Wireless transmission system

Abstract

Provided is a wireless transfer system which communicates in a gigahertz band, wherein the wireless transfer system reduces a dead zone and improves a radio wave propagation environment. In the wireless transfer system in which a base station located in a facility and a target apparatus transmit and receive radio waves at frequencies selected from a frequency band that is equal to or higher than 3 GHz and is lower than 6 GHz, the maximum gain of an antenna of the base station is from 5 dBi to 30 dBi, and when there is a dead zone in which the reception intensity is lower by at least 10 dB than the periphery in the facility, an electromagnetic wave reflection device, which includes one or more panels with reflection surfaces each having a size from 10 cm×10 cm to 3.0 m×3.0 m, is placed between the base station and the dead zone so that the center of reflection is at the height of 0.5 m or higher from the bottom of the facility. The sum of the straight-line distance between the antenna of the base station and the electromagnetic wave reflection device, and the straight-line distance between the electromagnetic wave reflection device and the dead zone is from 5.0 m to 300 m.

Description

Technical Field

[0001] The present invention relates to a wireless transmission system and an electromagnetic wave reflection device.

Background Art

[0002] Indoor base stations have been introduced in factories, plants, offices, commercial facilities, etc. due to the automation of manufacturing processes and office work, and the introduction of control and management by AI (Artificial Intelligence). The 5G mobile communication standard provides a frequency band of 6 GHz or less called "sub-6" and a 28 GHz band classified as the millimeter wave band. In the next-generation 6G mobile communication standard, an expansion to the sub-terahertz band is expected. By using such high-frequency bands, the communication bandwidth can be significantly expanded, and a large amount of data communication can be performed with low latency.

[0003] A configuration has been proposed in which an electromagnetic wave reflection device is arranged along at least a part of a process line (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Radio waves in the "sub-6" frequency band have high directivity due to the high frequency in the gigahertz band, a short propagation distance, and a large propagation loss. In indoor facilities such as factories, plants, and commercial facilities, and outdoor facilities such as railways and highways, there are various obstacles such as equipment and structures, making it difficult to maintain high communication quality. By using an electromagnetic wave reflection device, the radio wave propagation environment can be improved, but the position, size, number, etc. of obstacles vary for each facility, and the efficient arrangement of the electromagnetic wave reflection device is not generally determined.

[0006] In one aspect, the present invention aims to reduce dead zones and improve the radio wave propagation environment in a wireless transmission system that communicates in the gigahertz band. [Means for solving the problem]

[0007] In one embodiment, a wireless transmission system in which a base station located within a facility and a target device transmit and receive radio waves at a frequency selected from the frequency band of 3 GHz or higher and less than 6 GHz, The maximum gain of the base station antenna is 5 dBi or more and 30 dBi or less. When there is a dead zone within the facility where the received signal strength is 10 dB or more lower than the surrounding area, an electromagnetic wave reflecting device including one or more panels with a reflective surface size of 10 cm × 10 cm or more and 3.0 m × 3.0 m or less is placed between the base station and the dead zone, such that the center of reflection is at a height of 0.5 m or more from the floor of the facility. The sum of the straight-line distance between the base station antenna and the electromagnetic wave reflector and the straight-line distance between the electromagnetic wave reflector and the dead zone is 5.0m or more and 300m or less. [Effects of the Invention]

[0008] In wireless transmission systems that communicate in the gigahertz band, dead zones are reduced and the radio wave propagation environment is improved. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic plan view of a facility to which the wireless transmission system of the embodiment is applied. [Figure 2] This is a schematic plan view of a facility to which the wireless transmission system of the embodiment is applied. [Figure 3] This diagram shows the relative positions of the base station, electromagnetic wave reflector, and dead zone. [Figure 4] This figure shows the layout configuration and received power distribution of the wireless transmission system in Example 1 before the installation of the electromagnetic wave reflector. [Figure 5]This is a diagram showing the layout configuration and received power distribution when an electromagnetic wave reflection device is installed in the wireless transmission system of Example 1. [Figure 6A] This is a schematic diagram of the electromagnetic wave reflection device used in the calculation. [Figure 6B] This is a horizontal cross-sectional view of the panel of the electromagnetic wave reflection device. [Figure 7] This is a diagram showing the layout configuration and received power distribution before installing the electromagnetic wave reflection device in the wireless transmission system of Example 2. [Figure 8] This is a diagram showing the layout configuration and received power distribution when an electromagnetic wave reflection device is installed in the wireless transmission system of Example 2. [Figure 9] This is a diagram showing the layout configuration and received power distribution before installing the electromagnetic wave reflection device in the wireless transmission system of Example 3. [Figure 10] This is a diagram showing the layout configuration and received power distribution when an electromagnetic wave reflection device is installed in the wireless transmission system of Example 3. [Figure 11] This is a diagram showing the layout configuration and received power distribution before installing the electromagnetic wave reflection device in the wireless transmission system of Example 4. [Figure 12] This is a diagram showing the layout configuration and received power distribution when an electromagnetic wave reflection device is installed in the wireless transmission system of Example 4. [Figure 13] This is a diagram showing the layout configuration and received power distribution before installing the electromagnetic wave reflection device in the wireless transmission system of Example 5. [Figure 14] This is a diagram showing the layout configuration and received power distribution when an electromagnetic wave reflection device is installed in the wireless transmission system of Example 5. [Figure 15] This is a diagram showing a modified example of the panel of the electromagnetic wave reflection device. [Figure 16] This is a perspective view of an electromagnetic wave reflection device using a hollow panel. [[ID=3८]] [Figure 17] This is a perspective view of another electromagnetic wave reflection device using a hollow panel. [[ID=५०]] [[ID=५१]] [Figure 18] [[ID=५२]]This is a diagram showing the analysis space of the reflection characteristics. [[ID=५३]] [[ID=५४]] [Figure 19] [[ID=५५]]This is a diagram showing the analysis space of the reflection characteristics. [[ID=५६]] [[ID=५७]] [Figure 20]It is a model of a hollow panel used for analyzing reflection characteristics. [Figure 21] It is a diagram showing the reflection characteristics of the hollow panel of Example 11. [Figure 22] It is a diagram showing the reflection characteristics of the hollow panel of Example 12.

