provided between each of the adjacent columns. The conductor pattern 70 includes conductor patches 72 located between each of the 50 resonance cells adjacent to each other in the column direction. The conductor patch 72 is formed in a rectangular shape that is long in the row direction between the control line 62 and the ground line 64. The conductor patches 71 and 72 are configured to increase an upper limit of a phase shift of an emitted electromagnetic wave relative to an incident electromagnetic wave into the RIS module 10 to be greater than when the conductor pattern is not provided. The shape, location, and number of conductor patches 71 and 72 are arbitrary. The conductor pattern 70 may have only at least one of the conductor patches 71 and 72.

[0028] To reduce losses in the RIS module 10, high reflection (small absorption) of the incident electromagnetic (EM) wave is required. The reflection R(ω) and transmission T(ω) of the resonance cell 50 having structure shown in Fig. 2, can be calculated from extracted frequency-dependent S parameters obtained in the simulation of the resonance cell 50 as follows. S11(ω) is the reflection coefficient and S21(ω) is the transmission coefficient.

[0029] The total absorption A(ω) of the EM wave passing through the RIS module 10 is calculated from equation 8 below.

[0030] Since the transmission through the RIS module 10 is minimized by the reflector layer 30 of the thickness larger than the skin depth δ (equation 1) and the value of transmittance in the entire investigated frequency range is zero, the absorption equation 8 can be simplified to equation 9 below.

[0031] To achieve a high reflection of the RIS module 10, the absorption has to be minimized at the resonance frequency, where the phase change is the largest. In this embodiment, the very high reflection can be achieved due to the conductor pattern 70. The calculation of the effective impedance is based on the extracted reflection coefficient S11(ω) obtained from the simulation, according to equation 10 for S21(ω) = 0.

[0032] To achieve absorption close to 0, the real part of the relative wave impedance has to be possibly the highest achievable for the designed structure (Re(z)>>1), while the imaginary part should also be increased ((Im(z) >> 0)) at the resonance frequency. By adding a large impedance conductor pattern 70, the surface impedance of the RIS module 10 can be increased, resulting in a very small total absorption.

[0033] Here, a specific example of a resonance cell 50 is also described. Here, a 500 nm thick Au (ρ = 2.24 μΩ-cm) reflector layer 30 was used, and the reflector layer 30 was coated with a 75 μm thick dielectric layer 40 (εr = 2.8). The conductive films 51A and 51B was composed of a 500 nm thick gold thin film. The cell parameters are as follows: cell period size ax = ay = 380 μm, width w = 15 μm, the resonance cell 50’s size a = b = 240 μm, gap size g = 20 μm, the control lines 62’s w2= 4 μm. The characteristics obtained by this specific example are described below.

[0034] Fig. 4 and Fig. 5 illustrate the simulated reflection coefficient S11 and total phase variation of the resonance cell 50 without the conductor pattern 70. The variable capacitance CLof the variable capacitance element 52, was varied from 0 to 4 fF, with 0.5 fF steps. The increase of CLshows the shift of the resonance frequency towards lower frequency values from about 310 GHz to 240 GHz. For 0 fF and 0.5 fF, only small resonance peaks are observed, showing a total phase value of about 100°. Further increase of CLsignificantly increased the resonance peaks intensity from S11 = -23 dB at 1 fF to -13 dB at 2 fF, while the phase variation was also increased. The maximum total phase range that was observed between 235 and 310 GHz was 335°. However, high values of S11 peaks mean that high absorption of the incident EM wave occurs in the RIS module 10, and only a small part of the wave is reflected under the desired angle.

[0035] First, consider the case where only two of the conductor patches 71 among the conductor patches 71 and 72 in Fig. 2 are provided. The conductor patch 71’s length y1= 380 μm was the same as the cell period ay. The width x1varied from 10 to 50 μm, while the variable capacitance CLof the variable capacitance element 52 varied between 0 and 4 fF.

[0036] Fig. 6 illustrates the simulated relative impedance of the resonance cells 50 with conductor patches 71 when the x1size is varied from 10 μm to 50 μm at CL= 0 fF. and CLis varied from 0.5 fF to 4 fF. As can be seen, the change in the size of conductor patches 71 results in the change of the impedance in a wide range of Re(z) values from 28 to 58. The shift of the impedance peak can also be visible and can be associated with resonance frequency shift due to the addition of inductance and capacitance in the resonance cell 50, and it should be additionally tuned for the desired operating frequency. Also, with the change of the capacitance (0 to 4 fF), a significant decrease of the Re(z) is observed, illustrating small impedance peaks with values below 5.

