Patent Publication Number: US-2019200357-A1

Title: Wireless communication device

Description:
BACKGROUND OF THE DISCLOSURE 
     This is a continuation of International Application No. PCT/JP2017/029929 filed on Aug. 22, 2017 which claims priority from Japanese Patent Application No. 2016-169966 filed on Aug. 31, 2016. The contents of these applications are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to wireless communication devices and, more particularly, to a wireless communication device that performs wireless communication based on a plurality of different wireless communication systems that use the same frequency band. 
     DESCRIPTION OF THE RELATED ART 
     Wireless communication devices that perform wireless communication based on a plurality of different wireless communication systems have been proposed to meet a variety of wireless communication usage (for example, see Non-patent document 1). 
     Non-patent document 1 discloses a gateway compatible with two types of wireless communication systems, Wireless LAN (Wi-Fi (Registered trademark)) and Bluetooth (Registered trademark). Such gateway allows connection with devices compatible with wireless LAN as well as devices compatible with Bluetooth (Registered trademark). 
     Non-Patent Document 1: Ishibashi, Yasuhiro and two others (2002) “Wireless LAN Broadband Gateway Series WBG-1000, 1200”, Toshiba Review, Vol. 57, No. 10, pp. 28-32. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     In the gateway disclosed in the foregoing Non-patent document 1, however, there is an issue that a plurality of different wireless communication systems that use the same frequency band cannot be used together (used simultaneously) or when used together, smooth communication cannot be obtained. In other words, in the gateway disclosed in the foregoing Non-patent document 1, smooth communication cannot be obtained when wireless communication by Wi-Fi (Registered trademark) and wireless communication by Bluetooth (Registered trademark) are used together at the same frequency band. This is because radio wave interference causes Intra-channel interference where another station&#39;s signal directly enters a reception band to cause reception errors or Inter-channel interference where distortion or the like caused by reception of out-of-band noise of another station&#39;s signal or reception of another station&#39;s signal itself causes reception errors, or often causes the carrier sense where transmission of its own signal is suppressed (losing transmission opportunity) upon detection of another station&#39;s signal. 
     In recent years, various wireless communication systems are widely used. For example, the 2.4 GHz band is used in wireless communication systems such as Wi-Fi (Registered trademark), Bluetooth (Registered trademark), ZigBee (Registered trademark), and the like. There is demand for wireless communication devices that can perform smooth wireless communication based on at least two of the wireless communication systems simultaneously, which use this 2.4 GHz band, from among the wireless communication systems such as Wi-Fi (Registered trademark), Bluetooth (Registered trademark), ZigBee (Registered trademark), and the like. In wireless communication systems such as Wi-SUN (Registered trademark), EnOcean (Registered trademark), LoRa (Registered trademark), Sigfox (Registered trademark), and the like, the 800/900 MHz band (920 MHz band in Japan) is used. There is a demand for wireless communication devices that can perform smooth wireless communication based on at least two of the wireless communication systems simultaneously, which use this frequency band, from among the wireless communication systems such as Wi-SUN (Registered trademark), EnOcean (Registered trademark), LoRa (Registered trademark), Sigfox (Registered trademark), and the like. 
     Conventional wireless communication devices cannot use wireless communication based on a plurality of different wireless communication systems together, which use the same frequency band, or cannot perform smooth wireless communication based on a plurality of wireless communication systems simultaneously. This raises an issue that the conventional wireless communication device cannot be used in many usage cases such as, for example, receiving an operation instruction from a smartphone via wireless LAN based on Wi-Fi (Registered trademark) while controlling a lighting apparatus using wireless communication based on ZigBee (Registered trademark). 
     Accordingly, an object of the present disclosure is to provide a wireless communication device capable of performing smooth wireless communication based on a plurality of different wireless communication systems simultaneously, which use the same frequency band. 
     In order to achieve the foregoing object, a wireless communication device according to one aspect of the present disclosure includes a housing; and a first board placed inside the housing, wherein a first communication circuit and a first antenna that perform wireless communication based on a first wireless communication system that uses a first frequency band and a second communication circuit and a second antenna that perform wireless communication based on a second wireless communication system that uses the first frequency band are mounted on the first board, the second wireless communication system being different from the first wireless communication system, and the first antenna and the second antenna are respectively placed in a first region and a second region that are in a diagonal spatial relationship in plan view of the first board. 
     Because of this, the first antenna and the second antenna, which are compatible with a plurality of different wireless communication systems that use the same frequency band, are respectively placed in the first region and the second region that are in a diagonal spatial relationship in plan view of the first board to be placed at locations separate from each other, thereby suppressing radio wave interference between the wireless communication using the first antenna and the wireless communication using the second antenna. As a result, it becomes possible to perform smooth wireless communication based on a plurality of different wireless communication systems simultaneously, which use the same frequency band. Because of the suppression of radio wave interference, the retransmission rate during wireless communication caused by the interference is also suppressed, thereby reducing electric power consumption associated with the retransmission. 