Mode for Carrying Out the Invention

[0010] FIG. 1 is a schematic plan view of the inside of a facility to which the wireless transmission system 1 of the embodiment is applied. (A) in FIG. 1 is the state before installing the electromagnetic wave reflection device 20, and (B) in FIG. 1 is the state after installing the electromagnetic wave reflection device 20. In FIG. l, a production facility such as a factory or a plant is assumed, but the facility is not limited to indoor facilities and includes outdoor facilities such as railways and expressways.

[0011] In the production facility shown in FIG. 1, there are structures 12 such as storage shelves, racks, and manufacturing machines, and production equipment 14 such as automatic transfer devices, robot arms, and assembly devices. Structures involved in production, such as large manufacturing machines, are both structures 12 and can also be production equipment 14. In this specification and the claims, among the structures 12, those that are involved in production and transmit and receive radio waves with the base station 10 are also included in the production equipment 14.

[0012] The base station 10 is arranged inside the facility and transmits and receives radio waves with the production equipment 14 in a predetermined frequency band. The production equipment 14 is an example of a target device that transmits and receives radio waves with the base station 10. The number of base stations 10 provided in the facility is not limited to one, but from the viewpoints of installation work and cost, it is desirable that a single base station 10 can cover a service area of a certain size.

[0013] The base station 10 and the production equipment 14 transmit and receive radio waves at a desired frequency selected from a frequency band of 3 GHz or more and less than 6 GHz. The maximum gain of the antenna of the base station 10 is, for example, 5 dBi or more and 30 dBi or less. The maximum gain of the antenna of the base station 10 is expressed as an absolute gain based on a virtual isotropic antenna.

[0014] Depending on the transmission and reception angles between the base station 10 and the production equipment 14, the structure 12 can become an obstacle to radio wave propagation. Metal structures such as ducts and pipes also exist in the production site, and radio waves are reflected and scattered by these structures 12. As a result, a dead zone 30 occurs where the received strength of the radio waves radiated from the base station 10's antenna falls below a predetermined level. In particular, in the high-frequency band between 3 GHz and 6 GHz, radio waves have strong directional properties and little diffraction, making it difficult for radio waves from the base station 10 to reach the target.

[0015] In this specification and claims, "dead zone" refers to an area where the reception strength is reduced by 10 dB or more compared to the surrounding reception environment without shielding, due to the influence of shielding objects such as the structure 12. The dead zone 30 includes not only a two-dimensional area but also three-dimensional space. If the production equipment 14 is in the dead zone 30, it will be difficult to receive signals from the base station 10, which may reduce production efficiency. Therefore, an electromagnetic wave reflector 20 is introduced as shown in Figure 1(B).

[0016] By deploying the electromagnetic wave reflector 20, radio waves from the base station 10 can reach the dead zone 30. The reflective surface 21 of the electromagnetic wave reflector 20 is made of any material that can reflect the radio waves toward the dead zone 30 while maintaining the electric field strength of the incident radio waves as much as possible. For example, it can be made of a conductive film, conductive mesh, or periodic pattern of conductors formed on or inside a dielectric. The density of the conductive mesh and the period of the periodic pattern are designed to reflect radio waves between 3 GHz and 6 GHz, and have a pitch or period of 1 / 5 or less of the free-space wavelength in this band.

[0017] The reflective surface 21 of the electromagnetic wave reflector 20 may be a specular reflective surface that reflects radio waves at the same angle as the incident angle, or it may be an artificial surface that reflects radio waves in a desired direction at an angle different from the incident angle. In particular, if it is difficult to deliver radio waves to the production equipment 14 by specular reflection from the location of the base station 10 and the dead zone 30, and from the spatial location where the electromagnetic wave reflector 20 can be installed, it is desirable to use an artificial surface that achieves the desired reflection angle. The reflective surface 21 may contain a mixture of specular reflective surfaces and artificial surfaces. The arrangement configuration in Figure 1(B) reduces dead zones within the facility and improves the communication environment.

[0018] Figure 2 is a schematic plan view of another facility to which the wireless transmission system 1 is applied. Figure 2(A) shows the state before the installation of the electromagnetic wave reflector 20, and Figure 2(B) shows the state after the electromagnetic wave reflector 20 has been installed. Similar to Figure 1, the frequency of the radio waves radiated from the base station 10 is 3 GHz or higher and less than 6 GHz, and the maximum gain of the base station 10's antenna is 5 dBi or higher and 30 dBi or lower.

[0019] In process lines for assembling automobiles, home appliances, etc., multiple production equipment 14 such as robot arms are arranged within the line, and structures 12 such as fences or columns 13 may be placed outside of them. Reflection and scattering by the columns, fences, and robot arms can create a dead zone 30 in the central part along the longitudinal direction of the process line. In this case, as shown in Figure 2(B), by installing an electromagnetic wave reflector 20 along the process line, radio waves from the base station 10 can reach the dead zone 30.

[0020] The locations where dead zones 30 occur vary from facility to facility, and the optimal placement of the electromagnetic wave reflector 20 also differs from facility to facility. The inventors investigated and confirmed that by arranging the electromagnetic wave reflector 20 to satisfy predetermined conditions in relation to the base station 10 and the dead zone 30, it is possible to reduce dead zones to a certain extent efficiently, regardless of the arrangement of the structures 12 within the facility.

