Patent Publication Number: US-2015065051-A1

Title: Two-way communication system

Description:
CLAIM OF PRIORITY 
     This application claims benefit of priority to Japanese Patent Application No. 2013-181258 filed on Sep. 2, 2013, which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to a two-way communication system, and, in particular, relates to a two-way communication system including a plurality of in-vehicle communication units and a plurality of portable units. 
     2. Description of the Related Art 
     Recently, a two-way communication system that includes an in-vehicle unit installed in a vehicle and a portable unit capable of performing two-way communication with the in-vehicle unit, and is capable of performing an operation on the vehicle (hereinafter, abbreviated to a vehicle operation) using the two-way communication between the in-vehicle unit and the portable unit has been put to practical use. 
     In addition, recently, such a two-way communication system has been further developed, and a two-way communication system to which a function is added has been desired, examples of the function including stabilizing communication using a space diversity effect while causing a plurality of in-vehicle communication units and one portable unit to be communication-connected to each other, and simultaneously performing a plurality of vehicle operations while causing a plurality of in-vehicle communication units and a plurality of portable units to be communication-connected to each other on a one-to-one basis. As a technique relating to such a two-way communication system, an in-vehicle device according to Japanese Unexamined Patent Application Publication No. 2008-42577, and so forth have been proposed. In Japanese Unexamined Patent Application Publication No. 2008-42577, a method for communication connections in two-way communication between one in-vehicle device  200  (in-vehicle unit) and a plurality of mobile phones M (portable units) is illustrated.  FIG. 8  is an explanatory diagram illustrating the in-vehicle device  200  and the mobile phones M according to Japanese Unexamined Patent Application Publication No. 2008-42577. 
     As illustrated in  FIG. 8 , the in-vehicle device  200  is installed in a vehicle V. In addition, the in-vehicle device  200  includes a Bluetooth (registered trademark) communication unit  201  (in-vehicle communication unit), and a directional antenna  202 . The mobile phones M each include a Bluetooth communication unit  203 . The Bluetooth communication unit  201  and the Bluetooth communication units  203  are communication devices compatible with a short distance wireless communication standard called Bluetooth (registered trademark). The directional antenna  202  is set so the communication area of the Bluetooth communication unit  201  covers the driver seat area of the vehicle V. 
     By comparing received signal strength (received electric field strength) in communication between the in-vehicle device  200  and the mobile phones M with one another, the in-vehicle device  200  automatically selects one of the mobile phones M, considered to be most likely to be a driver&#39;s mobile phone M, from among the plural mobile phones M. In addition, the in-vehicle device  200  automatically performs a communication connection with the selected mobile phones M. In this way, the in-vehicle device  200  performs a communication connection by automatically selecting one mobile phone M from among the plural mobile phones M, and performs two-way communication with the communication-connected mobile phone M. 
     In addition, while Japanese Unexamined Patent Application Publication No. 2008-42577 is a technique relating to the two-way communication between the one in-vehicle communication unit and the plural portable units, it becomes possible to stabilize communication using the space diversity effect, by utilizing a plurality of such in-vehicle communication units and causing the in-vehicle communication units to be communication-connected to one portable unit out of the plural portable units. In addition, by utilizing a plurality of such in-vehicle communication units and causing the in-vehicle communication units to be communication-connected to a plurality of portable units on a one-to-one basis, it becomes possible to simultaneously perform a plurality of vehicle operations. 
     However, while, in such a method for a communication connection as illustrated in Japanese Unexamined Patent Application Publication No. 2008-42577, it is possible for the in-vehicle communication unit to select one portable unit from among the plural portable units and perform a communication connection therewith, the communication connection between the in-vehicle communication unit and the portable unit is not switched after the communication connection is established. Therefore, in order to stabilize communication using the space diversity effect while causing the plural in-vehicle communication units and one of the portable units to be communication-connected to each other and to simultaneously perform a plurality of vehicle operations while causing the plural in-vehicle communication units and the plural portable units to be communication-connected to each other on a one-to-one basis, the plural in-vehicle communication units corresponding to respective intended purposes become necessary. For example, in a case where two in-vehicle communication units are used in order to obtain the space diversity effect and two in-vehicle communication units are used in order to simultaneously perform the plural vehicle operations, four in-vehicle communication units become necessary. As a result, there is a problem that the number of in-vehicle communication units increases and the configuration of a system becomes complex. 
     SUMMARY 
     A two-way communication system according to an aspect of the invention includes an in-vehicle unit installed in a vehicle, and a plurality of portable units capable of performing two-way communication with the in-vehicle unit. The in-vehicle unit includes a plurality of in-vehicle communication units configured to perform two-way communication with the portable units, and an in-vehicle control unit configured to control communication connections between the in-vehicle communication units and the portable units. The in-vehicle unit or the portable units each include a received electric field strength detection unit configured to detect received electric field strength in the two-way communication between the in-vehicle unit and the portable units, and a received electric field strength determination unit configured to perform determination, based on the received electric field strength. In a case where the received electric field strength is less than a predetermined value, the plural in-vehicle communication units and one of the portable units are communication-connected to each other, and in a case where the received electric field strength is greater than or equal to the predetermined value, the plural in-vehicle communication units and the plural portable units are communication-connected to each other on a one-to-one basis. 