     Here, the first antenna and the second antenna may be arranged on the first board in such a way that principal radiation directions of the first antenna and the second antenna are orthogonal to each other. 
     Because of this, the principal radiation direction of the first antenna and the principal radiation direction of the second antenna that use the same frequency band are orthogonal to each other, and this enables further suppression of the radio wave interference between the wireless communication that uses the first antenna and the wireless communication that uses the second antenna, thereby realizing the wireless communication device having good reception. 
     A third communication circuit and a third antenna that perform wireless communication based on a third wireless communication system that uses a second frequency band, the second frequency band being different from the first frequency band, and a fourth communication circuit and a fourth antenna that perform wireless communication based on a fourth wireless communication system that uses the second frequency band, the fourth wireless communication system being different from the third wireless communication system, may be further mounted on the first board, and the third antenna and the fourth antenna may respectively be arranged in a third region and a fourth region that are in a diagonal spatial relationship in plan view of the first board, each of the third region and the fourth region being different from both the first region and the second region. 
     Because of this, as with the first antenna and the second antenna, the third antenna and the fourth antenna, which are compatible with the different wireless communication systems that use the second frequency band different from the first frequency band, respectively arranged in the third region and the fourth region that are in the diagonal spatial relationship in plan view of the first board to be placed at locations separate from each other, thereby suppressing radio wave interference between the wireless communication using the third antenna and the wireless communication using the fourth antenna. Accordingly, it becomes possible to perform smooth wireless communication based on two sets of two types of different wireless communication systems, namely, four types of wireless communication systems simultaneously, which use the same frequency band. 
     The third antenna and the fourth antenna may be arranged on the first board in such a way that principal radiation directions of the third antenna and the fourth antenna are orthogonal to each other. 
     Because of this, as with the first antenna and the second antenna, the principal radiation direction of the third antenna and the principal radiation direction of the fourth antenna that use the same frequency band are orthogonal to each other, and this enables further suppression of the radio wave interference between the wireless communication that uses the third antenna and the wireless communication that uses the fourth antenna, thereby realizing the wireless communication device having good reception. 
     Of communication circuits and antennas mounted on the first board, the first communication circuit and the first antenna may perform communication using a widest frequency band or perform most frequent communication, and of the communication circuits and the antennas mounted on the first board, the third communication circuit and the third antenna may be mounted at a location closest to the first communication circuit and the first antenna and arranged on a second plane whose height from the first board is different from that of a first plane on which the first communication circuit and the first antenna are mounted. 
     Because of this, the first communication circuit and the first antenna, which perform communication using a widest frequency band or perform most frequent communication in the communication circuits and antennas mounted on the first board, and the third communication circuit and the third antenna, which are mounted at a location closest to the first communication circuit and the first antenna, are placed at the planes of different heights. Thus, compared with a case where the first communication circuit and the first antenna and the third communication circuit and the third antenna are placed on the same plane, out-of-band radio wave interference between the wireless communication using the first communication circuit and the first antenna and the wireless communication using the third communication circuit and the third antenna is suppressed. 
     The first plane may be a principal surface of the first board, and the second plane may be a principal surface of the second board connected to the first board using a board joining connector. 
     Because of this, the first communication circuit and the first antenna and the third communication circuit and the third antenna are placed on different boards, which are connected using the board joining connector, and these boards are detachable. Thus, a combination of boards to be used together can be changed easily, thereby making it possible to flexibly meet various needs. 
     The third wireless communication system may be a wireless communication system of a type in which a frequency band to be used or a wireless communication system to be used or both are different depending on a delivery destination of the wireless communication device. 
     Because of this, the second board on which the third communication circuit and the third antenna are arranged is connected to the first board using the board joining connector. Thus, by replacing the second board with a type of board suitable in a delivery destination of the wireless communication device, the wireless communication device that meets specifications of the delivery destination can be manufactured without difficulty. 
     A shield case that houses an electric power supply circuit, a logic circuit, and at least one of the electric power supply circuit and the logic circuit therein may be further mounted on the first board; the shield case may have a contour including an interior angle larger than 180 degrees in plan view of the first board; an opening may be formed at a side surface corresponding to the interior angle of the shield case; and each of the first communication circuit, the first antenna, the second communication circuit, and the second antenna may be arranged on the first board at such a location that is out of sight when viewed outside through the opening from inside the shield case. 
     Because of this, despite the presence of the opening in the shield case introduced for downsizing and increasing the level of integration, stable communication is obtained because the first communication circuit and the first antenna and the second communication circuit and the second antenna are placed at locations where the first communication circuit and the first antenna and the second communication circuit and the second antenna are less likely to come under the influence of the electromagnetic wave noise leaked from the opening. 
     The first antenna and the second antenna may be arranged along a side of the first board in plan view of the first board; an external device connector for connecting the wireless communication device and an external device may further be mounted on the first board, the external device being placed outside the housing and performing wireless communication based on a wireless communication system that is different from both the first wireless communication system and the second wireless communication system; and the external device connector may be arranged along a side of the first board that is different from both the side of the first board along which the first antenna is arranged and the side of the first board along which the second antenna is arranged. 