[0021] Figure 3 shows the positional relationship between the base station 10, the electromagnetic wave reflector 20, and the dead zone 30. The base station 10, the electromagnetic wave reflector 20, and the dead zone 30 are located in a plane parallel to the XY plane. The phrase "in a plane parallel to the XY plane" is used because the base station 10 and the electromagnetic wave reflector 20 are not necessarily installed on the floor of the facility, and the dead zone 30 is not necessarily generated on the floor of the facility. The Z direction, which is perpendicular to the XY plane, is the height direction.

[0022] Let D1 be the straight-line distance between the reflection center R of the radio waves incident on the electromagnetic wave reflector 20 and the boundary line of the dead zone 30. Let D2 be the straight-line distance between the antenna of the base station 10 and the reflection center R of the electromagnetic wave reflector 20. Given the frequency of the radio waves radiated by the antenna of the base station 10 and the maximum gain of the antenna, the sum of D1 and D2 (D1+D2) satisfies a predetermined condition.

[0023] The electromagnetic wave reflector 20 is positioned at a location other than the straight line connecting the base station 10 and the dead zone 30. This is because if the electromagnetic wave reflector 20 were located on the straight line connecting the base station 10 and the dead zone 30, the electromagnetic wave reflector 20 would become an obstacle. The angle formed by the straight line connecting the antenna of the base station 10, the reflection center R on the electromagnetic wave reflector 20, and the dead zone 30 is any angle that allows the radio waves radiated from the antenna of the base station 10 to be reflected to the dead zone 30, for example, between 5° and 180°. If the angle of incidence of radio waves to the electromagnetic wave reflector 20 is less than 5°, radio wave interference may occur.

[0024] Since the configuration of an electromagnetic wave reflector and an electromagnetic wave shield are basically the same, the electromagnetic wave reflector 20 also has an electromagnetic wave shielding effect. The electromagnetic wave shielding effect of the electromagnetic wave reflector prevents electromagnetic waves of frequencies other than those of the base station 10 from entering the propagation path between the base station 10 and the dead zone 30 (and the production equipment 14 located in the dead zone 30).

[0025] The center of the reflective surface 21 of the electromagnetic wave reflector should preferably be at a height of 0.5 m or more from the floor of the facility, taking into consideration the size and position of the production equipment 14 and the position and height of the base station 10's antenna. The base station 10 may be positioned at a height of 1.0 m to 3.0 m from the floor in order to secure the widest possible service area within the facility. The inclination of the reflective surface 21 of the electromagnetic wave reflector 20 with respect to the floor surface, and the angle of the base station 10 with respect to the LOS (Line of Sight), are appropriately determined according to the shape and direction of the beam formed by the base station 10's antenna, the position of the receiving point of the production equipment 14, etc. Below, a specific example of indoor placement is described, and based on this example, conditions that can contribute to reducing dead zones are examined. [Examples]

[0026] Figure 4 shows the arrangement configuration and received power distribution of the wireless transmission system 1A of Example 1 before the installation of the electromagnetic wave reflector 20. Figure 5 shows the arrangement configuration and received power distribution of the wireless transmission system 1A of Example 1 after the installation of the electromagnetic wave reflector 20.

[0027] Figure 4(A) is a schematic plan view showing the positional relationship between base station 10 and dead zone 30. Figure 4(B) is a received power map of radio waves radiated from base station 10. Base station 10 is positioned at a height of 2m near the corner of an area measuring 11.0m in length and 7.5m in width. Base station 10 is equipped with an omnidirectional antenna with a maximum gain of 15dBi.

[0028] Base station 10 emits radio waves in the 4.8GHz band with a vertical beamwidth of 10° and a horizontal beamwidth of 50°. The floor, ceiling, and walls of the 11.0m x 7.5m area are constructed of materials compliant with ITU-R (International Telecommunication Union - Radiocommunication sector) Recommendation P.2040. The height of the omnidirectional antenna placed at the receiving point is 0.7m.

[0029] Within the area, metal structures 12-1 and 12-2, each measuring 2.1m in length, 0.7m in width, and 2.0m in height, are positioned. From the perspective of base station 10, the area behind structure 12-1 is a dead zone 30, where the received power is 10-30dB lower than the surrounding area. The existence of the dead zone 30 can also be confirmed from the received power map in Figure 4(B). The challenge lies in efficiently delivering radio waves to production equipment 14 (see Figure 1), which transmits and receives signals from base station 10, when it is located within the dead zone 30.

[0030] In Figure 5, an electromagnetic wave reflector 20 is positioned to reduce the dead zone 30. Figure 5(A) is a schematic plan view showing the electromagnetic wave reflector 20 positioned within the area, and Figure 5(B) is a received power map when the electromagnetic wave reflector 20 is used. In this layout, the reflective surface 21 of the electromagnetic wave reflector 20 is positioned at a 45° angle with respect to the line of sight (LOS) of the base station 10.

[0031] Figure 6A shows an example of an electromagnetic wave reflector 20 used in the wireless transmission system of the embodiment. In Embodiment 1, two electromagnetic wave reflectors 20-1 and 20-2, each 2.0 m long and 1.0 m wide, are connected horizontally to form a reflective surface of 2.0 m × 2.0 m. Each of the electromagnetic wave reflectors 20-1 and 20-2 includes a panel 200 and a frame 201 that holds the panel 200. When the electromagnetic wave reflectors 20-1 and 20-2 are installed on the floor, legs 202 may be provided on the frame 201. The frame 201 and legs 202 support the panel 200 at a predetermined height from the floor P. The height h1 from the floor P to the lower end of the panel 200 of the electromagnetic wave reflectors 20-1 and 20-2 is 13.5 cm, and the height from the floor P to the center of the reflective surface 21 is 113.5 cm.

