FREQUENCY-SELECTIVE SURFACE STRUCTURE AND ANTENNA DEVICE SO THAT

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to the field of antennas that radiate radio waves. In particular, the present disclosure relates to a frequency selective surface (FSS) structure configured to reflect and transmit a radio wave depending on an external input applied to the FSS structure, as well as to an antenna apparatus comprising the FSS structure.BACKGROUND

[0002] Reconfigurable intelligent surfaces (RISs) are of great interest for upcoming sixth generation (6G) communication systems due to their ability to influence an antenna radiation pattern in a controllable manner. The RISs may be based on various metamaterials, electromagnetic bandgap structures, or artificial magnetic conductors. Tunable metamaterials may be especially beneficial in beam scanning without front-end phase shifters. Instead of considering planar antenna arrays, antennas implemented by either 3D printing or molding of dielectric materials in 3D shapes, such as dielectric resonator antennas, may be an important area of research into high-performance 6G communication systems.

[0003] In general, the existing RISs are typically implemented as different kinds of graphene-based attenuators, absorbing structures, and reflecting surfaces. However, these RISs are all not tunable, i.e., they can be only transparent or reflective for radio waves. For example, this is because such a RIS usually uses a conductor structure incompatible with both transparent and reflecting states, and / or a ground connection (e.g., ground plane) which prevents achieving the transparent state. To resolve these issues, a complex multilayer conductor structure is needed.

[0004] One other possible solution is to use PIN diodes to connect conductor structures for the purpose of achieving the transparent and reflecting states. However, this solution is limited to a sub-10 GHz frequency range and results in high insertion loss with typical values of at least 3 dB. The performance of the PIN diodes is affected as they become capacitive at higher frequencies, which also limits their operational frequency range. PIN diode-based RISs also require a complex voltage feeding network which may be formed by thousands of components. When used in the so-called mm-wave range, the number of components increases drastically, thereby making the power consumption and practical implementation of the voltage feeding network unpractical and infeasible.

[0005] Document "Frequency Tunable Graphene Metamaterial Reflectarray" by S.H. Zainud-Deen et al, 2017 XXXIIND GENERAL ASSEMBLY AND SCIENTIFIC SYMPOSIUM OF THE INTERNATIONAL UNION OF RADIO SCIENCE (URSI GASS), URSI, 19 August 2017 (2017-08-19), pages 1-4, XP033252663, discloses radiation characteristics of frequency tunable graphene based metamaterial reflectarray.

[0006] Document "Recent Developments in Reflectarrays: Multi-Reconfiguration, Solar Cells, and Graphene" by Julien Perruisseau-Carrier, 2014 INTERNATIONAL WORKSHOP ON ANTENNA TECHNOLOGY: SMALL ANTENNAS, NOVEL EM STRUCTURES AND MATERIALS, AND APPLICATIONS (IWAT), IEEE, 4 March 2014 (2014-03-04), pages 119-121 , XP032683958, discloses research activities related to reflectarray antennas.SUMMARY

[0007] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure.

[0008] It is an objective of the present disclosure to provide an FSS structure that enables enhanced beam steering for radio waves.

[0009] The objective above is achieved by the features of the independent claims in the appended claims. Further embodiments and examples are apparent from the dependent claims, the detailed description and the accompanying drawings.

[0010] According to a first aspect, an FSS structure is provided. The FSS structure comprises a dielectric substrate and at least one sheet-like element provided on the dielectric substrate. Each of the at least one sheet-like element is made of a tunable material. The tunable material is switchable between a first state and a second state in response to an electric signal applied to the tunable material. The tunable material is configured to reflect a radio wave in the first state and transmit the radio wave in the second state. The FSS structure further comprises an electrically conductive sub-structure for applying the electric signal to the tunable material of each of the at least one sheet-like element. The electrically conductive sub-structure comprises at least one first conductor and at least one second conductor. The at least one first conductor is connected to each of the at least one sheet-like element. The at least one second conductor is arranged in vicinity of each of the at least one sheet-like element such that there is a gap between the at least one second conductor and each of the at least one sheet-like element. By using the FSS structure thus configured, it is possible to provide beam steering in a wide angular range (e.g., up to 360° for sub-6 GHz applications) and with high-gain achievement. Moreover, by using such an electrically conductive sub-structure in the FSS structure, it is possible to provide a simplified (compared to those used in the existing RISs) voltage feeding network for radio frequencies (e.g., mm-wave frequencies). Additionally, the FSS structure thus configured requires a reduced (contrasted with the existing RISs) number of components (e.g., 1-4 sheet-like elements) to achieve a desired beam-steering angular range. Finally, when used, for example, on walls and / or roofs of buildings, the FSS structure may provide a radio coverage to users inside the buildings (e.g., through a window glass) when the tunable material is in the second (transparent) state, as well as may redirect radio waves to users outside the buildings when the tunable material is in the first (reflecting) state.

[0011] In one embodiment of the first aspect, the at least one second conductor comprises a plurality of discrete conductors provided on the dielectric substrate around each of the at least one sheet-like element. By implementing the second conductor(s) in this way, it is possible to make the electrically conductive sub-structure (and, consequently, the FSS structure itself) less costly in terms of material consumption.

[0012] In one embodiment of the first aspect, the at least one second conductor is configured as a continuous electrically conductive layer provided on the dielectric substrate around each of the at least one sheet-like element. By implementing the second conductor(s) in this way, it is possible to apply the electric signal (e.g., voltage) uniformly to each tunable material.

[0013] In one embodiment of the first aspect, the at least one second conductor is provided over each of the at least one sheet-like element. In this embodiment, the gap between the at least one second conductor and each of the at least one sheet-like element is filled with a layer of dielectric material. This arrangement of the second conductor(s) may be advantageous when it is required to make the FSS structure more compact.