     Because of this, the external device connector, which is for connecting with an external device that performs wireless communication based on a wireless communication system different from the wireless communication systems employed in the wireless communication device, is placed at the location where interference with the wireless communication using the first antenna and interference the wireless communication using the second antenna are less likely to occur, thereby enabling stable wireless communication using the external device added by use of the external device connector. 
     The present disclosure enables the realization of the wireless communication device capable of performing smooth wireless communication based on a plurality of different wireless communication systems simultaneously, which use the same frequency band. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an application example of a wireless communication device according to an embodiment. 
         FIG. 2  is a mounting layout diagram illustrating a configuration of the wireless communication device illustrated in  FIG. 1 . 
         FIG. 3  is a block diagram illustrating a connection relationship between various circuit elements illustrated in  FIG. 2 . 
         FIG. 4  is a diagram illustrating one example of a development diagram of a shield case illustrated in  FIG. 1 . 
         FIG. 5  is a mounting layout diagram illustrating a configuration of a wireless communication device according to a modified example of an embodiment. 
         FIG. 6  is a block diagram illustrating a connection relationship between various circuit elements illustrated in  FIG. 5 . 
         FIG. 7  is a diagram illustrating a mounting example of a communication circuit and an antenna in a first board having an L-shape. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the embodiments which will be described below each illustrate a specific example of the present disclosure. Numeric values, frequency bands, wireless communication systems, shapes, materials, constituting elements, arrangement locations and connection modes of the constituting elements, processing procedures, and the like are mere examples, and not intended to limit the present disclosure. Of constituting elements in the following embodiments, the constituting elements that are not described in the independent claim representing the broadest concept of the present disclosure will be described as optional constituting elements. 
       FIG. 1  is a diagram illustrating an application example of a wireless communication device  50  according to an embodiment. Here, the wireless communication device  50  is illustrated as a device that functions as a gateway that uses wireless communication based on the total of four types of wireless communication systems together and also as a device controller for controlling various devices, the four types of wireless communication systems including wireless LAN (Wi-Fi (Registered trademark)) and Bluetooth (Registered trademark) that can be selectively used and are examples of a first wireless communication system that uses the 2.4 GHz band, which corresponds to a first frequency band, ZigBee (Registered trademark) that is an example of a second wireless communication system that uses the 2.4 GHz band, which corresponds to the same first frequency band, Wi-SUN (Registered trademark) that is an example of a third wireless communication system that uses the 920 MHz band, which corresponds to a second frequency band, and EnOcean (Registered trademark) that is an example of a fourth wireless communication system that uses the 920 MHz band, which corresponds to the same second frequency band. 
     Specifically, lighting apparatuses  12   a  and  12   b  form a ZigBee (Registered trademark) mesh network, and the wireless communication device  50  functions as a coordinator thereof. Battery-operated illuminance sensors  14  are installed at many indoor locations, and their data are also aggregated at the wireless communication device  50  via the ZigBee (Registered trademark) mesh network. 
     Light switches  22   a  and  22   b  that use an energy harvesting power supply by EnOcean (Registered trademark) are installed inside a room, and the transmission data from these light switches  22   a  and  22   b  are received with the wireless communication device  50 . 
     A smart meter  32   a  and a Home Energy Management System (HEMS) controller  32   b  are connected to the wireless communication device  50  based on Wi-SUN (Registered trademark), and the wireless communication device  50  continuously monitors electric power usage and transmits demand-response information such as shortage of electric power supply or the like via B route or the like. The wireless communication device  50  performs, based on this demand-response information, an energy saving control such as turning off indoor lighting at a location where a motion sensor outputs no response. 
     A smartphone  44  and the wireless communication device  50  are connected to each other based on Bluetooth (Registered trademark), and this enables the smartphone  44  to control lighting using a lighting control application. Further, a personal computer (PC)  42  and the smartphone  44  can access a setup screen (Web control) of the wireless communication device  50  based on Wi-Fi (Registered trademark) to set the lighting control or check usage thereof. 
       FIG. 2  is a diagram illustrating the configuration of the wireless communication device  50  illustrated in  FIG. 1 . Here, a mounting layout of circuit boards (first board  52  and second board  55 ) placed inside a housing  51  included in the wireless communication device  50  is illustrated. 