[0032] Figure 6B shows a horizontal cross-sectional view of an example configuration of a panel 200 used in an electromagnetic wave reflector 20. The panel 200 has a conductive layer 215 sandwiched between dielectric layers 208. The conductive layer 215 may be bonded and held between the two dielectric layers 208 by an adhesive layer 216. The conductive layer 215 forms the reflective surface 21 of the electromagnetic wave reflector (see Figure 5) and has a predetermined conductive pattern that reflects radio waves in the range of 24 GHz to 32 GHz. When multiple electromagnetic wave reflectors 20 are connected as shown in Figure 6A, the conductive layer 215 is electrically connected between adjacent panels 200 inside the frame 201. By electrically connecting the conductive layer 215, a reflective surface 21 with a continuous reflection potential can be formed between multiple panels 200. In the simulation, two polycarbonate plates are used as the dielectric layer 208, with the conductive layer 215 sandwiched between them. In actual operation, other resins that are transparent to 28 GHz electromagnetic waves may be used, or a conductive layer 215 may be formed on the surface of one dielectric layer 208.

[0033] Returning to Figure 5, as shown in Figure 5(B), it can be seen that by placing the electromagnetic wave reflector 20, the received signal strength behind the structure 12-1 is improved by 10-30 dB, and the dead zone 30 is reduced. The received signal strength is expressed as a relative strength with respect to the omnidirectional antenna. At this time, the straight-line distance D2 from the base station 10 antenna to the reflection center R of the electromagnetic wave reflector 20 is 7.0 m, and the straight-line distance D1 from the boundary of the dead zone 30 to the reflection center R of the electromagnetic wave reflector 20 is 3.9 m. The total distance of D1 and D2 (D1+D2) is 10.9 m. It is confirmed that the dead zone 30 is reduced and the radio wave propagation environment is improved under the conditions of Example 1. [Examples]

[0034] Figure 7 shows the arrangement configuration and received power distribution of the wireless transmission system 1B of Example 2 before the installation of the electromagnetic wave reflector 20. Figure 8 shows the arrangement configuration and received power distribution of the wireless transmission system 1B of Example 2 after the installation of the electromagnetic wave reflector 20.

[0035] Figure 7(A) is a schematic plan view showing the positional relationship between the base station 10 and the dead zone 30, and Figure 7(B) is a received power map of the radio waves radiated from the base station 10. The base station 10 is positioned at a height of 2m along the wall on the longer side of an area measuring 11.0m in length and 7.5m in width. The base station 10 is equipped with an omnidirectional antenna, and its maximum gain is 15dBi.

[0036] Base station 10 emits radio waves in the 4.8 GHz band with a vertical beamwidth of 10° and a horizontal beamwidth of 50°. The floor, ceiling, and walls of the 11.0m x 7.5m area are constructed of materials compliant with ITU-R Recommendation P.2040. The height of the receiving point is 0.7m.

[0037] Within the area, metal structures 12-1 and 12-2, each measuring 2.1m in length, 0.7m in width, and 2.0m in height, are positioned. Compared to Implementation 1, the positional relationship between structures 12-1 and 12-2 and the base station 10 is different. Behind structures 12-1 and 12-2 is a dead zone 30 where the received power is 10-30dB lower than the surrounding area. The existence of the dead zone 30 can also be confirmed from the received power map in Figure 7(B).

[0038] In Figure 8(A), an electromagnetic wave reflector 20 is placed within the area. As shown in Figure 6A, the electromagnetic wave reflector 20 consists of two electromagnetic wave reflectors 20-1 and 20-2, each 2.0m long and 1.0m wide, connected horizontally, and has a reflective surface 21 measuring 2.0m x 2.0m. The height h1 from the floor surface P at the bottom of the reflective surface 21, i.e., the panel 200, is 13.5cm, and the height from the floor surface P to the center of the reflective surface 21 is 113.5cm.

[0039] The layout of Example 2 assumes that, as viewed from the base station 10, production equipment 14 (see Figure 1) is located behind structure 12-2, and production equipment 14 is not located behind structure 12-1. An electromagnetic wave reflector 20 is introduced to reduce the dead zone 30 behind structure 12-2. The electromagnetic wave reflector 20 is positioned parallel to structures 12-1 and 12-2, and is located closer to structure 12-2 than the center of the two structures 12-1 and 12-2. Radio waves from the base station 10 are incident on the reflective surface 21 of the electromagnetic wave reflector 20 at an incident angle greater than 45° and are reflected toward the dead zone 30 behind structure 12-2.

[0040] From Figure 8(B), it can be seen that the received signal strength is improved by 10 to 50 dB in the dead zone 30 behind structure 12-2. At this time, the straight-line distance D2 from the base station 10 antenna to the reflection center R of the electromagnetic wave reflector 20 is 13.5 m. The straight-line distance D1 from the boundary within the dead zone 30 to the reflection center R of the electromagnetic wave reflector 20 is 1.5 m. The sum of the distances D1 and D2 (D1 + D2) is 15.0 m.

[0041] Under the conditions of Example 2, it was confirmed that the dead zone 30 in the area where the production equipment 14 is located is reduced, and the radio wave propagation environment is improved. [Examples]

[0042] Figure 9 shows the arrangement configuration and received power distribution of the wireless transmission system 1C of Example 3 before the installation of the electromagnetic wave reflector 20. Figure 10 shows the arrangement configuration and received power distribution of the wireless transmission system 1C of Example 3 after the installation of the electromagnetic wave reflector 20.

[0043] Figure 9(A) is a schematic plan view showing the positional relationship between the base station 10 and the dead zone 30, and Figure 9(B) is a received power map of the radio waves radiated from the base station 10. The base station 10 is positioned at a height of 2m along the wall on the longer side of an area measuring 11.0m in length and 7.5m in width. The base station 10 is equipped with an omnidirectional antenna, and its maximum gain is 10dBi.