     The two-way communication system of this configuration includes the in-vehicle unit installed in the vehicle, and the plural portable units capable of performing two-way communication with the in-vehicle unit. In addition, the in-vehicle unit includes the plural in-vehicle communication units configured to perform two-way communication with the portable units, and the in-vehicle control unit configured to control communication connections between the in-vehicle communication units and the portable units. Therefore, in the two-way communication system, it is possible to perform selective communication connections between the plural in-vehicle communication units and the plural portable units. In addition, it is possible to stabilize communication using the space diversity effect while causing the plural in-vehicle communication units and one of the portable units to be communication-connected to each other, and it is possible to simultaneously perform a plurality of vehicle operations while causing the plural in-vehicle communication units and the plural portable units to be communication-connected to each other on a one-to-one basis. 
     Furthermore, in the two-way communication system of this configuration, the in-vehicle unit or the portable units each include the received electric field strength detection unit configured to detect received electric field strength in the two-way communication between the in-vehicle unit and the portable units, and the received electric field strength determination unit configured to perform determination, based on the received electric field strength. In addition, based on the received electric field strength, communication connections between the plural in-vehicle communication units and the plural portable units are switched, and the plural in-vehicle communication units and one of the portable units are communication-connected to each other, or the plural in-vehicle communication units and the plural portable units are communication-connected to each other on a one-to-one basis. Therefore, it is possible to cause the in-vehicle communication units for obtaining the space diversity effect to double as the in-vehicle communication units for performing the two-way communication with the plural portable units on a one-to-one basis. As a result, it is possible to reduce the number of the in-vehicle communication units and to simplify the configuration of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are configuration diagrams illustrating a configuration of a two-way communication system according to a first embodiment of the present invention; 
         FIG. 2  is an explanatory diagram illustrating an example of use of the two-way communication system illustrated in  FIGS. 1A and 1B ; 
         FIG. 3  is a flowchart illustrating a procedure of a communication connection according to the first embodiment of the present invention; 
         FIGS. 4A and 4B  are first explanatory diagrams illustrating specific examples of communication connections according to the first embodiment of the present invention; 
         FIGS. 5A and 5B  are second explanatory diagrams illustrating specific examples of communication connections according to the first embodiment of the present invention; 
         FIG. 6  is a flowchart illustrating a procedure of a vehicle operation according to the first embodiment of the present invention; 
         FIGS. 7A and 7B  are configuration diagrams illustrating a configuration of a two-way communication system according to a second embodiment of the present invention; and 
         FIG. 8  is an explanatory diagram illustrating an in-vehicle device and mobile phones according to Japanese Unexamined Patent Application Publication No. 2008-42577. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Hereinafter, a first embodiment of the present invention will be described with reference to drawings. In addition, in the present embodiment, in order to easily understand the description, a two-way communication system including two in-vehicle communication units and two portable units will be described. 
     First, the configuration of a two-way communication system  1  according to the first embodiment of the present invention will be described with reference to  FIGS. 1A and 1B  and  FIG. 2 .  FIGS. 1A and 1B  are configuration diagrams illustrating the configuration of the two-way communication system  1  according to the first embodiment of the present invention.  FIG. 1A  is a configuration diagram illustrating the entire configuration, and  FIG. 1B  is a configuration diagram illustrating the configuration of each of portable units  20  illustrated in  FIG. 1A .  FIG. 2  is an explanatory diagram illustrating an example of use of the two-way communication system  1  illustrated in  FIGS. 1A and 1B . 
     As illustrated in  FIGS. 1A and 1B , the two-way communication system  1  includes an in-vehicle unit  10  and the two portable units  20 . The in-vehicle unit  10  and the portable units  20  are able to perform two-way communication with each other. As illustrated in  FIG. 2 , the in-vehicle unit  10  is installed in a predetermined position in a vehicle  30 . Two users  40  take along the respective two portable units  20 . 
     Hereinafter, it is assumed that one of the two portable units  20  is a first portable unit  20   a  and the other thereof is a second portable unit  20   b . In addition, it is assumed that one of the two users  40  is a first user  40   a  (driver) and the other thereof is a second user  40   b  (fellow passenger). In addition, it is assumed that the first user  40   a  drives the vehicle  30  while taking along the first portable unit  20   a  and the second user  40   b  rides in the vehicle  30  with the first user  40   a  while taking along the second portable unit  20   b.    
     Next, the in-vehicle unit  10  will be described. As illustrated in  FIGS. 1A and 1B , the in-vehicle unit  10  includes an in-vehicle control unit  11 , two in-vehicle communication units  12 , two received electric field strength detection units  13 , and a received electric field strength determination unit  14 . Hereinafter, it is assumed that one of the two in-vehicle communication units  12  is a first in-vehicle communication unit  12   a  and the other thereof is a second in-vehicle communication unit  12   b . While not illustrated, the first in-vehicle communication unit  12   a  and the second in-vehicle communication unit  12   b  are spaced at a predetermined interval within the vehicle  30 . 