     The following four sets of communication circuits and antennas are mounted on the first board  52 ; a first communication circuit  61   a  and a first antenna  61   b  that perform wireless communication based on Wi-Fi (Registered trademark) and Bluetooth (Registered trademark) that can be used selectively and both use the 2.4 GHz band; a second communication circuit  62   a  and a second antenna  62   b  that perform wireless communication based on ZigBee (Registered trademark) that uses the 2.4 GHz band; a third communication circuit  63   a  and a third antenna  63   b  that perform wireless communication based on Wi-SUN (Registered trademark) that uses the 920 MHz band; and a fourth communication circuit  64   a  and a fourth antenna  64   b  that perform wireless communication based on EnOcean (Registered trademark) that uses the 920 MHz band. Here, the first communication circuit  61   a  is a combo chip compatible with wireless communication systems of Wi-Fi (Registered trademark) and Bluetooth (Registered trademark), which use a single band of the 2.4 GHz band. The third communication circuit  63   a  and the third antenna  63   b  are placed on the second board  55 , which serves as a sub board and is connected to the first board  52 , which serves as a main board, using a board joining connector (B to B)  54 . Note that the first antenna  61   b,  the second antenna  62   b,  the third antenna  63   b,  and the fourth antenna  64   b  may be wiring patterns formed on a principal surface (namely, a surface on which circuit components are mounted) of the first board  52  or the second board  55 , or conductors formed so as to stand up from the board, or mixtures of the wiring pattern and the conductor. The first board  52  has a rectangular contour in the present embodiment. However, the shape is not limited only to such contour as described below. 
     On the first board  52 , as logic circuits, a micro-processing unit (MPU)  70 , a RAM  72 , a flash memory  73 , an ethernet (Registered trademark) IC (Ether PHY)  74 , and a real time clock (RTC)  57  are mounted, and as an electric power supply circuit, an electric power management IC (PM)  71  is mounted. Here, the MPU  70 , the electric power management IC  71 , the RAM  72 , the Flash memory  73 , and the ethernet (Registered trademark) IC  74  are placed inside the shield case  53  having a lidded structure to suppress unwanted electromagnetic wave noise toward communication circuits in the surrounding area. The details of the shield case  53  will be described below. 
     Further, on the first board  52 , as connectors of various types for connecting to an electric power supply or an external device, an electric power supply terminal (power in)  56 , an ethernet (Registered trademark) ports  58   a  and  58   b,  and a microSD (Registered trademark)  60  are mounted. 
     Note that, in the present specification, “mounted (or placed) on a board” means not only a case where a component is directly mounted (or placed) on the board, but also includes a case where a component is indirectly mounted (or placed) on the board using another component such as the board joining connector  54  or the like. 
       FIG. 3  is a block diagram illustrating a connection relationship between various circuit elements mounted on the first board  52  and the second board  55  illustrated in  FIG. 2 . As illustrated in the drawing, the first communication circuit  61   a,  the second communication circuit  62   a,  the fourth communication circuit  64   a,  the ethernet (Registered trademark) ports  58   a  and  58   b,  the RAM  72 , the flash memory  73 , the real time clock  57 , the electric power management IC  71 , and the microSD (Registered trademark)  60  are connected to the MPU  70 , and the third communication circuit  63   a  is connected to the MPU  70  using the board joining connector  54 . 
     The electric power management IC  71  receives an externally-supplied DC electric power via the electric power supply terminal  56 , generates DC voltages of various types required in the wireless communication device  50  at appropriate timings, and supplies the generated DC voltages to circuit elements of various types. The ethernet (Registered trademark) ports  58   a  and  58   b  are used as WAN ports for connecting with the internet outside or LAN ports forming a home network, or are used as LAN HUB. 
     The MPU  70  includes a ROM in which a control program is stored, a processor that executes the control program, and the like. When the processor executes the control program, the processor sends and receives data and control signals to and from circuit elements of various types connected to the MPU  70 , thereby making the wireless communication device  50  function as the gateway and the device controller illustrated in  FIG. 1 . 
     The wireless communication device  50  in the present embodiment configured as described above has the following characteristic features. 
     The first antenna  61   b  for Wi-Fi (Registered trademark) and Bluetooth (Registered trademark) that use the 2.4 GHz band and the the second antenna  62   b  for ZigBee (Registered trademark) that uses the same 2.4 GHz are placed in a first region and a second region of the first board  52 , respectively, wherein the first region and the second region are in a diagonal spatial relationship in plan view of the first board  52 . In the present embodiment, as illustrated in  FIG. 2 , the first antenna  61   b  is placed in an upper right region (first region) of the first board  52  whereas the second antenna  62   b  is placed in a lower left region (second region) of the first board  52 . 
     Because of this, the first antenna  61   b  and the second antenna  62   b,  which are compatible with a plurality of different wireless communication systems that use the same frequency band, are placed at locations separate from each other. Accordingly, radio wave interference is suppressed between the wireless communication by Wi-Fi (Registered trademark) and Bluetooth (Registered trademark) that use the first antenna  61   b  and the wireless communication by ZigBee (Registered trademark) that uses the second antenna  62   b.  As a result, it becomes possible to perform smooth wireless communication based on the plurality of different wireless communication systems (Wi-Fi (Registered trademark) and Bluetooth (Registered trademark) and ZigBee (Registered trademark)) simultaneously, which use the same frequency band. Because of the suppression of the radio wave interference, the first board  52  can be made smaller in size compared with that of the prior art, thereby making it possible to make the wireless communication device  50  smaller in size and higher in integration level. Further, because of the suppression of the radio wave interference, the retransmission rate during wireless communication caused by the interference is also suppressed, thereby reducing the electric power consumption associated with the retransmission. 