[0014] In one embodiment of the first aspect, the at least one second conductor is configured as an electrically conductive mesh arranged on the layer of dielectric material. By using the electrically conductive mesh, it is possible to apply the electric signal uniformly to the whole surface of the tunable material, while almost not interfering with the second (transparent) state of the tunable material.

[0015] In one embodiment of the first aspect, each of the dielectric substrate and the at least one sheet-like element has a thickness selected based on a wavelength of the radio wave. By so doing, it is possible to implement the FSS structure that is fully transparent when the tunable material is in the second (transparent) state.

[0016] In one embodiment of the first aspect, the tunable material comprises a zero-gap semiconductor, a Metal Insulator Transition (MIT)-based material, a Transition Metal Oxide (TMO)-based material, or any combination thereof. These materials exhibit better characteristics at mm-wave frequencies, thereby improving the performance of the FSS structure at these frequencies.

[0017] In one embodiment of the first aspect, the tunable material is further switchable between the first state and the second state in response to a thermal action on each of the at least one sheet-like element. By using such a tunable material, it is possible to use (separately or in combination) two different methods for controlling the FSS structure (i.e., by applying electric and heat input signals), thereby making the FSS structure more flexible to use.

[0018] In one embodiment of the first aspect, the at least one sheet-like element comprises multiple sheet-like elements arranged adjacent to each other on the dielectric substrate. In this embodiment, the at least one first conductor of the electrically conductive sub-structure comprises multiple first conductors each connected to one of the multiple sheet-like elements, and the at least one second conductor of the electrically conductive sub-structure comprises multiple second conductors each arranged in vicinity of one of the multiple sheet-like elements and with the gap therefrom. By using such a multi-element FSS structure, it is possible to simultaneously reflect and transmit the radio wave by switching between the first and second states of individual sheet-like elements of the FSS structure. Thus, the FSS structure may simultaneously provide the radio coverage, for example, to users inside and outside buildings.

[0019] In one embodiment of the first aspect, a number of the multiple sheet-like elements is selected such that an electric field generated in each of the multiple sheet-like elements in response to the electric signal is evenly distributed across the sheet-like element. By so doing, each of the multiple sheet-like element may reflect or transmit the radio wave more efficiently.

[0020] According to a second aspect, an antenna apparatus is provided. The antenna apparatus comprises a housing and at least one antenna element arranged in the housing and configured to emit a radio wave. The antenna apparatus further comprises at least one FSS structure according to the first aspect. Each of the at least one FSS structure is arranged at least partly in the housing on a propagation path of the radio wave. The antenna apparatus further comprises a power source configured to apply an electric signal to the tunable material of each of the at least one sheet-like element in each of the at least one FSS structure via the electrically conductive sub-structure. By using the FSS structure(s) in the antenna apparatus, it is possible to efficiently adjust the radiation pattern of the antenna apparatus.

[0021] In one embodiment of the second aspect, each of the at least one FSS structure is arranged between a near field of the at least one antenna element and a far field of the at least one antenna element. By arranging the FSS structure(s) in this way, it is possible to avoid considerable impedance matching degradation. Furthermore, this arrangement of the FSS structure(s) may ensure that grating lobes do not appear in the radiation pattern of the antenna apparatus. Finally, the arrangement of the FSS structure(s) somewhere between the near and far fields of the antenna element(s) may allow one to make the antenna apparatus more compact.

[0022] In one embodiment of the second aspect, each of the at least one FSS structure is arranged in the far field of the at least one antenna element. By arranging the FSS structure(s) in the far field of the antenna element(s), it is possible to obtain a better performance of the antenna apparatus (compared to the situation when the FSS structure(s) is(are) arranged between the far and near fields of the antenna element(s)), but at the cost of an increase in the total size of the antenna apparatus.

[0023] In one embodiment of the second aspect, the antenna apparatus further comprises a temperature control element configured to exert a thermal action on each of the at least one sheet-like element in each of the at least one FSS structure. By using such a temperature control element, it is possible to provide an additional method for controlling the FSS structure(s) (i.e., by exerting a thermal action on the FSS structure(s)).

[0024] In one embodiment of the second aspect, the at least one FSS structure is shaped as a hollow box or tube in which the at least one antenna element is arranged. This configuration of the FSS structure(s) may allow one to adjust the radiation pattern of the antenna apparatus more efficiently.

[0025] Other features and advantages of the present disclosure will be apparent upon reading the following detailed description and reviewing the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present disclosure is explained below with reference to the accompanying drawings in which: FIG. 1 shows a schematic block diagram of a typical wireless communication system for indoor scenarios; FIG. 2 shows a schematic block diagram of a frequency selective surface (FSS) structure in accordance with a first exemplary embodiment; FIG. 3 shows a schematic block diagram of a wireless communication system for indoor scenarios, in which the FSS structure shown in FIG. 2 is used; FIG. 4 shows a schematic block diagram of an FSS structure in accordance with a second exemplary embodiment; FIG. 5 shows a schematic block diagram of an FSS structure in accordance with a third exemplary embodiment; FIG. 6 shows a schematic block diagram of an FSS structure in accordance with a fourth exemplary embodiment; FIG. 7 shows a schematic block diagram of a wireless communication system based on multiple indoor CPEs in accordance with one exemplary embodiment; FIG. 8 shows a schematic block diagram of an antenna apparatus in accordance with a first exemplary embodiment; FIG. 9 shows a schematic block diagram of an antenna apparatus in accordance with a second exemplary embodiment; FIG. 10 shows a schematic block diagram of an antenna apparatus in accordance with a third exemplary embodiment; and FIG. 11 shows a schematic block diagram of an antenna apparatus in accordance with a fourth exemplary embodiment. DETAILED DESCRIPTION

[0027] Various embodiments of the present disclosure are further described in more detail with reference to the accompanying drawings. However, the present disclosure may be embodied in many other forms and should not be construed as limited to any certain structure or function discussed in the following description. In contrast, these embodiments are provided to make the description of the present disclosure detailed and complete.