     The in-vehicle communication unit  12  is a communication device compatible with a short distance wireless communication standard such as Bluetooth (registered trademark). The in-vehicle communication unit  12  is able to perform two-way communication utilizing an electromagnetic wave signal for short distance wireless communication. In addition, the electromagnetic wave signal for the short distance wireless communication is a weak electromagnetic wave signal, and a distance the electromagnetic wave signal reaches (hereinafter, abbreviated to a communication distance) is limited to about several meters. 
     The received electric field strength detection units  13  are embedded in the respective two in-vehicle communication units  12 , and detect the received electric field strengths of electromagnetic wave signals received by the in-vehicle communication units  12  when the in-vehicle unit  10  and the portable units  20  perform two-way communication with each other. Based on the received electric field strengths detected by the received electric field strength detection units  13 , the received electric field strength determination unit  14  determines whether or not the received electric field strengths are greater than or equal to a predetermined value. 
     The in-vehicle control unit  11  controls the in-vehicle communication units  12 , the received electric field strength detection units  13 , and the received electric field strength determination unit  14 . In addition, the in-vehicle control unit  11  acquires various kinds of information from the in-vehicle communication units  12 , the received electric field strength detection units  13 , and the received electric field strength determination unit  14 , and performs various kinds of determination, based on the acquired information. In addition, the in-vehicle control unit  11  is connected to various kinds of in-vehicle devices not illustrated, and issues an instruction relating to a vehicle operation, and acquires vehicle information. 
     Next, the portable units  20  will be described. As the portable units  20 , personal digital assistants called smartphones are used. In addition, as illustrated in  FIGS. 1A and 1B , the portable units  20  each include a control unit  21 , a communication unit  22 , an input unit  23 , a display unit  24 , and an arithmetic unit  25 . 
     The communication unit  22  is a communication device embedded in the personal digital assistant and compatible with the short distance wireless communication standard such as Bluetooth. The communication unit  22  is able to perform two-way communication utilizing an electromagnetic wave signal for the short distance wireless communication. In addition, the electromagnetic wave signal for the short distance wireless communication is a weak electromagnetic wave signal, and a communication distance is limited to about several meters. 
     The input unit  23  detects an input operation by the user  40 . As the input unit  23 , an input device called a touch panel, or the like is used. The display unit  24  displays various kinds of information. As the display unit  24 , a display device such as a liquid crystal panel is used. The arithmetic unit  25  performs various kinds of arithmetic operations. The control unit  21  controls the communication unit  22 , the input unit  23 , the display unit  24 , and the arithmetic unit  25 . 
     In addition, while not illustrated, the portable unit  20  further includes a communication unit other than the communication unit  22 , which is able to perform wireless communication with a communication network, a rechargeable battery, a charging unit for charging the battery, and so forth. 
     Next, the two-way communication between the in-vehicle unit  10  and one of the portable units  20  will be described. In the two-way communication system  1 , the two-way communication is able to be performed between the in-vehicle unit  10  and the portable unit  20 . The two-way communication between the in-vehicle unit  10  and the portable unit  20  is performed using one of the in-vehicle communication units  12  in the in-vehicle unit  10  and the communication unit  22  in the portable unit  20 . When the two-way communication between the in-vehicle unit  10  and the portable unit  20  is performed, a communication connection is performed between one of the in-vehicle communication units  12  in the in-vehicle unit  10  and the communication unit  22  in the portable unit  20 . 
     Hereinafter, the two-way communication between one of the in-vehicle communication units  12  in the in-vehicle unit  10  and the communication unit  22  in the portable unit  20  is abbreviated to two-way communication between the in-vehicle communication unit  12  and the portable unit  20 . In addition, the communication connection between one of the in-vehicle communication units  12  in the in-vehicle unit  10  and the communication unit  22  in the portable unit  20  is abbreviated to a communication connection between the in-vehicle communication unit  12  and the portable unit  20 . 
     Next, the procedure of the communication connection between one of the in-vehicle communication units  12  and one of the portable units  20  will be described with reference to  FIG. 3 .  FIG. 3  is a flowchart illustrating the procedure of a communication connection according to the first embodiment of the present invention. 
     It is desirable that the in-vehicle communication unit  12  is able to switch between a detection mode and a non-detection mode. In addition, in the detection mode, the in-vehicle communication unit  12  periodically transmits a notification signal informing that a communication connection is available, and in the non-detection mode, the in-vehicle communication unit  12  does not transmit the notification signal. By receiving the notification signal, the portable unit  20  detects the in-vehicle communication unit  12  with which a communication connection is able to be established. Therefore, in the detection mode, the in-vehicle communication unit  12  may be put into a state in which the portable unit  20  is able to detect the in-vehicle communication unit  12 , and in the non-detection mode, the in-vehicle communication unit  12  may be put into a state in which the portable unit  20  is not able to detect the in-vehicle communication unit  12 . 