     The same thing as with the first antenna  61   b  and the second antenna  62   b  can be said with the third antenna  63   b  and the fourth antenna  64   b.  In other words, the third antenna  63   b  for Wi-SUN (Registered trademark) that uses the 920 MHz band and the the fourth antenna  64   b  for EnOcean (Registered trademark) that uses the same 920 MHz are placed in a third region and a fourth region, respectively, wherein the third region and the fourth region are each different from both the first region and the second region and are in the diagonal spatial relationship in plan view of the first board  52 . In the present embodiment, as illustrated in  FIG. 2 , the third antenna  63   b  is placed in a lower right region (third region) of the first board  52  whereas the fourth antenna  64   b  is placed in an upper left region (fourth region) of the first board  52 . 
     Because of this, the third antenna  63   b  and the fourth antenna  64   b,  which are compatible with a plurality of different wireless communication systems that use the same frequency band, are placed at separate locations from each other, as with the first antenna  61   b  and the second antenna  62   b.  Accordingly, radio wave interference is suppressed between the wireless communication by Wi-SUN (Registered trademark) that uses the third antenna  63   b  and the wireless communication by EnOcean (Registered trademark) that uses the fourth antenna  64   b.  As a result, it becomes possible to perform smooth wireless communication based on the plurality of different wireless communication systems (Wi-SUN (Registered trademark) and EnOcean (Registered trademark)) simultaneously, which use the same frequency band. Because of the suppression of the radio wave interference, the first board  52  can be made smaller in size compared with that of the prior art, thereby making it possible to make the wireless communication device  50  smaller in size and higher in integration level. Further, because of the suppression of the radio wave interference, the retransmission rate during wireless communication caused by the interference is also suppressed, thereby reducing the electric power consumption associated with the retransmission. 
     Note that in the present specification “two regions that are in a diagonal spatial relationship” may be, in plan view of the first board  52 , regions within a predetermined range (for example, a distance corresponding to 10% of the diagonal line) from respective ones of two vertices of the first board  52  that are in the diagonal spatial relationship, or may means that, of the antennas mounted on the first board  52 , antennas are placed at locations closest to respective ones of these two vertices, or may be regions along the sides of the first board  52  that are connected to respective ones of these two vertices. 
     The first antenna  61   b  for Wi-Fi (Registered trademark) and Bluetooth (Registered trademark) that use the 2.4 GHz band and the the second antenna  62   b  for ZigBee (Registered trademark) that uses the same 2.4 GHz are arranged on the first board  52  in such a way that principal radiation directions thereof are orthogonal to each other. In the present embodiment, as illustrated in  FIG. 2 , the first antenna  61   b  is placed in such a manner as to extend along the right-hand side of the first board  52  having a rectangle contour whereas the second antenna  62   b  is placed in such a manner as to extend along the bottom side of the first board  52 . As a result, the principal radiation directions of the first antenna  61   b  and the second antenna  62   b  are orthogonal to each other. 
     Because of this, the principal radiation direction of the first antenna  61   b  and the principal radiation direction of the second antenna  62   b  that use the same frequency band are orthogonal to each other, and the radio wave interference is further suppressed between the wireless communication by Wi-Fi (Registered trademark) and Bluetooth (Registered trademark) that use the first antenna  61   b  and the wireless communication by ZigBee (Registered trademark) that uses the second antenna  62   b,  thereby realizing the wireless communication device  50  having good reception. 
     The same thing as with the first antenna  61   b  and the second antenna  62   b  can be said with the third antenna  63 b and the fourth antenna  64   b.  In other words, the third antenna  63   b  for Wi-SUN (Registered trademark) that uses the 920 MHz band and the the fourth antenna  64   b  for EnOcean (Registered trademark) that uses the same 920 MHz are arranged on the first board  52  in such a way that principal radiation directions thereof are orthogonal to each other. In the present embodiment, as illustrated in  FIG. 2 , the third antenna  63   b  is placed in such a manner as to extend along the bottom side of the first board  52  having a rectangle contour whereas the fourth antenna  64   b  is placed in such a manner as to extend along the left-hand side of the first board  52 . As a result, the principal radiation directions of the third antenna  63   b  and the fourth antenna  64   b  are orthogonal to each other. 
     Because of this, as with the first antenna  61   b  and the second antenna  62   b,  the principal radiation direction of the third antenna  63   b  and the principal radiation direction of the fourth antenna  64   b  that use the same frequency band are orthogonal to each other, and the radio wave interference is further suppressed between the wireless communication by Wi-SUN (Registered trademark) that uses the third antenna  63 b and the wireless communication by EnOcean (Registered trademark) that uses the fourth antenna  64   b,  thereby realizing the wireless communication device  50  having good reception. 
     Note that, in the present specification, “orthogonal” refers to a state where directions are substantively orthogonal to the extent that the suppression of radio wave interference is effective, and also includes a state where directions cross at an angle of 80 to 100 degrees, for example. 