[0044] Base station 10 emits radio waves in the 4.8GHz band with a vertical beamwidth of 10° and a horizontal beamwidth of 50°. The floor, ceiling, and walls of the 11.0m x 7.5m area are constructed of materials compliant with ITU-R Recommendation P.2040. The height of the omnidirectional antenna placed at the receiving point is 0.7m.

[0045] Within the area, metal structures 12-1 and 12-2, each measuring 2.1m in length, 0.7m in width, and 2.0m in height, are positioned. From the perspective of base station 10, the area behind structures 12-1 and 12-2 is a dead zone 30 where the received power is 10-30dB lower than the surrounding area. The existence of the dead zone 30 can also be confirmed from the received power map in Figure 9(B).

[0046] In Figure 10(A), an electromagnetic wave reflector 20 is placed within the area. As shown in Figure 6A, the electromagnetic wave reflector 20 consists of two electromagnetic wave reflectors 20-1 and 20-2, each 2.0 m high and 1.0 m wide, connected horizontally, and has a reflective surface 21 measuring 2.0 m × 2.0 m. The height h1 from the floor surface P at the lower end of the reflective surface 21 is 13.5 cm, and the height from the floor surface P to the center of the reflective surface 21 is 113.5 cm.

[0047] The layout of Example 3 assumes that, as viewed from the base station 10, production equipment 14 (see Figure 1) is located behind structure 12-2, and production equipment 14 is not located behind structure 12-1. An electromagnetic wave reflector 20 is introduced to reduce the dead zone 30 behind structure 12-2. The electromagnetic wave reflector 20 is positioned parallel to structures 12-1 and 12-2, and is located closer to structure 12-2 than the center of the two structures 12-1 and 12-2. Radio waves from the base station 10 are incident on the reflective surface 21 of the electromagnetic wave reflector 20 at an incident angle greater than 45° and are reflected towards the dead zone 30 behind structure 12-2.

[0048] From Figure 10(B), it can be seen that the received signal strength is improved by 10-30 dB in the dead zone 30 behind structure 12-2. At this time, the straight-line distance D2 from the base station 10 antenna to the reflection center R of the electromagnetic wave reflector 20 is 13.5 m. The straight-line distance D1 from the boundary of the dead zone 30 to the reflection center R of the electromagnetic wave reflector 20 is 1.5 m. The sum of the distances D1 and D2 (D1+D2) is 15.0 m.

[0049] Under the conditions of Example 3, it is confirmed that the dead zone 30 is reduced in the area where the production equipment 14 is located, and the radio wave propagation environment is improved. From Example 2 and Example 3, it can be seen that even if there is a 5 dBi difference in the maximum gain of the base station 10 antenna, the dead zone can be reduced to the same extent. In the layout of Example 1, even when the maximum gain of the base station 10 antenna is 10 dBi, which is 5 dBi lower than 15 dBi, a dead zone reduction effect similar to that of Figure 5 (B) can be expected. Furthermore, in Examples 1 and 2, even when the maximum gain of the base station 10 antenna is 5 dBi greater than 15 dBi, a dead zone reduction effect greater than that of Figure 5 (B) and Figure 8 (B) can be expected. [Examples]

[0050] Figure 11 shows the arrangement configuration and received power distribution of the wireless transmission system 1D of Example 4 before the installation of the electromagnetic wave reflector 20. Figure 12 shows the arrangement configuration and received power distribution of the wireless transmission system 1D of Example 4 after the installation of the electromagnetic wave reflector 20. In the arrangement configuration of Figure 11(A), the wireless transmission system 1D includes a process line where parts 125, a robot arm 123, etc., are located. A polycarbonate safety fence 121 is provided along the process line for a length of 50m. In Figure 12, the electromagnetic wave reflector 20 is provided in place of the safety fence 121 for the same length of 45m.

[0051] Base station 10 is located at one end of a process line on a 70m x 35m floor. Base station 10 is equipped with an omnidirectional antenna, with an antenna height of 3.0m and a maximum gain of 15dBi. Base station 10 radiates radio waves in the 4.7GHz band with a vertical beamwidth of 28° and a horizontal beamwidth of 28°. The floor, ceiling, walls, and columns of the floor are made of concrete. The receiving point is at a height of 0.8m, and the receiving antenna is an omnidirectional antenna. From the received power distribution in Figure 11(B), it can be seen that the area behind the structure far from base station 10 is a dead zone with received power 10-60dB lower than the surrounding area.

[0052] In Figure 12(A), an electromagnetic wave reflector 20 is used instead of the safety fence 121. Fifty electromagnetic wave reflectors 20, each 1.0 m wide and 2.0 m high, are connected and installed. The lower end of the panel 200 is 0.15 m above the floor, and the reflection center is at a height of 0.15 m or more above the floor. From the received power map in Figure 12(B), it can be seen that the received signal strength is improved by about 10 dB to 20 dB inside the process line, including the dead zone. The straight-line distance D1 from the dead zone to the reflection center of the electromagnetic wave reflector 20 at this time is 5.0 m. The straight-line distance D2 from the base station 10 antenna to the reflection center R of the electromagnetic wave reflector 20 is 60.0 m. The total distance of D1 and D2 (D1 + D2) is 65.0 m. [Examples]

[0053] Figure 13 shows the layout configuration and received power distribution of the wireless transmission system 1E of Example 5 before the installation of the electromagnetic wave reflector 20. Figure 14 shows the layout configuration and received power distribution of the wireless transmission system 1B of Example 4 when the electromagnetic wave reflector 20 is installed. In the layout configuration of Figure 13 (A), the base station 10 is placed at the end of a floor measuring 300.0 m in length and 100.0 m in width. The base station 10 is equipped with an omnidirectional antenna, with the antenna height being 3.0 m from the floor and a maximum gain of 15 dBi.