     When the in-vehicle communication unit  12  and the portable unit  20  perform a communication connection with each other, first the in-vehicle communication unit  12  serving as a target of a communication connection may be put into the detection mode, as illustrated in  FIG. 3  (step Sa 1 ). In addition, the in-vehicle communication unit  12  transmits the notification signal (step Sa 2 ). Next, the portable unit  20  searches for the in-vehicle communication units  12 , and detects one of the in-vehicle communication units  12  with which a communication connection is able to established (step Sa 3 ). In addition, the portable unit  20  transmits an answer signal corresponding to the notification signal (step Sa 4 ). The answer signal includes ID information used for identifying whether the portable unit  20  is the first portable unit  20   a  or the second portable unit  20   b.    
     Next, the in-vehicle communication unit  12  receives the answer signal (step Sa 5 ). Next, the in-vehicle communication unit  12  performs matching, based on the ID information included in the answer signal, and in a case where the answer signal is a signal from the portable unit  20  serving as a target of a communication connection, the in-vehicle communication unit  12  requests that portable unit  20  to establish a communication connection (step Sa 6 ). In addition, the portable unit  20  allows the communication connection (step Sa 7 ), and the communication connection between the in-vehicle communication unit  12  and the portable unit  20  is established (step Sa 8 ). In the two-way communication system  1 , in accordance with such a procedure, the communication connection between the in-vehicle communication unit  12  and the portable unit  20  is performed. 
     Next, a specific example of communication connections when the communication connections are performed between the two in-vehicle communication units  12  and the two portable units  20  will be described. First, selective communication connections between the two in-vehicle communication units  12  and the two portable units  20 , based on received electric field strength, will be described with reference to  FIGS. 4A and 4B .  FIGS. 4A and 4B  are first explanatory diagrams illustrating specific examples of communication connections according to the first embodiment of the present invention.  FIG. 4A  is an explanatory diagram in a case where the received electric field strength is less than a predetermined value, and  FIG. 4B  is an explanatory diagram in a case where the received electric field strength is greater than or equal to the predetermined value. 
     In the two-way communication system  1 , as illustrated in  FIGS. 4A and 4B , first, regardless of whether or not the received electric field strength is greater than or equal to the predetermined value, the first in-vehicle communication unit  12   a  and the first portable unit  20   a  are communication-connected to each other. After that, based on received electric field strength in two-way communication between the first in-vehicle communication unit  12   a  and the first portable unit  20   a , selective communication connections are performed between the second in-vehicle communication unit  12   b  and the first portable unit  20   a  and second portable unit  20   b.    
     In a case where the received electric field strength is less than the predetermined value, the second in-vehicle communication unit  12   b  is communication-connected to the first portable unit  20   a , as illustrated in  FIG. 4A . As a result, the first in-vehicle communication unit  12   a  and the first portable unit  20   a  are communication-connected to each other, and the second in-vehicle communication unit  12   b  and the first portable unit  20   a  are communication-connected to each other. In this way, in a case where the received electric field strength is less than the predetermined value, the two in-vehicle communication units  12  and one of the portable units  20  are communication-connected to each other. The two in-vehicle communication units  12  are spaced at the predetermined interval, as described above. Therefore, the two-way communication utilizing the space diversity effect becomes available between the two in-vehicle communication units  12  and one of the portable units  20 . In addition, the second portable unit  20   b  is not communication-connected to the first in-vehicle communication unit  12   a  or the second in-vehicle communication unit  12   b.    
     On the other hand, in a case where the received electric field strength is greater than or equal to the predetermined value, the second in-vehicle communication unit  12   b  is communication-connected to the second portable unit  20   b , as illustrated in  FIG. 4B . As a result, the first in-vehicle communication unit  12   a  and the first portable unit  20   a  are communication-connected to each other, and the second in-vehicle communication unit  12   b  and the second portable unit  20   b  are communication-connected to each other. In this way, in a case where the received electric field strength is greater than or equal to the predetermined value, the two in-vehicle communication units  12  and the respective two portable units  20  are communication-connected to each other on a one-to-one basis. In addition, it becomes possible for the two in-vehicle communication units  12  to simultaneously receive two kinds of signals simultaneously transmitted from the respective two portable units  20 . 
     The detection of the received electric field strength is performed by the received electric field strength detection unit  13 . After the first in-vehicle communication unit  12   a  and the first portable unit  20   a  are communication-connected to each other, the received electric field strength detection unit  13  detects the received electric field strength of a signal received from the first portable unit  20   a  by the first in-vehicle communication unit  12   a . In addition, the received electric field strength determination unit  14  determines whether or not the received electric field strength is greater than or equal to the predetermined value, and the in-vehicle control unit  11  determines one of the portable units  20 , which is to be communication-connected to the in-vehicle communication unit  12 . 