     A communication circuit and an antenna (in this case, the third communication circuit  63   a  and the third antenna  63   b  that perform wireless communication by Wi-SUN (Registered trademark) that uses the 920 MHz band) mounted at a location closest to the the first communication circuit  61   a  and the first antenna  61   b  are placed on a second plane (in this case, the principal surface of the second board  55 ) whose height from the first board  52  is different from that of a first plane (in this case, the principal surface of the first board  52 ) on which the first communication circuit  61   a  and the first antenna  61   b  are mounted, wherein the first communication circuit  61   a  and the first antenna  61   b  perform wireless communication by Wi-Fi (Registered trademark), which use the widest frequency band in the communication circuits and antennas mounted on the first board  52  or perform most frequent wireless communication in the communication circuits and antennas mounted on the first board  52 . In other words, the third communication circuit  63   a  and the third antenna  63   b  that perform wireless communication by Wi-SUN (Registered trademark) that uses the 920 MHz band are not on-board mounted on the first board  52 , but are mounted as a built-in antenna module (module including the second board  55 ) to be connected to the first board  52  using the board joining connector  54 . 
     Wi-Fi (Registered trademark) and Wi-SUN (Registered trademark) use different frequency bands. However, since communication by Wi-Fi (Registered trademark) uses large bandwidth and is performed highly frequently, there is a possibility of causing out-of-band interference at a communication circuit and an antenna placed close to the first communication circuit  61   a  and the first antenna  61   b  that perform wireless communication by Wi-Fi (Registered trademark) on the first board  52 . Accordingly, in the present embodiment, the first communication circuit  61   a  and the first antenna  61   b  and the third communication circuit  63   a  and the third antenna  63   b,  which are mounted at a location closest to the first communication circuit  61   a  and the first antenna  61   b,  are placed at the planes of different heights. Accordingly, compared with a case where the first communication circuit  61   a  and the first antenna  61   b  and the third communication circuit  63   a  and the third antenna  63   b  are placed on the same plane, out-of-band radio wave interference is suppressed between the wireless communication using the first communication circuit  61   a  and the first antenna  61   b  (namely, wireless communication by Wi-Fi (Registered trademark) and Bluetooth (Registered trademark) that use the 2.4 GHz band) and the wireless communication using the third communication circuit  63   a  and the third antenna  63   b  (namely, wireless communication by Wi-SUN (Registered trademark) that uses the 920 MHz band). 
     The first communication circuit  61   a  and the first antenna  61   b  and the third communication circuit  63   a  and the third antenna  63   b  are placed on different boards, which are connected using the board joining connector  54 , and these boards are detachable. Thus, a combination of boards to be used together can be changed easily, thereby making it possible to flexibly meet various needs. 
     On the first board  52 , the shield case  53  that houses at least one of the electric power supply circuit and the logic circuits therein is mounted. The shield case  53  has a contour including interior angles  53   a  and  53   b  larger than 180 degrees in plan view of the first board  52 , and openings  53   c  and  53   d  are formed at side surfaces corresponding to the interior angles  53   a  and  53   b  of the shield case  53 , respectively. The first communication circuit  61   a  and the first antenna  61   b,  the second communication circuit  62   a  and the second antenna  62   b,  the third communication circuit  63   a  and the third antenna  63   b,  and the fourth communication circuit  64   a  and the fourth antenna  64   b  are each placed on the first board  52  at such a location that is out of sight when viewed outside through the opening  53   c  or  53   d  from inside the shield case  53 . Here, “out of sight” when viewed outside from inside the shield case  53  means the spatial relationship such that the communication circuits and the antennas do not receive direct radiation of electromagnetic wave noise leaked from the opening  53   c  or  53   d,  and also includes cases where other circuit components are present between the openings  53   c  and  53   d  and the communication circuits and the antennas. 
     Specifically, the shield case  53  is molded from a solid plate of nickel silver or the like by use of inexpensive punching and bending processing, as illustrated in a development diagram illustrated in  FIG. 4 . The shield case  53  is fixed by soldering after inserting its nail portions into through holes formed in the first board  52  while covering the logic circuits such as the MPU  70 , the electric power management IC  71 , the RAM  72 , the Flash memory  73 , and the ethernet (Registered trademark) IC  74 . Here, the shield case  53  has a protruded plane shape (namely, the interior angles  53   a  and  53   b  larger than 180 degrees are present in the contour in plan view), and therefore, partial gaps (openings  53   c  and  53   d ) are formed at the side surfaces of inner side corners (see  FIG. 2 ). In the present embodiment, the openings  53   c  and  53   d  are directed to (open toward) the electric power supply terminal  56  and the real time clock  57 . This suppresses, at each of four types of communication circuits and antennas, the direct radiation of the electromagnetic wave noise leaked from the openings  53   c  and  53   d.    
     Because of this, despite the presence of the openings in the shield case  53  introduced for downsizing and increasing the level of integration, stable communication is obtained because all the communication circuits and the antennas are placed at locations where the communication circuit and the antenna are less likely to come under the influence of the electromagnetic wave noise leaked from the openings. 