[0028] According to the detailed description, it will be apparent to the ones skilled in the art that the scope of the present disclosure encompasses any embodiment thereof, which is disclosed herein, irrespective of whether this embodiment is implemented independently or in concert with any other embodiment of the present disclosure. For example, the apparatuses disclosed herein may be implemented in practice by using any numbers of the embodiments provided herein. Furthermore, it should be understood that any embodiment of the present disclosure may be implemented using one or more of the features presented in the appended claims.

[0029] The word "exemplary" is used herein in the meaning of "used as an illustration". Unless otherwise stated, any embodiment described herein as "exemplary" should not be construed as preferable or having an advantage over other embodiments.

[0030] Any positioning terminology, such as "left", "right", "top", "bottom", "above" "below", "upper", "lower", "horizontal", "vertical", etc., may be used herein for convenience to describe one element's or feature's relationship to one or more other elements or features in accordance with the figures. It should be apparent that the positioning terminology is intended to encompass different orientations of the apparatus disclosed herein, in addition to the orientation(s) depicted in the figures. As an example, if one imaginatively rotates the apparatus in the figures 90 degrees clockwise, elements or features described as "left" and "right" relative to other elements or features would then be oriented, respectively, "above" and "below" the other elements or features. Therefore, the positioning terminology used herein should not be construed as any limitation of the invention.

[0031] Although the numerative terminology, such as "first", "second", "third", "fourth", etc., may be used herein to describe various embodiments and features, it should be understood that these embodiments and features should not be limited by this numerative terminology. This numerative terminology is used herein only to distinguish one feature or embodiment from another feature or embodiment. For example, a first state and a second state which are discussed herein could be renamed a second state and a first state, respectively, without departing from the teachings of the invention.

[0032] As used in the embodiments disclosed herein, an antenna apparatus may refer to an apparatus configured to radiate and receive radio waves. The radio waves may refer to a type of electromagnetic radiation that occurs in the so-called centimeter-wave (cm-wave) and millimeter-wave (mm-wave) bands. The radio waves have been used, for example, in wireless communications, such as point-to-point communications, intersatellite links, and point-to-multipoint communications, etc. However, the application of the radio waves is not limited to wireless communications only, and they may be also used, for example, for (air, ground or marine) vehicle navigation and control, road obstacle detection, etc. For this reason, the antenna apparatus according to the embodiments disclosed herein may be used in the same use scenarios as the radio waves. More specifically, the antenna apparatus may be implemented as part of a user equipment (UE) that may refer to a wireless customer premises equipment (CPE) (e.g., a wireless router, switch, etc.), a mobile device, a mobile station, a terminal, a subscriber unit, a mobile phone, a cellular phone, a smart phone, a cordless phone, a personal digital assistant (PDA), a wireless communication device, a desktop computer, a laptop computer, a tablet computer, a single-board computer (SBC) (e.g., a Raspberry Pi device), a gaming device, a netbook, a smartbook, an ultrabook, a medical device or medical equipment, a biometric sensor, a wearable device (e.g., a smart watch, smart glasses, a smart wrist band, etc.), an entertainment device (e.g., an audio player, a video player, etc.), a vehicular component or sensor (e.g., a driver-assistance system), a smart meter / sensor, an unmanned vehicle (e.g., an industrial robot, a quadcopter, etc.) and its component (e.g., a self-driving car computer), industrial manufacturing equipment, a global positioning system (GPS) device, an Internet-of-Things (IoT) device, an Industrial IoT (IIoT) device, a machine-type communication (MTC) device, a group of Massive IoT (MIoT) or Massive MTC (mMTC) devices / sensors, or any other suitable device that uses the radio waves for operation. In some embodiments, the UE may refer to at least two collocated and inter-connected UEs thus defined.

[0033] As used in the embodiments disclosed herein, a frequency selective surface (FSS) structure may refer to a structure that may be used to control the propagation of radio waves by changing the electric and magnetic properties of the FSS structure. More specifically, the FSS structure may be used to reflect, transmit or absorb the radio waves at a specific frequency or frequency subband (e.g., from the mm-wave bands).

[0034] FIG. 1 shows a schematic block diagram of a typical wireless communication system 100 for indoor scenarios. As shown in FIG. 1, the system 100 comprises a first CPE (e.g., a wireless router) 102 installed on a wall 104 inside a room (e.g., a conference room). The first CPE 102 has a limited radio coverage 106, and it is first assumed that two users are within the radio coverage 106 of the first CPE 102. If the users decide to move around the room (as schematically shown by the arrow in FIG. 1), they will go beyond the radio coverage 106 of the first CPE 102, thereby loosing wireless connectivity. To resolve this issue, the system 100 comprises a second CPE 108 which, for example, is similar to the first CPE 102. However, if the users continue going further in the same direction, they will depart the radio coverage of the second CPE 108 as well, for which reason at least one more additional CPE will be needed.

[0035] This approach based on using multiple CPEs inside the room is expensive and undesirable from the user point of view.

[0036] The exemplary embodiments disclosed herein provide a technical solution that allows mitigating or even eliminating the above-sounded drawbacks peculiar to the prior art. In particular, the exemplary embodiments disclosed herein relate to an FSS structure comprising one or more sheet-like elements, each of which is made of a tunable material (e.g., graphene) switchable between a reflecting state and a non-reflecting (transparent) state in response to an external input applied thereto. The external input may be represented by an electric signal, a thermal action, or their combination. In case of multiple sheet-like elements, they are arranged adjacent to each other, and each of them is provided with an independently controlled electrically conductive sub-structure which is arranged so as not to interfere with the transparent state of the sheet-like element(s). The FSS structure may be installed in an antenna apparatus to provide the possibility of adjusting its radiation pattern. Moreover, the FSS thus configured may provide an extended coverage for users, so that the users may be served with only one such FSS structure (e.g., even if they move around the room, as shown in FIG. 1).