     Next, a communication connection in a case where the two in-vehicle communication units  12  and one of the portable units  20  perform communication connections with each other will be described with reference to  FIGS. 5A and 5B .  FIGS. 5A and 5B  are second explanatory diagrams illustrating specific examples of communication connections according to the first embodiment of the present invention.  FIGS. 5A and 5B  illustrate an example of a case where, first, the first in-vehicle communication unit  12   a  and the first portable unit  20   a  perform a communication connection with each other and after that, the second in-vehicle communication unit  12   b  and the first portable unit  20   a  perform a communication connection with each other.  FIG. 5A  is an explanatory diagram in a case where the first in-vehicle communication unit  12   a  and the first portable unit  20   a  perform a communication connection with each other, and  FIG. 5B  is an explanatory diagram in a case where, after that, the second in-vehicle communication unit  12   b  and the first portable unit  20   a  perform a communication connection with each other. 
     First, in a case where the first in-vehicle communication unit  12   a  and the first portable unit  20   a  perform a communication connection with each other, the first in-vehicle communication unit  12   a  may be put into the detection mode and the second in-vehicle communication unit  12   b  may be put into the non-detection mode, as illustrated in  FIG. 5A . In addition, the first portable unit  20   a  detects the first in-vehicle communication unit  12   a , and a communication connection between the first in-vehicle communication unit  12   a  and the first portable unit  20   a  is established. The second in-vehicle communication unit  12   b  is in the non-detection mode, and even if the first portable unit  20   a  searches for the corresponding in-vehicle communication unit  12 , it may be difficult to detect the second in-vehicle communication unit  12   b.    
     Next, in a case where the second in-vehicle communication unit  12   b  and the first portable unit  20   a  perform a communication connection with each other, the first in-vehicle communication unit  12   a  where the communication connection is established may switch from the detection mode to the non-detection mode, as illustrated in  FIG. 5B . In addition, the second in-vehicle communication unit  12   b  may switch from the non-detection mode to the detection mode. In addition, the first portable unit  20   a  detects the second in-vehicle communication unit  12   b , and a communication connection between the second in-vehicle communication unit  12   b  and the first portable unit  20   a  is established. 
     In the two-way communication system  1 , in this way, when the two in-vehicle communication units  12  and one of the portable units  20  perform communication connections with each other, one in-vehicle communication unit  12  out of the two in-vehicle communication units  12  may be put into the detection mode, and the other in-vehicle communication unit  12  may be put into the non-detection mode. Therefore, the number of the in-vehicle communication units  12  detectable by the portable unit  20  is constantly one. 
     In addition, while the description of other combinations of communication connections is omitted, one in-vehicle communication unit  12  out of the two in-vehicle communication units  12  may be put into the detection mode and the other in-vehicle communication unit  12  may be put into the non-detection mode, in the same way as in a case of the above-mentioned communication connections. Accordingly, it is possible to smoothly perform communication connections between the in-vehicle communication units  12  and the portable units  20 . 
     The determination of the in-vehicle communication unit  12  and the portable unit  20  that are to serve as targets of a communication connection is performed by the in-vehicle control unit  11 . In addition, switching between the detection mode and the non-detection mode is performed by the in-vehicle control unit  11 . 
     Next, a vehicle operation utilizing the two-way communication system  1  will be described. In the present embodiment, using the two-way communication system  1 , it is possible to perform various kinds of vehicle operations on the vehicle  30 . The vehicle operations utilizing the two-way communication system  1  include vehicle operations such as locking and unlocking of a door, starting and stopping of an engine, opening and closing of a window, and an operation of an air conditioner. In addition, the vehicle operations utilizing the two-way communication system  1  further include confirmation of pieces of vehicle information relating to the vehicle  30  such as location information of the vehicle  30 , remaining amount information of fuel, and the information of air pressure in a tire. 
     One of the users  40  performs predetermined input operations on the input unit  23  in the corresponding portable unit  20 , and hence, the vehicle operations utilizing the two-way communication system  1  are executed. In addition, in a case where the two in-vehicle communication units  12  and the two portable units  20  are communication-connected to each other on a one-to-one basis, the two users  40  simultaneously operate the respective two portable units  20 , and hence, it is possible to simultaneously perform two vehicle operations. In addition, which portable unit  20  of the two portable units  20  is caused to perform which of the vehicle operations is arbitrarily set in accordance with the specification or the like of the system. 
     Next, the procedure of a vehicle operation utilizing the two-way communication system  1  will be described with reference to  FIG. 6 .  FIG. 6  is a flowchart illustrating the procedure of a vehicle operation according to the first embodiment of the present invention. In addition, it is assumed that the vehicle operation performed in  FIG. 6  is an operation including the confirmation of the vehicle information, and the description thereof is advanced. 
     As illustrated in  FIG. 6 , first, the first in-vehicle communication unit  12   a  and the first portable unit  20   a  are communication-connected to each other (step Sb 1 ). Next, the second in-vehicle communication unit  12   b  and the first portable unit  20   a  are communication-connected to each other (step Sb 2 ). Next, the received electric field strength detection unit  13  detects the received electric field strength (step Sb 3 ). In addition, the received electric field strength determination unit  14  determines whether or not the received electric field strength is greater than or equal to the predetermined value, and the in-vehicle control unit  11  performs determination relating to a communication connection (step Sb 4 ). 