     MODIFIED EXAMPLE 
     Next, a wireless communication device  50   a  according to a modified example of the foregoing embodiment is described. 
       FIG. 5  is a diagram illustrating a configuration of the wireless communication device  50   a  according to the modified example of the embodiment. Here, the mounting layout of circuit boards (first board  52   a  and second board  55   a ) placed inside the housing  51  of the wireless communication device  50   a  is illustrated.  FIG. 6  is a block diagram illustrating a connection relationship between various circuit elements mounted on the first board  52   a  and the second board  55   a  illustrated in  FIG. 5 . 
     In the present modified example, unlike the foregoing embodiment, a USB port  59  is mounted on the first board  52   a,  and on the second board  55   a,  a different third communication circuit  65   a  and third antenna  65   b  compatible with wireless communication system LoRa (Registered trademark) are mounted in place of Wi-SUN (Registered trademark), which serves as the third wireless communication system in the foregoing embodiment. In the following description, constituting elements identical to those in the foregoing embodiment are given the same reference numerals, and their descriptions are omitted. The descriptions regarding constituting elements different from the foregoing embodiment are provided below. 
     The USB port  59  is placed outside the housing  51  and is one example of an external device connector for connecting the wireless communication device  50   a  and an external device (namely, a communication circuit and an antenna) that performs wireless communication based on a wireless communication system different from any of the wireless communication systems (first wireless communication system (Wi-Fi (Registered trademark) and Bluetooth (Registered trademark)), second wireless communication system (ZigBee (Registered trademark)), third wireless communication system (LoRa (Registered trademark)), and fourth wireless communication system (EnOcean (Registered trademark))) used by the wireless communication device  50   a.  External devices to be connected to the USB port  59  include a 3G/LTE dongle, for example. Because of this, it becomes possible to perform wireless communication based on a newly added wireless communication system that is different from the wireless communication systems employed in the wireless communication device  50   a.    
     Here, as illustrated in  FIG. 5 , the USB port  59  is placed along a side of the first board  52   a  (namely, the upper side) that is different from any sides of the first board  52   a  along which the antennas included in the wireless communication device  50   a  are placed (namely, the right-hand side of the first board  52   a  along which the first antenna  61   b  is placed, the bottom side of the first board  52   a  along which the second antenna  62   b  is placed, the bottom side of the first board  52   a  along which the third antenna  65   b  is placed, and the left-hand side of the first board  52   a  along which the fourth antenna  64   b  is placed). 
     Because of this, the USB port  59 , which is for connecting with an external device (for example, 3G/LTE dongle) that performs wireless communication based on the wireless communication system that is different from the wireless communication systems employed in the wireless communication device  50   a,  is placed at the location where interference with wireless communication using any of the four antennas included in the wireless communication device  50   a  is less likely to occur, thereby enabling stable wireless communication by the external device that is added using the USB port  59 . 
     Note that the communication circuit and the antenna (external devices) to be added are not limited to a dongle by USB and may be a card or the like, which is connected to its own unique connector, Secure Digital Input/Output (SDIO), or the like. 
     On the second board  55   a  of the present modified example, the third communication circuit  65   a  and the third antenna  65   b  that perform wireless communication based on LoRa (Registered trademark), which is another example of the third wireless communication system that uses the 920 MHz band, are mounted so as to be compatible in the delivery destination where LoRa (Registered trademark) is employed as a wireless communication system in smart grid. 
     In this way, the second board  55   a  on which the third communication circuit  65   a  and the third antenna  65   b  are placed is connected to the first board  52   a  using the board joining connector  54 . Thus, by replacing the second board  55   a  with a type of board suitable in the delivery destination of the wireless communication device  50   a,  the wireless communication device  50   a  that meets the specifications of the delivery destination can be manufactured without difficulty. 
     Note that, depending on the delivery destination of the wireless communication device  50   a,  a communication circuit and an antenna that perform wireless communication based on Sigfox (Registered trademark), which is another example of the third wireless communication system that uses the 800/900 MHz band, may be mounted as the communication circuit and the antenna mounted on the second board  55   a.    
     Thus far, the wireless communication devices of the present disclosure are described based on the embodiments and modified examples. However, the present disclosure is not limited to these embodiments and the modified examples. Embodiments obtained by applying various modifications apparent to those skilled in the art to the embodiments and the modified examples and different embodiments formed by combining parts of constituting elements of the embodiments and the modified examples are included in the scope of the present disclosure so long as they do not depart from the gist of the present disclosure. 
     For example, in the embodiments and the modified examples described above, a plurality of different wireless communication systems are employed for each of two frequency bands (2.4 GHz band and 800/900 MHz band). However, embodiments are not limited to such combinations and only need to employ a plurality of different wireless communication systems in at least one of frequency bands. For example, two different wireless communication systems that use the 2.4 GHz band may be employed, or two different wireless communication systems that use the 2.4 GHz band and one wireless communication system that uses the 800/900 MHz band may be employed. 