[0037] FIG. 2 shows a schematic block diagram of an FSS structure 200 in accordance with a first exemplary embodiment. As shown in FIG. 2, the FSS structure 200 comprises a dielectric substrate 202 (e.g., SiO 2 ) and a sheet-like element 204 provided on the dielectric substrate 202. The sheet-like element 204 is made of a tunable material having a first state and a second state. In the first state, the tunable material is configured to reflect a radio wave incident on the sheet-like element 204. In the second state, the tunable material is configured to transmit the radio wave incident on the sheet-like element 204. The transition between the first and second states may be provided in response to an electric signal applied to the tunable material. The tunable material may be represented by a zero-gap semiconductor, a MIT-based material, a TMO-based material, any combination of these materials or any other materials with similar switchable properties. One non-restrictive example of the zero-gap semiconductor may be graphene. Examples of the MIT-based and TMO-based materials may include, but not limited to, vanadium dioxide and molybdenum disulfide. The sheet-like element 204 may be either transferred to the dielectric substrate 202 in solid state or deposited on the dielectric substrate 202 by means of any appropriate fabrication method. The selection of the fabrication method depends on the tunable material involved and may include, but not limited to, Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD).

[0038] To apply the electric signal to the tunable material of the sheet-like element 204, the FSS structure 200 further comprises an electrically conductive sub-structure that is formed by the combination of a first conductor 206 and a second conductor 208. The first conductor 206 is implemented as an electrode directly connected to the sheet-like element 204 (i.e., to the tunable material). The second conductor 208 is implemented as a continuous thin electrically conductive layer surrounding the sheet-like element 204 such that there is a gap 210 between the second conductor 208 and the sheet-like element 204. More specifically, the second conductor 208 is implemented as a conducting ring or loop around the sheet-like element 204. Each of the first and second conductors 206, 208 may be made of any metal (e.g., gold, silver, aluminum, copper, titanium, hafnium, nickel, zirconium, or any alloy or combination thereof), conducting polymer, conducting co-polymer, or semiconductor material. As for the gap 210, it has a width that is suitable for a wavelength of the radio wave and optimized for creating a desired voltage difference between the first and second conductors 206 and 208 or, in other words, between the sheet-like element 204 and the second conductor 208.

[0039] It is worth noting that each of the dielectric substrate 202 and the sheet-like element 204 has a thickness selected based on a wavelength of the radio wave. In this case, the FSS structure 200 may be fully transparent when the tunable material is in the second (transparent) state. More specifically, the thickness of the dielectric substrate 202 used to grow the tunable material (e.g., graphene) thereon needs to be considered for providing the transparent operation of the FSS structure 200; otherwise, the FSS structure 200 will be always reflective (even if the tunable material is in the second state). To enable both the reflective and transparent operation of the FSS structure 200, the thickness of the dielectric substrate 202 having a permittivity dk > 3 (e.g., Si / SiO 2 used as a common substrate for graphene has dk = 12) may be (n * λ_g) / 2, where λ _ g = c / f ∗ dk is the wavelength of a radio wave in a guided medium, f is the frequency of the radio wave, c is the speed of light in vacuum, and n is any multiple of λ_g, meaning that the material will be periodically transparent at multiple frequencies. When dk is lower, or the thickness of the dielectric substrate 202 is extremely low, the thickness of the dielectric substrate 202 may not correspond to the half wavelength of the radio wave, as indicated above.

[0040] In general, the selection of graphene as the tunable material is caused by its unique quantum properties leading to the phenomenon where the number of charge carriers in graphene depends on the polarity and values of a gate voltage (i.e., the voltage fed via the first conductor 206. If there is a voltage difference between graphene and the ring- or loop-like second conductor 208 separated by the gap 310 from graphene, a resulting electric field will change the quantity of mobile charge carries (e.g., electrons) and will cause an excess of charge carries in comparison to a neutral state (i.e., when there is no such voltage difference). The concentration of the charge carriers is proportional to the applied voltage. Graphene is neutral at a zero gate voltage, at which its resistivity is at maximum. The dependency of the resistivity on the gate voltage may vary with the presence of defects in a graphene layer and is not that strong in multilayer structures.

[0041] When the inter-state transition of the tunable material is based on the gate voltage, the presence of the ring- or loop-like second conductor 208 in the FSS structure 200 is advantageous, since the ring- or loop-like second conductor 208 may provide a plurality of voltage feeding points in order to achieve the constant surface resistance of the tunable material. Hence, only one sheet-like element 204 may be sufficient, instead of having multiple smaller elements for the same area. In some cases, one voltage feeding point (i.e., the second conductor 208 implemented as a point electrode) may not be enough for providing the constant surface resistance, since there may be a smaller surface resistance in the middle of the sheet-like element 204 compared to that at its edges.

[0042] As an alternative or addition to graphene, vanadium dioxide (VO 2 ) may be used as a phase-change switch providing the above-described transition from the first state to the second state. The phase-change switch exploits an abrupt MIT that happens in TMOs, such as vanadium dioxide. These two different material states (i.e., metal and insulator states) of vanadium dioxide possess different crystal structures and are defined as two different phases of vanadium dioxide, with one being conductive (thus, reflecting) and another being dielectric (thus, transparent). In contrast to graphene, the state or phase transition in vanadium dioxide or similar MIT-based materials may be controlled by using not only an electric signal, but also a thermal action (e.g., exerted by a laser pointer). MIT-based materials are extremely fast, for which reason phase change devices based thereon deliver deep sub-thermal switching (<10 mV / decade at room temperature). Vanadium dioxide enables a low-temperature phase change transition, wherein vanadium dioxide changes its state from "dielectric" to "conductor", as well as its energy efficiency and scalability.