     In a case where, in the step Sb 4 , the received electric field strength is less than the predetermined value, the communication connection between the second in-vehicle communication unit  12   b  and the first portable unit  20   a  is maintained (step Sb 5 ). As a result, the first in-vehicle communication unit  12   a  and the first portable unit  20   a  are communication-connected to each other, and the second in-vehicle communication unit  12   b  and the first portable unit  20   a  are communication-connected to each other. In addition, the procedure moves to a step Sb 7 . 
     In a case where, in the step Sb 4 , the received electric field strength is greater than or equal to the predetermined value, the communication connection between the second in-vehicle communication unit  12   b  and the first portable unit  20   a  is terminated, and after that, the second in-vehicle communication unit  12   b  and the second portable unit  20   b  are communication-connected to each other (step Sb 6 ). As a result, the first in-vehicle communication unit  12   a  and the first portable unit  20   a  are communication-connected to each other, and the second in-vehicle communication unit  12   b  and the second portable unit  20   b  are communication-connected to each other. In addition, the procedure moves to the step Sb 7 . 
     Next, in the step Sb 7 , the communication connection between the in-vehicle communication unit  12  and the portable unit  20  is completed, and the in-vehicle unit  10  and the portable unit  20  stand by in states of being capable of performing the two-way communication. Next, the input unit  23  in the portable unit  20  detects an input operation by the user  40  (step Sb 8 ). Next, the communication unit  22  in the portable unit  20  transmits a request signal corresponding to the input operation, to the in-vehicle communication unit  12  in the in-vehicle unit  10  (step Sb 9 ). Next, by issuing an instruction corresponding to the request signal, the in-vehicle control unit  11  in the in-vehicle unit  10  causes a predetermined vehicle operation to be performed, and acquires predetermined vehicle information (step Sb 10 ). 
     Next, the in-vehicle communication unit  12  in the in-vehicle unit  10  transmits, to the portable unit  20 , the vehicle information acquired by the in-vehicle control unit  11  in the in-vehicle unit  10  (step Sb 11 ). Next, the arithmetic unit  25  in the portable unit  20  converts the vehicle information received by the portable unit  20 , into a displayable form, and the display unit  24  displays the vehicle information (step Sb 12 ). 
     After that, returning to the step Sb 3 , the received electric field strength detection unit  13  detects the received electric field strength again, and switches the communication connection between the in-vehicle communication unit  12  and the portable unit  20  as appropriate. In the two-way communication system  1 , the vehicle operation is performed in this way. 
     In addition, since, in the two-way communication system  1 , the in-vehicle communication unit  12  and the portable unit  20  perform the two-way communication with each other using the short distance wireless communication, the received electric field strength is reduced if the user  40  moves about several meters away from the vehicle  30 . In addition, in a case where the received electric field strength is reduced and becomes less than the predetermined value, the two in-vehicle communication units  12  and one of the portable units  20  are communication-connected to each other as described above, and the two-way communication utilizing the space diversity effect becomes available between the two in-vehicle communication units  12  and one of the portable units  20 . As a result, even in a case where the received electric field strength is reduced, it becomes possible to stabilize the communication. 
     On the other hand, if the two users  40  ride in the vehicle  30  and distances between the in-vehicle communication unit  12  and the portable units  20  become sufficiently small, the received electric field strength becomes high. In addition, in a case where the received electric field strength becomes high and greater than or equal to the predetermined value, the two in-vehicle communication units  12  and the two portable units  20  are communication-connected to each other on a one-to-one basis, and it becomes possible for the two in-vehicle communication units  12  to simultaneously receive two kinds of signals simultaneously transmitted from the respective two portable units  20 . As a result, it becomes possible for the two users  40  to simultaneously perform a plurality of vehicle operations using the respective two portable units  20 . 
     Next, advantageous effects of the present embodiment will be described. The two-way communication system  1  of the present embodiment includes the in-vehicle unit  10  installed in the vehicle  30 , and the two portable units  20  capable of performing two-way communication with the in-vehicle unit  10 . In addition, the in-vehicle unit  10  includes the two in-vehicle communication units  12  that perform two-way communication with the portable units  20 , and the in-vehicle control unit  11  that controls communication connections between the two in-vehicle communication units  12  and the two portable units  20 . Therefore, in the two-way communication system  1 , it is possible to perform selective communication connections between the two in-vehicle communication units  12  and the two portable units  20 . In addition, it is possible to stabilize communication using the space diversity effect while causing the two in-vehicle communication units  12  and one of the portable units  20  to be communication-connected to each other, and it is possible to simultaneously perform a plurality of vehicle operations while causing the two in-vehicle communication units  12  and the two portable units  20  to be communication-connected to each other on a one-to-one basis. 