[0054] Base station 10 emits radio waves in the 4.7GHz band with a vertical beamwidth of 28° and a horizontal beamwidth of 90°. The floor, ceiling, walls, and columns of the base station are made of concrete. The height of the receiving point is 1.0m, and the receiving antenna is an omnidirectional antenna.

[0055] A structure 12 measuring 50.0m in length, 30.0m in width, and 3.0m in height is placed on the floor. From the received power map in Figure 13 (B), it can be seen that the area behind the structure 12, as seen from the base station 10, is a dead zone with received power 10dB to 10-40dB lower than the surrounding area.

[0056] In Figure 14(A), the electromagnetic wave reflector 20 is positioned at an oblique angle to the structure 12. The electromagnetic wave reflector 20 is made up of 28 units of the electromagnetic wave reflector 20 shown in Figure 6A, each 2.0 m long and 1.0 m wide, connected horizontally. The height of the lower end of the reflective surface 21 (panel 200) from the floor surface P is 0.135 m. From Figure 14(B), it can be seen that the received signal strength is improved by about 10 dB to 20 dB in the dead zone behind the structure 12. At this time, the straight-line distance D1 from the dead zone to the reflection center of the electromagnetic wave reflector 20 is 50.0 m. The straight-line distance D2 from the base station 10 antenna to the reflection center of the electromagnetic wave reflector 20 is 250.0 m. The total distance of D1 and D2 (D1 + D2) is 300.0 m.

[0057] In Examples 1 to 5, when the positional relationship between the base station 10, the electromagnetic wave reflector 20, and the dead zone 30 to be eliminated satisfies predetermined conditions, the dead zone 30 where target equipment such as production equipment 14 is located can be reduced. The conditions that the positional relationship between the base station 10, the electromagnetic wave reflector 20, and the dead zone 30 must satisfy change depending on the frequency band of the radio waves emitted from the base station 10. However, within the range of 4.5 GHz ± 1.5 GHz, i.e., in the frequency band of 3 GHz or higher and less than 6 GHz, there is no significant change in the conditions, and the conditions of Examples 1 to 5 apply within the range of 3 GHz or higher and less than 6 GHz.

[0058] The electromagnetic wave reflector 20 does not have to be configured as two electromagnetic wave reflectors 20-1 and 20-2 combined, as shown in Figure 6A. Furthermore, it does not necessarily have to be supported by legs 202. As long as it can reflect radio waves from the base station 10 towards the dead zone 30, an electromagnetic wave reflector 20 without legs 202 may be placed leaning against a desk, shelf, or stand. The manner in which the electromagnetic wave reflector 20 is supported is not particularly important. Therefore, legs 202 may be used as shown in Figure 6A, or only a frame 201 surrounding the panel 200 of the electromagnetic wave reflector 20 may be used. As in Examples 4 and 5, the required number of electromagnetic wave reflectors 20 may be linked together. A configuration linking multiple electromagnetic wave reflectors 20 is effective in reducing dead zones in the process lines shown in Figures 2 and 12.

[0059] The size of the electromagnetic wave reflector 20 only needs to be large enough to deliver the radio waves radiated from the base station 10 to the dead zone 30, and is at least large enough to fill the first Fresnel zone of the radio wave propagation path. The radius r of the first Fresnel zone is given by the straight-line distances D1 and D2 and the wavelength λ, r = [λ × D1 × D2 / (D1 + D2)] 1 / 2 This is expressed as follows. The wavelength of radio waves in the 4.8GHz band is approximately 62mm, and if D1 and D2 are the distances obtained in Examples 1 to 5, the radius r is approximately 5cm to 40cm. The area of ​​the reflective surface 21 per panel of the electromagnetic wave reflector 20 is preferably at least 10cm × 10cm. Increasing the size of the reflective surface 21 per panel allows radio waves to reach a wider area with a single panel, but if the area of ​​the panel 200 is too large, the ease of handling during transportation and installation may be impaired. The size of the panel 200 of the electromagnetic wave reflector 20 is preferably 3m × 3m or less. Therefore, the size of the panel 200 can be appropriately determined considering the weight, strength, and ease of handling of the electromagnetic wave reflector, such as 15cm × 15cm or more, 2.5m × 2.5m or less, or 20cm × 20cm or more, 2.0m × 2.0m or less.

[0060] The height of the center of the reflective surface 21 is determined by the height of the base station 10's antenna and the height of the receiving point. In Examples 1 to 5, the base station 10's antenna is positioned at a height of 2 to 3 m from the floor, but considering the typical ceiling height of indoor facilities such as factories and commercial facilities, the base station 10's antenna can be positioned at a height of 1.5 m or more and 10 m or less from the floor. In addition to being installed near the ceiling, the base station 10 can also be suspended from the ceiling or installed on a pole on the floor. Assuming that the receiving antenna of the production equipment 14 is positioned at a height of 0.7 m or more and 2.0 m or less from the floor, from the viewpoint of ensuring sufficient reception strength at the production equipment 14, it is desirable that the height of the center of the reflective surface 21 of the electromagnetic wave reflector 20, or the reflection center R (see Figure 3), be 0.5 m or more. In this case, depending on the position of the base station 10, the height of the reflection center R may be lower than the height of the receiving point of the production equipment 14, or it may be higher than the height of the receiving point. In the former case, radio waves from the base station 10, which is installed at a high position, can be effectively delivered to the production equipment 14 located in the dead zone 30.