[0043] Those skilled in the art would recognize that graphene and vanadium dioxide are only two examples which should not be considered as any limitation of the present disclosure. Other possible materials, such as molybdenum disulfide, other zero-gap semiconductors, MIT-based materials, TMO-based materials and 2D materials with similar switchable properties, or any combination of these or other possible materials (e.g., the combination of graphene with vanadium dioxide) may be used as the tunable material in the FSS structure 200. The only requirement is that a candidate tunable material has such properties that the transition from the first (reflecting) state to the second (transparent) state is possible in response to an external input. As noted above, the external input may be, for example, an electric signal or a thermal action. Depending on which of the two control methods is used for the tunable material, the sheet-like element 204 may be surrounded with the ring- or loop-like second conductor 208, or there may be some integrated or separate means (e.g., an optical infrared heater) configured to exert the thermal action on the tunable material to provide the above-described inter-state transition.

[0044] Since the surface resistance of the tunable material used in the FSS structure 200 is substantially higher in the second (transparent) state than in the first (reflecting) state, the tunable material acts either as a conductor or a dielectric (or insulator). Being in the second state, substantially all the electromagnetic (EM) energy of the radio wave is transmitted through the FSS structure 200, and the resistance of the whole FSS structure 200 may be above 700 Q / sq (e.g., 1000 Q / sq). However, when an electric signal (or any other state-switching input depending on the tunable material used) is applied, the FSS structure 200 becomes conductive (i.e., the first state is provided), thereby reflecting all the EM energy of the radio wave. Being in the first state, the resistance of the whole FSS structure 200 may be below 50 Ω / sq (e.g., 20 Q / sq).

[0045] It should also be noted that the number, shape and arrangement of the constructive elements constituting the FSS structure 200, which are shown in FIG. 2, are not intended to be any limitation of the present disclosure, but merely used to provide a general idea of how the constructive elements may be implemented within the FSS structure 200. For example, in some other embodiments, the FSS structure 200 may comprise multiple sheet-like elements 204 arranged adjacent to each other on the dielectric substrate 202, and each of the multiple sheet-like elements 204 may be provided with the same electrically conductive sub-structure as the one discussed above. In general, a number (and, correspondingly, size) of the multiple sheet-like elements 204 may be selected such that an electric field generated in each of the multiple sheet-like elements in response to the electric signal is evenly distributed across the sheet-like element 204. Moreover, the ring- or loop-like second conductor 208 may be shaped differently; for example, the shape of the second conductor 208 may resemble the shape of the sheet-like element 204 surrounded thereby.

[0046] FIG. 3 shows a schematic block diagram of a wireless communication system 300 for indoor scenarios, in which the FSS structure 200 is used. As shown in FIG. 3, the system 300 comprises a CPE (e.g., a wireless router) 302 installed on a wall 304 inside a room (e.g., a conference room). The CPE 302 is assumed to be equipped with the FSS structure 200. For this reason, unlike the CPE 102 shown in FIG. 1, the CPE 302 has an extended radio coverage 306 which allows all users present in the room to be connected to a wireless network irrespective of their location in the room. In other words, all the users may be served with only one CPE 302.

[0047] FIG. 4 shows a schematic block diagram of an FSS structure 400 in accordance with a second exemplary embodiment. As shown in FIG. 4, the FSS structure 400 comprises a dielectric substrate 402 (e.g., Si / SiO 2 ) and two identical sheet-like elements 404 arranged adjacent to each other on the dielectric substrate 402. The dielectric substrate 402 may have a thickness selected in the above-described manner, i.e., by making it corresponding to the half wavelength of an incident radio wave. Each of the two sheet-like elements 404 are implemented similar to the sheet-like element 204 of the FSS structure 200. The FSS structure 400 further comprises two identical electrically conductive sub-structures each configured to apply an electric signal to one of the two sheet-like elements 404. Each of the two electrically conductive sub-structures has a first conductor 406 and a second conductor 408. The first and second conductors 406, 408 are implemented similar to the first and second conductors 206, 208, respectively. In other words, the first conductors 406 are implemented as electrodes directly connected to the sheet-like elements 404, while the second conductors 408 are implemented as conducting rings or loops surrounding the sheet-like elements 404 with gaps 410 therebetween.

[0048] Unlike the FSS structure 100, the FSS structure 400 may be simultaneously reflective and transparent for the incident radio wave. This is because the presence of the two independent electrically conductive sub-structures allows one to control the two sheet-like elements 404 differently. That is, one of the sheet-like elements 404 may be switched to the first state, while another of the sheet-like elements 404 may be switched to the second state.

[0049] FIG. 5 shows a schematic block diagram of an FSS structure 500 in accordance with a third exemplary embodiment. As shown in FIG. 5, the FSS structure 500 comprises a dielectric substrate 502 (e.g., SiO 2 ) and a sheet-like element 504 provided on the dielectric substrate 502. The dielectric substrate 502 and the sheet-like element 504 may be implemented similar to the dielectric substrate 202 and the sheet-like element 204, respectively. Similar to the FSS structure 200, the FSS structure 500 also comprises an electrically conductive sub-structure for applying an electric signal to the sheet-like element 504 (i.e., the tunable material which the sheet-like element 504 is made of). However, the electrically conductive sub-structure of the FSS structure 500 differs from that of the FSS structure 200. In particular, the electrically conductive sub-structure of the FSS structure 500 comprises a first conductor 506 and four second conductors 508-1, 508-2, 508-3, and 508-4. Similar to the first conductor 206, the first conductor 506 is implemented as an electrode directly connected to the sheet-like element 504. As for the second conductors 508-1, 508-2, 508-3, and 508-4, they are implemented as discrete conductors arranged on the dielectric substrate 502 in vicinity of the sheet-like element 504 and with an equal gap 510 therefrom. The gap 510 may be selected in the same manner as the gap 201. It should be noted that the number and shape of the second conductors shown in FIG. 5 should not be considered as any limitation of the present disclosure. In one other embodiment, the FSS structure 500 may comprise two second conductors, with one being configured as a half-ring surrounding the sheet-like element 504 from the left side and another being configured as a similar half-ring surrounding the sheet-like element 504 from the right side. In one more other embodiment, the FSS structure 500 may comprise multiple second conductors evenly or unevenly arranged around the periphery of the sheet-like element 504. Furthermore, in some other embodiments, the FSS structure 500 may comprise multiple first conductors and multiple second conductors, with each first conductor being arranged between two adjacent second conductors (similar to the first conductor 506 that is arranged between the adjacent second conductors 508-1 and 508-4).