     Furthermore, in the two-way communication system  1  of the present embodiment, the in-vehicle unit  10  includes the received electric field strength detection unit  13  that detects received electric field strength in the two-way communication between the in-vehicle unit  10  and the portable units  20 , and the received electric field strength determination unit  14  that performs determination, based on the received electric field strength. In addition, based on the received electric field strength, communication connections between the two in-vehicle communication units  12  and the two portable units  20  are switched, and the two in-vehicle communication units  12  and one of the portable units  20  are communication-connected to each other, or the two in-vehicle communication units  12  and the two portable units  20  are communication-connected to each other on a one-to-one basis. Therefore, it is possible to cause the in-vehicle communication units  12  for obtaining the space diversity effect to double as the in-vehicle communication units  12  for performing the two-way communication with the two portable units  20  on a one-to-one basis. As a result, it is possible to reduce the number of the in-vehicle communication units  12  and to simplify the configuration of the system. 
     In addition, in the two-way communication system  1  of the present embodiment, a function for stabilizing communication while causing the two in-vehicle communication units  12  and one of the portable units  20  to be communication-connected to each other is usually used in, for example, a case where the first user  40   a  locks or unlocks a door at a point distant from the vehicle  30 . Since, in such a situation, vehicle operations performed by the user  40  are restricted, a function for simultaneously performing a plurality of vehicle operations becomes unnecessary. On the other hand, a function for simultaneously performing a plurality of vehicle operations while causing the two in-vehicle communication units  12  and the two portable units  20  to be communication-connected to each other on a one-to-one basis is usually used in, for example, a case where the two users  40  ride in the vehicle  30  and perform various kinds of vehicle operations inside the vehicle. Since, in such a situation, distances between the in-vehicle communication unit  12  and the portable units  20  become sufficiently small and a communication state becomes favorable, the function for stabilizing communication while causing the two in-vehicle communication units  12  and one of the portable units  20  to be communication-connected to each other becomes unnecessary. Therefore, in the two-way communication system  1  of the present embodiment, it is possible to switch communication connections between the two in-vehicle communication units  12  and the two portable units  20  without lowering convenience in the vehicle operations. 
     In addition, in the two-way communication system  1  of the present embodiment, in a case where the two in-vehicle communication units  12  are in states of being able to be simultaneously detected when, for example, one of the portable units  20  performs communication connections with the two in-vehicle communication units  12 , it is necessary for one of the users  40  to manually select which of the two in-vehicle communication units  12  the portable unit  20  is to be communication-connected to. As a result, a burden occurs when a communication connection between the in-vehicle communication unit  12  and the portable unit  20  is performed. 
     However, in the two-way communication system  1  of the present embodiment, it is desirable that the in-vehicle communication unit  12  is able to switch between the detection mode of being able to be detected by the portable unit  20  and the non-detection mode of being unable to be detected by the portable unit  20 . In addition, when one of the portable units  20  performs communication connections with the two in-vehicle communication units  12 , one in-vehicle communication unit  12  may be put into the detection mode, and the other in-vehicle communication unit  12  may be put into the non-detection mode. Therefore, the number of the in-vehicle communication units  12  detectable by the portable unit  20  is constantly one. In such a case, it becomes unnecessary for one of the users  40  to select which of the two in-vehicle communication units  12  the portable unit  20  is to be communication-connected to. In addition, it is possible to cause the portable unit  20  to automatically perform a communication connection with the detected one of the in-vehicle communication units  12 . As a result, it is possible to smoothly perform communication connections between the in-vehicle communication units  12  and the portable units  20 . 
     In addition, in a second embodiment, in a case of adopting the same configuration as that in the first embodiment, the same symbol is assigned thereto, and the description thereof will be omitted. 
     First, the configuration of a two-way communication system  101  according to the second embodiment of the present invention will be described with reference to  FIGS. 7A and 7B .  FIGS. 7A and 7B  are configuration diagrams illustrating the configuration of the two-way communication system  101  according to the second embodiment of the present invention.  FIG. 7A  is a configuration diagram illustrating the entire configuration, and  FIG. 7B  is a configuration diagram illustrating the configuration of one of portable units  120  illustrated in  FIG. 7A . 
     As illustrated in  FIGS. 7A and 7B , the two-way communication system  101  includes an in-vehicle unit  110  and the two portable units  120 . One of the two portable units  120  is a first portable unit  120   a  and the other thereof is a second portable unit  120   b . In this way, in the two-way communication system  101 , the in-vehicle unit  10  and the portable units  20  in the two-way communication system  1  according to the first embodiment are replaced with the corresponding in-vehicle unit  110  and the respective portable units  120 . In addition, the first portable unit  20   a  and the second portable unit  20   b  are replaced with the first portable unit  120   a  and the second portable unit  120   b , respectively. 