[0061] Based on the simulation results of Examples 1 to 5, the following conditions can be derived. (1) The frequency band of the radio waves emitted from the base station 10 located within the facility is between 3 GHz and 6 GHz. (2) The maximum gain of the antenna of base station 10 is 5 dBi or more and 30 dBi or less, and may be 10 dBi or more and 30 dBi or less from the viewpoint of radio wave range. (3) A dead zone 30 is an area where the received signal strength is 10 dB or more lower than the surrounding propagation environment. (4) The size of the reflective surface per panel of the electromagnetic wave reflector 20 is 10cm x 10cm or more and 3.0m x 3.0m or less, and can be set to 15cm x 15cm or more, 2.5m x 2.5m or less, 20cm x 20cm or more, or 2.0m x 2.0m or more from the viewpoint of panel weight, strength, ease of handling, etc. The height of the reflection center from the floor surface P can be set to 0.135m or more, 0.15m or more, 0.5m or more, etc. (5) The combined length of D1 and D2 is between 5.0m and 300m. (6) The height of the reflection center of the electromagnetic wave reflector 20 may be less than or equal to the height of the receiving point of the production equipment 14 (target equipment).

[0062] Thus, when the frequency band of radio waves handled by the base station 10 and the maximum gain of the base station 10's antenna are determined within the facility, an electromagnetic wave reflector 20 having a reflective surface 21 of a predetermined size is used. The electromagnetic wave reflector 20 is positioned to satisfy the condition that the sum of the straight-line distance D2 from the base station 10's antenna to the reflection center R of the electromagnetic wave reflector 20 (see Figure 3) and the straight-line distance D1 from the reflection center R to the dead zone 30 is between 5.0m and 300m. This reduces the dead zone 30, improving the radio wave propagation environment indoors where shielding objects such as structures 12 exist. The sum of D1 and D2 is the distance from the transmitting antenna to the receiving antenna via the reflection center R. The value of D1+D2 corresponds to the range of radio waves of a specific frequency band from the base station 10. Assuming use indoors, including in process lines, a maximum of 300m for D1+D2 is reasonable. Under the above conditions, the dead zone 30 can be reduced indoors where shielding objects such as the structure 12 exist, thereby improving the radio wave propagation environment.

[0063] The wireless transmission system described above is not limited to production facilities such as factories, but can also be applied to facilities such as sound barriers used on railways and highways. In event venues with many structures and obstacles, and in areas of roads with many vehicles and obstacles, dead zones can occur frequently. By using the wireless transmission system of this embodiment, dead zones can be reduced and the radio wave propagation environment can be improved.

[0064] <Panel variation> In Examples 1 to 5, a flat transparent resin plate was used as the dielectric layer 208. This panel 200 is referred to as a "flat panel". The dielectric layer does not necessarily have to be a solid plate and may have a hollow interior. In the following modified example, an electromagnetic wave reflector is provided having a hollow panel 200A using a dielectric layer 210 having a hollow interior 213. "Hollow" literally means that there is a cavity inside the dielectric layer 210, and its shape is not specified.

[0065] Figure 15 shows a modified example of the electromagnetic wave reflector panel. Figure 15(A) is a schematic diagram showing a part of the hollow panel 200A, and (B) is a horizontal cross-sectional view of the hollow panel 200A along line II. The hollow panel 200A has a conductive layer 215 sandwiched between two dielectric layers 210 via an adhesive layer 216. The conductive layer 215 is formed in a pattern that reflects radio waves with frequencies of 2 GHz to 15 GHz, more preferably 3 GHz or higher and 6 GHz or lower. The dielectric layer 210 has a hollow 213 extending in a predetermined direction inside the plate 211. The cross-sectional shape of the hollow 213 is not limited to a rectangle, but may be polygonal, elliptical, circular, etc. By having a hollow 213 inside, the weight of the hollow panel 200A is reduced, and the dielectric constant of the dielectric layer 210 approaches that of air.

[0066] Figure 16 is a perspective view of an electromagnetic wave reflector 20A using a hollow panel 200A. The electromagnetic wave reflector 20A includes a hollow panel 200A and a frame 201 that holds the hollow panel 200A. When the electromagnetic wave reflector 20A is installed on the floor as shown in Figure 16, legs 202 may be provided on the frame 201. Multiple electromagnetic wave reflectors 20A are connected by the frame 201, and the hollow panel 200A is supported at a predetermined height from the floor by the legs 202 and the frame 201. Also, inside the frame 201, the conductive layers 215 of adjacent hollow panels 200A are electrically connected and maintained at the same reflection potential. In Figure 16, the hollow panel 200A has a number of vertically extending hollows 213 (see Figure 15). For convenience of illustration, the hollows 213 are shown as vertical lines, but the hollow panel 200A is transparent to visible light and gigahertz-band radio waves. The hollow sections 213 may extend parallel to each other within the hollow panel 200A.

[0067] In the hollow panel 200A, the direction in which the hollows 213 extend is not limited to the vertical direction. As shown in Figure 17, multiple hollows 213 may extend parallel to the horizontal direction of the hollow panel 200A. In this case as well, the weight of the hollow panel 200A is reduced, and the dielectric constant approaches that of air.

[0068] Figures 18 and 19 show the analysis space 31 for the reflection characteristics of the embodiment described below. The in-plane of the panel is the XY plane, and the thickness direction is the Z direction. When the analysis space is expressed as (size in the X direction) × (size in the Y direction) × (size in the Z direction), the size of the analysis space 31 when the frequency is 2 to 15 GHz is 500 mm × 500 mm × 21.7 mm. As shown in Figure 19, the boundary conditions are designed with electromagnetic wave absorbers 32 placed around the analysis space 31.

[0069] Figure 20 is a model diagram of a hollow panel used in electromagnetic field simulations. The thickness of the dielectric layer 210 is t1, the thickness of the plate wall is t2, the thickness of the hollow 213 is L1, and the lateral width of the hollow 213, i.e., the pitch, is L2. In the electromagnetic field simulation using this model, a 4.7 GHz plane wave is incident on the hollow panel 200A and reflected from the panel surface. The scattering cross section (RCS), which represents the reflectivity of the panel, is analyzed using general-purpose 3D electromagnetic field simulation software.