[0050] FIG. 6 shows a schematic block diagram of an FSS structure 600 in accordance with a fourth exemplary embodiment. As shown in FIG. 6, the FSS structure 600 comprises a dielectric substrate 602 (e.g., Si / SiO 2 ) and a sheet-like element 604 provided on the dielectric substrate 602. The dielectric substrate 602 and the sheet-like element 604 may be implemented similar to the dielectric substrate 202 and the sheet-like element 304, respectively. Similar to the FSS structure 200, the FSS structure 600 also comprises an electrically conductive sub-structure for applying an electric signal to the sheet-like element 604 (i.e., the tunable material which the sheet-like element 604 is made of). However, the electrically conductive sub-structure of the FSS structure 600 differs from that of the FSS structure 200. In particular, the electrically conductive sub-structure of the FSS structure 600 comprises a first conductor 606 and a second conductor 608. Similar to the first conductor 206, the first conductor 606 is implemented as an electrode directly connected to the sheet-like element 604. As for the second conductor 608, it is configured as an electrically conductive mesh provided over the sheet-like element 604 such that there is a gap (not shown in FIG. 6) between the second conductor 608 and the sheet-like element 604. The gap may be selected in the same manner as the gap 210. The gap may be filled with a layer of dielectric material such that the second conductor 608 is arranged on the layer of dielectric material. In case of using the layer of dielectric material, its thickness should also be considered (in addition to the thicknesses of the dielectric substrate 602 and the sheet-like element 604) for providing the transparent operation of the FSS structure 600 when the tunable material is in the second state. It is worth noting that the configuration and shape of the second conductor shown in FIG. 6 should not be considered as any limitation of the present disclosure. In other embodiments, the second conductor 608 may be implemented as a plurality of discrete conductors provided over the sheet-like element 604 in an even or uneven manner (e.g., arranged over the edge regions of the sheet-like element 604). Moreover, the meshed configuration of the second conductor 608 may be implemented differently. For example, the electrically conductive mesh may have any polygonal-shaped unit cell, such, for example, as triangular-shaped, square-shaped, rectangular-shaped, diamond-shaped, etc., if required and depending on particular applications. In general, the shape and size of the unit cell of the electrically conductive mesh (e.g., the square-shaped unit cell) may be determined based on the installation site of the FSS structure 600 (e.g., if the FSS structure 600 is intended to be mounted on a window glass, the electrically conductive mesh may be configured such that it is optically invisible).

[0051] FIG. 7 shows a schematic block diagram of a wireless communication system 700 based on multiple indoor CPEs. More specifically, the system 700 comprises two network nodes 702-1 and 702-2, as well as three indoor CPEs each installed in one of houses 704-1, 704-2, and 704-3. Each of the network nodes 702-1 and 702-2 may refer to a fixed point of communication for a CPE or any other UE in a particular wireless communication network. The network node may be referred to as a base transceiver station (BTS) in terms of the 2G communication technology, a NodeB in terms of the 3G communication technology, an evolved NodeB (eNodeB) in terms of the 4G communication technology, and a gNB in terms of the 5G New Radio (NR) communication technology. Each of the three indoor CPEs is assumed to be equipped with any of the FSS structures 200, 400, 500, and 600. In this case, the energy of radio waves radiated by each indoor CPE may be directed towards virtually any direction. As shown in FIG. 7, the radio wave from the CPE installed in the house 704-1 is directed towards the network node 702-1, while the radio wave from the CPE installed in the house 704-3 is directed towards the network node 702-2. As for the radio wave from the CPE installed in the house 704-2, it is directed towards both the first and second network nodes 702-1 and 702-2, as well as towards other desired directions (i.e., a multiple-input multiple-output (MIMO) scenario is possible). Thus, by using any of the FSS structures 200, 400, 500, and 600, it is possible to perform improved beam steering and achieve an increased gain (e.g., from 2-3 dBi to 7-8 dBi). If the indoor CPEs are not equipped with any of the FSS structures 200, 400, 500, and 600, they will provide an omnidirectional radiation pattern (to all directions), which may be useless in some use scenarios because an achievable gain may be significantly less compared to the CPEs with the FSS structure.

[0052] FIG. 8 shows a schematic block diagram of an antenna apparatus 800 in accordance with a first exemplary embodiment. As shown in FIG. 8, the antenna apparatus 800 comprises an antenna element 802 arranged in a housing (not shown in FIG. 8) and configured to emit a radio wave. The antenna apparatus 800 further comprises an FSS structure 804 arranged in the housing on a propagation path of the radio wave (i.e., over the antenna element 802 in the apparatus configuration shown in FIG. 8). The FSS structure 804 may be implemented as any of the FSS structures 200, 400, 500, and 600. The FSS structure 804 may be additionally arranged either in a far field of the antenna element 802, or between a near field of the antenna element 802 and the far field of the antenna element 802. The antenna apparatus 800 is also intended to comprise a power source (not shown in FIG. 8) configured to apply an electric signal to a tunable material used in the FSS structure 804, thereby providing the above-described inter-state transition and, consequently, beam steering. When the tunable material of the FSS structure 804 is in the first (reflecting) state, the radio wave emitted by the antenna element 802 is reflected by the FSS structure 804 to the left, i.e., the antenna apparatus 800 has an end-fire radiation pattern 806-1. When the tunable material of the FSS structure 804 is in the second (transparent) state, the radio wave emitted by the antenna element 802 transmits through the FSS structure 804, i.e., the antenna apparatus 800 has a broadside radiation pattern 806-2.