     The in-vehicle unit  110  includes the in-vehicle control unit  11  and the two in-vehicle communication units  12 . The portable unit  120  includes the control unit  21 , the communication unit  22 , the input unit  23 , the display unit  24 , the arithmetic unit  25 , a received electric field strength detection unit  126 , and a received electric field strength determination unit  127 . In this way, in the two-way communication system  101 , the received electric field strength detection unit  126  and the received electric field strength determination unit  127  are included not in the in-vehicle unit  110  but in each of the portable units  120 . 
     In the present embodiment, the received electric field strength detection unit  126  detects received electric field strength when the in-vehicle unit  110  and the corresponding portable unit  120  perform two-way communication with each other. In addition, based on the received electric field strength detected by the received electric field strength detection unit  126 , the received electric field strength determination unit  127  determines whether or not the received electric field strength is greater than or equal to a predetermined value. The result of the determination performed by the received electric field strength determination unit  127  is transmitted to an in-vehicle unit  110  side, using the two-way communication performed between the in-vehicle unit  110  and the corresponding portable unit  120 . In addition, on the basis of the transmitted determination result, the in-vehicle control unit  11  determines one of the in-vehicle communication units  12  and one of the portable units  120 , which are to serve as communication targets. 
     In the two-way communication system  101 , a selective communication connection is performed in this way. In addition, in the present embodiment, the same advantageous effects as those of the first embodiment are obtained. 
     While, as above, the embodiments of the present invention have been described, the present invention is not limited to the above-mentioned embodiments, and may be arbitrarily altered without departing from the scope of an object of the present invention. 
     For example, in one of the embodiments of the present invention, one user  40  may simultaneously perform a plurality of vehicle operations using the two portable units  20  (or the two portable units  120 ). In addition, the first user  40   a  may take along the second portable unit  20   b , and the second user  40   b  may take along the first portable unit  20   a . In that case, a setting of which portable unit  20  of the two portable units  20  is caused to perform which of vehicle operations may be arbitrarily changed. 
     In addition, in one of the embodiments of the present invention, the number of the in-vehicle communication units  12  may be three or more. In addition, the number of the portable units  20  (or the portable units  120 ) may be three or more. In addition, the in-vehicle unit  10  (or the in-vehicle unit  110 ) and these portable units  20  (or the portable units  120 ) may be communication-connected to each other on a one-to-one basis, and more vehicle operations may be simultaneously performed. In addition, by arranging more in-vehicle communication units  12  in the vehicle  30 , the space diversity effect may be further enhanced. In addition, in a case where the number of the portable unit  20  (or the portable units  120 ) is fewer than the number of the in-vehicle communication units  12 , even if the received electric field strength is greater than or equal to the predetermined value, it is not necessary for all the in-vehicle communication units  12  to be communication-connected to the portable units  20  (or the portable units  120 ). 
     In addition, in one of the embodiments of the present invention, the in-vehicle communication units  12  in the in-vehicle unit  10  and the communication units  22  in the portable units  20  (or the portable units  120 ) may be communication devices compatible with a communication standard other than Bluetooth. In recent years, for example, a communication device compatible with a communication standard that is called Bluetooth low energy and in which electric power necessary for communication is further reduced has been put into practical use. Using such a communication device, it is possible to suppress power consumption when the in-vehicle communication units  12  and the portable units  20  (or the portable units  120 ) perform two-way communication with each other. In addition, the in-vehicle communication units  12  in the in-vehicle unit  10  and the communication units  22  in the portable units  20  (or the portable units  120 ) may be communication devices compatible with a communication method other than the short distance wireless communication. 
     In addition, in one of the embodiments of the present invention, the two in-vehicle communication units  12  in the in-vehicle unit  10  (or the in-vehicle unit  110 ) may each include a main body unit and an antenna unit, the two antenna units may be spaced at a predetermined interval, and the two main body units may be arranged together in one point. 
     In addition, in the first embodiment of the present invention, the portable units  20  may each include one of the received electric field strength detection unit  13  and the received electric field strength determination unit  14 . In addition, in the second embodiment of the present invention, the in-vehicle unit  10  may include one of the received electric field strength detection unit  126  and the received electric field strength determination unit  127 . 
     In addition, in one of the embodiments of the present invention, the communication connection utilizing switching between the detection mode and the non-detection mode of the in-vehicle communication unit  12  may be performed in a case other than a case where the two in-vehicle communication units  12  and one of the portable units  20  perform communication connections with each other. Even in a case where, by performing a setting so that the in-vehicle communication units  12  each select and transmit, for example, a first notification signal detectable by only the first portable unit  20   a  and a second notification signal detectable by only the second portable unit  20   b , the two in-vehicle communication units  12  and the two portable units  20  perform communication connections with each other, the communication connection utilizing switching between the detection mode and the non-detection mode of the in-vehicle communication unit  12  may become available. In addition, in addition to a case where the two in-vehicle communication units  12  and one of the portable units  20  perform communication connections used for performing a vehicle operation, the communication connection utilizing switching between the detection mode and the non-detection mode of the in-vehicle communication unit  12  may be performed in a case where the two in-vehicle communication units  12  and one of the portable units  20  perform communication connections used for authentication and registration.