[0037] Knowing that the value of the impedance should be the highest for the lowest absorption of the EM wave in the RIS module 10, x1= 40 μm which corresponds to Re(z) = 58 was used for the simulation of the active RIS module 10 with phase variation.

[0038] Fig. 7 and Fig. 8 illustrate the simulated reflection coefficient S11(ω) and total phase variation of the resonance cell 50 optimized with only two conductor patches 71. CLwas varied between 0 to 4 fF, with 0.5 fF step. The change of CLshifted the resonance frequency towards lower frequency values from about 325 GHz to 210 GHz. For 0 fF only small resonance peaks were observed, showing a total phase value of about 100° at the 300 GHz band. Further increase of CLincreased the resonance peaks intensity, from -11 dB at 0.5 fF to -4 dB at 2 fF, with the simultaneous increase of the total phase range that was observed between 210 and 325 GHz to 342° at 275 GHz. These results correspond to the above impedance changes. The intensity of S11(ω) peaks was decreased compared to the resonance cell 50 without the conductor pattern 70, resulting in lower absorption of the incident EM wave. Furthermore, a larger shift of resonance peaks allows for the control of the RIS module 10 in a wider frequency range, allowing for a broadband operation.

[0039] Next, consider the case where only two of the conductor patches 72 among the conductor patches 71 and 72 in Fig. 2 are provided. The size of the conductor patches 72 was varied for x2= 10 - 50 μm, and y2= 40 - 200 μm, in the x and y-direction, respectively. the variable capacitance CLof the variable capacitance element 52 varied between 0 and 4 fF. The conductor patches 72 affects the resonance frequency of the resonance cell 50 throughout the introduction of additional inductance capacitance; thus, the optimization of the resonance cell 50 for desired operating frequency band is required.

[0040] Fig. 9 illustrates the simulated relative impedance Re(z) of the resonance cell 50 when CL= 0 fF, y2= 70 μm, and the size of x2varied from 40 μm to 200 μm. Fig. 10 illustrates the simulated relative impedance of the the resonance cell 50 when CL= 0 fF, x2= 10 μm, and the size of y2varies from 10 to 50 μm. The change in the size of the conductor patch 72 results in a variation of the impedance values for a constant CL= 0 fF. In Fig. 9, for constant y2= 70 μm, the x2size varied between 40 μm and 200 μm, with a step of 20 μm. The values of relative impedance Re(z) were high with a maximum of 55 for x2= 40 μm. In Fig. 10, for constant x2= 140 μm, the size of y2varied between 10 and 50 μm, with a 5 μm step. All values illustrated relative impedance Re(z) about 50, with a maximum of 58 for y2= 10 μm.

[0041] The optimization of the size of the conductor patches 72 showed that the highest relative impedance Re(z) can be obtained for the combination of x2= 40 μm, and y2= 35 μm, which was used for the calculation of the total phase variation with the capacitance CLvalue change.

[0042] Fig. 11 illustrates the simulated relative impedance of the resonance cell 50 for the x2= 40 μm, and y2= 35 μm and the capacitance CLvariation between 0 and 4 fF. As can be seen, with the increase of the capacitance CLthe intensity of peaks is reduced, showing maximum values of 6 at CL= 1.5 fF. The values of the Re(z) for the conductor patches 72 are smaller than for the conductor patches 71; thus, the intensity of S11(ω) peaks in Fig. 12 is increased.

[0043] Fig. 12 and Fig. 13 illustrates the simulated reflection coefficient S11(ω) and total phase variation for the optimized resonance cell 50 with only the conductor patches 72. CLwas varied between 0 to 4 fF, with 0.5 fF step. The change of CLshifted the resonance frequency towards lower frequency values from about 360 GHz to 210 GHz. For 0 fF only a small resonance peak of about -2 dB was observed, showing a total phase value of about 100° at the 300 GHz band. Further increase of the capacitance CLincreased the resonance peaks intensity, from -15 dB at 0.5 fF to -4 dB at 2 fF, with the simultaneous increase of the total phase range that was observed between 210 and 360 GHz to 346° at 275 GHz. The intensity of S11(ω) peaks was decreased compared to the resonance cell 50 without the conductor pattern 70. At the same time, the intensity of S11(ω) peaks was increased compared to the resonance cell 50 with only the conductor patches 71.