[0053] FIG. 9 shows a schematic block diagram of an antenna apparatus 900 in accordance with a second exemplary embodiment. The antenna apparatus 900 may be implemented as a CPE or any other UE. As shown in FIG. 9, the antenna apparatus 900 comprises an antenna element 902 arranged in a housing (not shown in FIG. 9) and configured to emit a radio wave. The antenna apparatus 900 further comprises three independently controlled FSS structures 904-1, 904-2, 904-3, which are arranged in the housing on a propagation path of the radio wave (i.e., over the antenna element 902 in the apparatus configuration shown in FIG. 9). Again, the FSS structures 904-1, 904-2, 904-3 may be arranged either in a far field of the antenna element 902, or between a near field of the antenna element 902 and the far field of the antenna element 902. The antenna apparatus 900 is also intended to comprise a power source (not shown in FIG. 9) configured to apply an electric signal to a tunable material used in each of the FSS structure 904-1, 904-2, 904-3, thereby providing the above-described inter-state transition and, consequently, beam steering. More specifically, each of the FSS structures 904-1, 904-2, 904-3 may be independently turned on / off for reflective / transparent operation. The antenna apparatus 900 has an end-fire radiation pattern 906-1 when the tunable material of the FSS structure 904-3 is in the first (reflecting) state and the tunable materials of the FSS structures 904-1 and 904-2 are in the second (transparent) state. Further, the antenna apparatus 900 has a broadside radiation pattern 906-2 when the tunable materials of the FSS structures 904-1, 904-2, 904-3 are all in the second state. Finally, the antenna apparatus 900 has an end-fire radiation pattern 906-3 when the tunable material of the FSS structure 904-1 is in the first state and the tunable materials of the FSS structures 904-2 and 904-3 are in the second state.

[0054] FIG. 10 shows a schematic block diagram of an antenna apparatus 1000 in accordance with a third exemplary embodiment. The antenna apparatus 1000 may be implemented as a CPE or any other UE. As shown in FIG. 10, the antenna apparatus 1000 comprises a housing 1002 and an antenna module 1004 arranged in the housing 1002 and comprising multiple antenna elements each configured to emit a radio wave. The antenna apparatus 1000 further comprises three independently controlled FSS structures 1006-1, 1006-2, 1006-3, which are fully arranged in the housing 1002 on the propagation path of the radio waves emitted by the antenna elements (i.e., over the antenna module 1004 in the apparatus configuration shown in FIG. 10). Again, the FSS structures 1006-1, 1006-2, 1006-3 may be arranged either in a far field of the antenna module 1004, or between a near field of the antenna module 1004 and the far field of the antenna module 1004. The antenna apparatus 1000 is also intended to comprise a power source (not shown in FIG. 10) configured to apply an electric signal to a tunable material used in each of the FSS structure 1006-1, 1006-2, 1006-3, thereby providing the above-described inter-state transition and, consequently, beam steering. In general, the operation of the antenna apparatus 1000 may be similar to that of the antenna apparatus 900.

[0055] FIG. 11 shows a schematic block diagram of an antenna apparatus 1100 in accordance with a fourth exemplary embodiment. The antenna apparatus 1100 may be implemented as a CPE or any other UE. As shown in FIG. 11, the antenna apparatus 1100 comprises a housing 1102 and an antenna module 1104 arranged in the housing 1102 and comprising multiple antenna elements each configured to emit a radio wave. The antenna apparatus 1100 further comprises three independently controlled FSS structures 1106-1, 1106-2, 1106-3, which are arranged on the propagation path of the radio waves emitted by the antenna elements (i.e., over the antenna module 1104 in the apparatus configuration shown in FIG. 11). However, unlike the apparatus configuration shown in FIG. 10, the FSS structures 1106-1, 1106-2, 1106-3 are arranged a bit further away from the antenna module 1104, so that each of the FSS structures 1106-1 and 1106-3 partly extends beyond the housing 1102 at a distance b (i.e., which a height in the apparatus configuration shown in FIG. 11) and the FSS structure 1106-2 is fully outside the housing 1102. This arrangement of the FSS structures 1106-1, 1106-2, 1106-3 allows one to achieve better operation results compared to the apparatus configuration shown in FIG. 10, but the overall required apparatus size becomes a bit higher. In general, an optimal position for the FSS structures 1106-1, 1106-2, 1106-3 may be in the far field of the antenna module 1104. However, for example, to minimize the overall apparatus size, the FSS structures 1106-1, 1106-2, 1106-3 may be also arranged in the near field of the antenna module 1104, while still allowing one to achieve proper operation results. Furthermore, instead of arranging the FSS structures 1106-1, 1106-2, 1106-3 a bit further away from the antenna module 1104, it is also possible to push down the antenna module 1104 itself inside the antenna apparatus 1100, thereby achieving the same operation results of the antenna apparatus 1100.

[0056] The table below shows the results of comparing the performance (i.e., beam steering ranges) of different antenna apparatuses. In particular, "Reference" corresponds to an antenna apparatus similar to the antenna apparatuses 1000 but having no FSS structure, "Case 1" corresponds to the antenna apparatus 1000, and "Case 2" corresponds to the antenna apparatus 1100 with the distance b equal to 5 mm. Moreover, "H-Pol" means a horizontal polarization, and "V-Pol" means a vertical polarization. To obtain the results shown in the table below, the FSS structures in each of the cases were switched between the transparent and reflecting states in the same manner as discussed above with reference to FIG. 9. One can see that the antenna apparatuses 1000 and 1100 provide better beam steering ranges compared to the FSS structure-free antenna apparatus (see the "improvement compared to "Reference" case" in the table below). In turn, the antenna apparatus 1100 provides the best beam steering range but at the cost of an increase in the overall apparatus size. Beam steering range (25 / 27 / 29 GHz), H-PolBeam steering range (25 / 27 / 29 GHz), V-PolSize increaseReference (i.e., the antenna apparatus 1000 with no FSS structure)50° / 40° / 30°50° / 50° / 40°NoCase 1 (i.e., the antenna apparatus 1000 with the FSS structures)70° / 5° / 5° (20° / 10° / 20° - improvement compared to "Reference" case)70° / 55° / 50° (20° / 5° / 10° - improvement compared to "Reference" case)NoCase 2 (i.e., the antenna apparatus 1100 with the FSS structures and b = 5 mm)80° / 60° / 55° (30° / 20° / 25° - improvement compared to "Reference" case)70° / 60° / 55° (20° / 10° / 15° - improvement compared to "Reference" case)Yes, there is an increase in the total apparatus size (i.e., the FSS structures 1106-1 - 1106-3 go beyond the housing 1102 such that there is a protruding triangular segment having an area of about 60 mm 2< )