     Further, the frequency bands to be used in wireless communication are not limited to the 2.4 GHz band and the 800/900 MHz band, and a different frequency band such as the 5 GHz band or the like may also be used. For example, the first communication circuit  61   a  and the first antenna  61   b  may be compatible with Wi-Fi (Registered trademark) that uses the 2.4 GHz band and the 5 GHz band and Bluetooth (Registered trademark) that uses the 2.4 GHz band. 
     In the embodiments and the modified examples described above, the shapes (plane shape) of the first boards  52  and  52   a  are rectangular. However, the shapes of the first boards  52  and  52   a  are not limited to such shape and may be various types of polygon such as a circle, an oval, a hexagon, and the like, an L-shape, a recessed shape, a protruded shape, a shape obtained by clipping part of these, or various types of different shapes obtained by combining these. 
       FIG. 7  is a diagram illustrating a mounting example of a communication circuit and an antenna in a first board  52   b  having an L-shape. In this drawing, for the sake of simplicity of illustration, only mounting locations of four sets of communication circuits and antennas (first communication circuit  61   a  and first antenna  61   b,  second communication circuit  62   a  and second antenna  62   b,  third communication circuit  63   a  and third antenna  63   b,  fourth communication circuit  64   a  and fourth antenna  64   b ) are illustrated. 
     Even in this case, it can be said that the first antenna  61   b  and the second antenna  62   b  are respectively placed in the first region and the second region that are in the diagonal spatial relationship in plan view of the first board  52   b.  This is because the first antenna  61   b  and the second antenna  62   b  are placed in the regions that are in the diagonal spatial relationship in a circumscribed rectangular region  82  of this L-shaped first board  52   b.    
     It can be said that the third antenna  63   b  and the fourth antenna  64   b  are also respectively placed in the third region and the fourth region that are in the diagonal spatial relationship in plan view of the first board  52   b.  This is because the third antenna  63   b  and the fourth antenna  64   b  are placed in the regions that are in the diagonal spatial relationship in one (rectangular region  81   b ) of two rectangular regions  81   a  and  81   b  obtained by dividing this L-shaped first board  52   b  with a partition line  80 . 
     In this way, “regions that are in the diagonal spatial relationship in plan view of the first board” include not only the regions that are in the diagonal spatial relationship when the first board is a rectangular shape, but also include the regions that are in the diagonal spatial relationship in the circumscribed rectangular region of the first board, and the regions that are in the diagonal spatial relationship in the rectangular region obtained by dividing the first board into a plurality of rectangular regions. 
     Note that, according to the first board  52   b  illustrated in  FIG. 7 , the first communication circuit  61   a  and the first antenna  61   b  are mounted on the rectangular region  81   a  that is different from the rectangular region  81   b  on which other three types of the communication circuits and the antennas are mounted, wherein the first communication circuit  61   a  and the first antenna  61   b  perform wireless communication by Wi-Fi (Registered trademark), which use the widest frequency band in the communication circuits and antennas mounted on the first board  52   b  or perform most frequent communication in the communication circuits and antennas mounted on the first board  52   b.  Because of this, in-band interference and out-of-band interference are suppressed between the first communication circuit  61   a  and the first antenna  61   b,  which perform wireless communication using large bandwidth or perform wireless communication highly frequently, and the other three types of communication circuits and antennas. 
     The present disclosure can be used as wireless communication devices or more specifically as devices that perform wireless communication based on a plurality of different wireless communication systems that use the same frequency band, and, for example, as a gateway or a device controller that controls devices by performing wireless communication with various devices such as a lighting apparatus, an illuminance sensor, a light switch, a smart meter, a smartphone, a PC, and the like. 
       12   a,    12   b  Lighting apparatus 
       14  Illuminance sensor 
       22   a,    22   b  Light switch 
       32   a  Smart meter 
       32   b  HEMS controller 
       42  Personal computer (PC) 
       44  Smartphone 
       50 ,  50   a  Wireless communication device 
       51  Housing 
       52 ,  52   a,    52   b  First board 
     Shield case 
       53   a,    53   b  Interior angle larger than 180 degrees 
       53   c,    53   d  Opening 
     Board joining connector 
       55 ,  55   a  Second board 
       56  Electric power supply terminal 
       57  Real time clock 
       58   a,    58   b  Ethernet (Registered trademark) port 
       59  USB port 
       60  microSD (Registered trademark) 
       61   a  First communication circuit 
       61   b  First antenna 
       62   a  Second communication circuit 
       62   b  Second antenna 
       63 a,  65   a  Third communication circuit 
       63   b,    65   b  Third antenna 
       64   a  Fourth communication circuit 
       64   b  Fourth antenna 
       70  MPU 
       71  Electric power management IC 
       72  RAM 
       73  Flash memory 
       74  Ethernet (Registered trademark) IC 
       80  Partition line 
       81   a,    81   b  Rectangular region 
     Circumscribed rectangular region