[0044] In this embodiment, conductor patches 71 and 72 are combined with the resonance cell 50 to provide the highest range of capacitance variation at the interesting frequency band. The optimized dimensions of patches in this invention were x1= 50 μm, x2= 160 μm, and y2= 10 μm.

[0045] In Fig. 13 to 16, the simulated impedance, reflection coefficient S11(ω), and total phase variation for the optimized resonance cell 50 with 4 conductor patches 71 and 72 are illustrated. The variation of the capacitance CLin the range from 0 fF to 4 fF resulted in the shift of resonance peaks toward lower frequencies, from 315 GHz to 205 GHz, respectively. Also, as can be seen in the impedance graph, a much higher intensity of peaks is observed, resulting in the general decrease in the intensity of S11(ω) peaks from -8.5 dB to -2.7 dB, for 0.5 fF and 2 fF, respectively, as compared to above example. The absorption of the incident EM wave was reduced up to 4 times (CL= 2 fF) compared to the resonance cell 50 without conductor pattern 70. Furthermore, the total phase variation change was further increased up to 351° at 275 GHz.

[0046] It should be noted, that the general range of phase change between 210 GHz and 320 GHz allows for broadband devices, in which the operation band can be easily switched, just by the change of the capacitance value. For example, in this embodiment, at the 300 GHz band, the capacitance CLvariation from 0 to 2 fF yields about 350° phase change, while at the 250 GHz band, a similar range can be obtained for CLvalue from 0.5 fF to 3 fF.

[0047] The invention is applicable to transmissive RIS modules. In such a case, the reflector layer 30 is omitted. For example, as illustrated in Fig. 17, a structure in which sets of dielectric layer 40, resonance cell 50, wiring pattern 60, and conductor pattern 70 are stacked is employed. To reduce the number of layers, it is desirable to increase the phase change transmitted through each layer, as well as to increase the transmission range by reducing reflection. As above, the reflection of incident electromagnetic waves can be significantly reduced by adjusting and maximizing the relative impedance. According to Equation 8 above, minimizing reflection and maximizing transmission can reduce the total loss (absorption) in the RIS module. The lower the reflection, the wider the transmission window available for beamforming.

[0048] The present invention is applicable to RIS modules in general. The conductor pattern as in the above embodiment reduces the limitation of the characteristics of the RIS module due to the wiring pattern (improved characteristics).

[0049] Although the invention has been described above with reference to embodiments and variations, the invention is not limited to the above embodiments and variations. For example, the present invention includes various modifications to the above embodiments and variations that can be understood by those skilled in the art within the scope of the technical concept of the invention. Each of the configurations listed in the above embodiments and variations can be combined as appropriate to the extent that there is no contradiction.

[0050] 10...RIS module, 20...substrate, 30...reflector layer, 40...dielectric layer, 50...resonance cell, 51...resonator, 51A ...first conductive film, 51B...second conductive film, 52...variable capacitance element, 60...wiring pattern, 70...conductor pattern, 71...conductor patch, 72 ...conductor patch, C...controller, W...wiring.

Claims

1. A reconfigurable intelligent surface module comprising: 　　　　a dielectric layer including a main surface; 　　　　a plurality of resonance cells periodically arranged on the main surface, each of the plurality of resonance cells having variable capacitance and a resonance frequency that varies with a value of the variable capacitance; 　　　　a wiring pattern formed on the main surface and configured to provide an electrical signal for controlling the variable capacitance to each of the plurality of resonance cells; and 　　　　a conductor pattern formed on the main surface and located between adjacent resonance cells of the plurality of resonance cells and not in contact with the plurality of resonance cells or the wiring pattern.

2. The reconfigurable intelligent surface module according to claim 1, wherein 　　　　the conductor pattern includes a first conductor patch located between adjacent resonance cells adjacent in a first direction among the plurality of resonance cells.

3. The reconfigurable intelligent surface module according to claim 2, wherein 　　　　the conductor pattern further includes a second conductor patch located between adjacent resonance cells adjacent in a second direction different from the first direction among the plurality of resonance cells.

4. The reconfigurable intelligent surface module according to claim 1, wherein 　　　　the conductor pattern includes a plurality of conductor patches configured to increase an upper limit of a phase shift of an emitted electromagnetic wave relative to an incident electromagnetic wave into the reconfigurable intelligent surface module to be greater than when the conductor pattern is not provided.

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