[0057] It should be noted that none of the antenna apparatuses 800-1100 is limited to the shown number, shape and arrangement of the FSS structures. In some other embodiments, each of the antenna apparatuses 800-1100 may have an FSS structure shaped as a hollow box or tube in which one or more antenna elements are arranged or may have a plurality of FSS structures attached to each other such that this hollow box or tube configuration is obtained. Furthermore, the size, shape and number of the FSS structures may vary depending on particular applications, as well as their positioning in relation to each other and in relation to the antenna element(s).

[0058] In one embodiment, each of the antenna apparatuses 800-1100 may further comprise a temperature control element configured to exert a thermal action on the tunable material in each FSS structure. Examples of the temperature control element may include, but not limited to, a heat source, a heat sink, an optical heat source, etc. Of course, the use of the temperature control element is reasonable only when the tunable material may provide the above-described inter-state transition in response to a temperature change (e.g., when vanadium dioxide is used as the tunable material).

[0059] Although the exemplary embodiments of the present disclosure are described herein, it should be noted that any various changes and modifications could be made in the embodiments of the present disclosure, without departing from the scope of legal protection which is defined by the appended claims. In the appended claims, the word "comprising" does not exclude other elements or operations, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A frequency selective surface, FSS, structure (200) comprising: a dielectric substrate (202); at least one sheet-like element (204) provided on the dielectric substrate (202), each of the at least one sheet-like element (204) being made of a tunable material, the tunable material being switchable between a first state and a second state in response to an electric signal applied to the tunable material, the tunable material being configured to reflect a radio wave in the first state and transmit the radio wave in the second state; and an electrically conductive sub-structure for applying the electric signal to the tunable material of each of the at least one sheet-like element (204), the electrically conductive sub-structure comprising at least one first conductor (206) and at least one second conductor (208), characterized by the at least one first conductor (206) being connected to each of the at least one sheet-like element (204), the at least one second conductor (208) being arranged in vicinity of each of the at least one sheet-like element (204) such that there is a gap between the at least one second conductor (208) and each of the at least one sheet-like element (204), wherein the at least one second conductor (208) comprises a plurality of discrete conductors (508-1, 508-2, 508-3, 508-4) provided on the dielectric substrate (202) around each of the at least one sheet-like element (204); or wherein the at least one second conductor (208) is configured as a continuous electrically conductive layer provided on the dielectric substrate (202) around each of the at least one sheet-like element (204); or wherein the at least one second conductor (208) is provided over each of the at least one sheet-like element (204), the gap between the at least one second conductor (208) and each of the at least one sheet-like element (204) is filled with a layer of dielectric material, and the at least one second conductor (208) is configured as an electrically conductive mesh arranged on the layer of dielectric material.

2. The FSS structure of claim 1, wherein each of the dielectric substrate (202) and the at least one sheet-like element (204) has a thickness selected based on a wavelength of the radio wave.

3. The FSS structure of claim 1 or 2, wherein the tunable material comprises at least one of a zero-gap semiconductor, a Metal Insulator Transition (MIT)-based material, and a Transition Metal Oxide (TMO)-based material.

4. The FSS structure of any one of claims 1 to 3, wherein the tunable material is further switchable between the first state and the second state in response to a thermal action on each of the at least one sheet-like element.

5. The FSS structure of any one of claims 1 to 4, wherein the at least one sheet-like element (204) comprises multiple sheet-like elements (404) arranged adjacent to each other on the dielectric substrate (202), and wherein the at least one first conductor (206) of the electrically conductive sub-structure comprises multiple first conductors (406) each connected to one of the multiple sheet-like elements (404), and the at least one second conductor (208) of the electrically conductive sub-structure comprises multiple second conductors (408) each arranged in vicinity of one of the multiple sheet-like elements (404) and with the gap therefrom.

6. The FSS structure of claim 5, wherein a number of the multiple sheet-like elements (404) is selected such that an electric field generated in each of the multiple sheet-like elements (404) in response to the electric signal is evenly distributed across the sheet-like element.

7. An antenna apparatus (800) comprising: a housing; at least one antenna element (802) arranged in the housing and configured to emit a radio wave; at least one frequency selective surface, FSS, structure (200) according to any one of claims 1 to 6, each of the at least one FSS structure (200) being arranged at least partly in the housing on a propagation path of the radio wave; and a power source configured to apply an electric signal to the tunable material of each of the at least one sheet-like element (204) in each of the at least one FSS structure (200) via the electrically conductive sub-structure.

8. The antenna apparatus of claim 7, wherein each of the at least one FSS structure (200) is arranged between a near field of the at least one antenna element (802) and a far field of the at least one antenna element (802).

9. The antenna apparatus of claim 7, wherein each of the at least one FSS structure (200) is arranged in a far field of the at least one antenna element (802).

10. The antenna apparatus of any one of claims 7 to 9, further comprising a temperature control element configured to exert a thermal action on each of the at least one sheet-like element (204) in each of the at least one FSS structure (200).

11. The antenna apparatus of any one of claims 7 to 10, wherein the at least one FSS structure (200) is shaped as a hollow box or tube in which the at least one antenna element (802) is arranged.