[0070] Figure 21 shows the reflection characteristics of the hollow panel 200A of Example 11. In Example 11, the dielectric layer 210 has a thickness t1 of 5.0 mm, a plate wall thickness t2 of 0.5 mm, a hollow 213 thickness L1 of 4.0 mm, and a pitch L2 of 3.5 mm, and is formed of polycarbonate with a cross-sectional hollowness of 70.0%. A panel is used in which a conductive layer 215 (see Figure 20) is sandwiched between two of these polycarbonate layers. Simulations are performed by changing the incident angle from -60° to +60° in 10° increments. The dB values ​​of the main peak of the scattering cross-section are as shown in Figure 21. A scattering cross-section exceeding 20 dB is obtained in the range of ±40°, and a scattering cross-section exceeding 22 dB is obtained in the range of ±20°, and the scattering cross-section is almost symmetrical in the negative and positive directions with respect to 0°.

[0071] Figure 22 shows the reflection characteristics of the flat panel of Example 12. As a model, two 5 mm thick polycarbonate sheets are bonded together with a conductive layer 215 in between, as shown in Figure 6B. The hollow ratio is 0%. Simulations are performed by changing the incident angle from -60° to +60° in 10° increments. The dB values ​​of the main peak of the scattering cross-section are as shown in Figure 22. Similar to the hollow panel 200A, a scattering cross-section exceeding 20 dB is obtained in the range of ±40°, and a scattering cross-section exceeding 22 dB is obtained in the range of ±20°, resulting in a scattering cross-section that is almost symmetrical on the negative and positive sides with respect to 0°.

[0072] For incident radio waves in the 3GHz to 6GHz range, there is no significant difference in the scattering cross-sectional area obtained between the hollow panel 200A and the flat panel 200. Either panel can be used depending on whether lightness and ease of handling are prioritized, or the mechanical strength of the panel is prioritized. When using the flat panel 200, the panel thickness is not limited to 5.0mm; it may be set to an appropriate thickness in the range of 1.0mm to 10.0mm, taking into consideration the panel's material, strength, transparency, etc. Similarly, the thickness of the hollow panel 200A is not limited to 5.0mm; it can be set to an appropriate thickness within the range that allows for the formation of a hollow space 213 inside.

[0073] Although the present invention has been described above based on specific embodiments, the present invention is not limited to the above-described configurations. The hollow ratio of the cross-section of the hollow panel can be appropriately set in the range of 10% to 75%, taking into consideration the strength, weight, dielectric constant, etc. of the panel. The panel size of the electromagnetic wave reflector used in the wireless transmission system is not limited to 2m x 1m, but can be appropriately selected in the range of 10cm x 10cm to 3m x 3m, as described above. When multiple panels are used in conjunction, they can be connected using the frame 201 so that the conductive layers are electrically connected between adjacent panels. When using the hollow panel 200A, the hollow 213 does not necessarily have to extend continuously from the top end to the bottom end of the hollow panel 200A. The hollow 213 may be provided in a part of the hollow panel 200A, or the hollow 213 may be provided separately in the upper half and lower half of the hollow panel 200A. The wireless transmission system of the embodiment is effectively used in facilities that include production equipment, etc., and process lines where dead zones are likely to occur. In addition to process lines, it can also be effectively used in indoor and outdoor event facilities where numerous exhibits and long lines of people tend to form.

[0074] This application claims priority based on Japanese Patent Application No. 2021-206521, filed on December 20, 2021, and includes the entire contents of that Japanese Patent Application. [Explanation of symbols]

[0075] 1. 1A~1E Wireless Transmission System 10 base station 12, 12-1, 12-2 Structure (shielding object) 14. Production equipment (target equipment) 20, 20-1, 20-2, 20A, 20B Electromagnetic wave reflector 21 Reflective surface 30 Dead Zone 200 panels 200A Hollow Panel 208, 210 Dielectric layer 211 Plate 213 Hollow 215 Conductive layer R reflection center D1 Straight-line distance from the dead zone to the reflection center of the electromagnetic wave reflector. D2: Straight-line distance from the base station antenna to the reflection center of the electromagnetic wave reflector.

Claims

1. In a wireless transmission system in which a base station located within a facility and a target device transmit and receive radio waves at a frequency selected from the frequency band between 3 GHz and below 6 GHz, The maximum gain of the base station antenna is 5 dBi or more and 30 dBi or less. When there is a dead zone within the facility where the received signal strength is 10 dB or more lower than the surrounding area, an electromagnetic wave reflecting device including one or more panels with a reflective surface size of 10 cm x 10 cm or more and 3.0 m x 3.0 m or less is placed between the base station and the dead zone, such that the center of reflection is at a height of 0.5 m or more from the floor of the facility. The sum of the straight-line distance between the base station antenna and the electromagnetic wave reflector and the straight-line distance between the electromagnetic wave reflector and the dead zone is 5.0 m or more and 300 m or less. A wireless transmission system in which the height of the reflection center of the electromagnetic wave reflector is less than or equal to the height of the receiving point of the target device.

2. The horizontal beam width of the radio waves emitted from the base station is 10° or more and 180° or less. The wireless transmission system according to claim 1.

3. The reflective surface of the electromagnetic wave reflector has at least one of a specular reflective surface that reflects the radio waves at the same angle as the incident angle and an artificial surface that reflects them at an angle different from the incident angle. The wireless transmission system according to claim 1.

4. The aforementioned facility is a production facility, and the aforementioned target equipment is production equipment. The dead zone is located along the process line within the production facility. The electromagnetic wave reflecting device is arranged along the process line. The wireless transmission system according to claim 1.

5. The panel of the electromagnetic wave reflector has a hollow interior. The wireless transmission system according to claim 1.

6. The cross-sectional hollow ratio of the aforementioned panel is 10% or more and 75% or less. The wireless transmission system according to claim 5.

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