Abstract:
This short-distance radio communication system for a vehicle includes an in-vehicle unit having more than one antenna and a portable unit performing radio communications with the in-vehicle unit. The in-vehicle unit transmits a first burst together with a call signal for calling the portable unit through a first antenna, and transmits a second burst subsequent to the first burst through a second antenna. The portable unit receives the signals transmitted from the in-vehicle unit, measures respective received signal strength indicators from the first burst and the second burst contained in the received signals, and responds to the in-vehicle unit according to results of comparing the respective indicators with given thresholds.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a short-distance radio communication system for a vehicle that performs short-distance radio communications between a key carried by a user and a vehicle. 
     BACKGROUND ART 
     In recent years, a smart entry system evolved from a keyless entry system is achieving widespread use as a system for locking and unlocking doors of a vehicle. A keyless entry system locks and unlocks vehicle doors with a button provided on the key pressed, where the key needs to be taken out from a bag or pocket for example. 
     On the other hand, a smart entry system, provided with a short-distance radio communication capability between a vehicle and a key (referred to as a smart key hereinafter), locks and unlocks vehicle doors by means of radio communications between the vehicle and vehicle doors. Concretely, a user unlocks vehicle doors simply by touching a touch sensor on the vehicle with a smart key remaining in a bag or pocket while the vehicle doors are locked; the user locks vehicle doors simply by touching the touch sensor while the vehicle doors are unlocked. With a smart entry system, if a smart key is detected inside the vehicle, the engine can be started without the need of inserting the key into the key hole of the vehicle. Under such circumstances, whether a smart key is present inside or outside a vehicle needs to be properly detected. 
     This smart entry system is provided with more than one antennas on the vehicle and an antenna control unit that controls to determine which antenna is to be used. When the antenna control unit transmits a high-power signal to an intended antenna, a weak signal can be accordingly sent to other antennas that must be under no-signal conditions, a problem called crosstalk. 
     For example, to detect a smart key, when an antenna provided inside a vehicle transmits a high-power signal toward the smart key, an antenna provided outside the vehicle as well transmits a signal that is low-power but can be detected by the smart key. At this moment, if the smart key is present immediately near the outside antenna, determination and positioning is made that the smart key is present inside the vehicle. For example, in a case where a user is outside the vehicle with their back facing a door handle, with a smart key remaining in a rear pocket for instance, determination is made that the smart key is inside the vehicle. In such a state, a small child may accidentally start the engine. 
     To avoid such a situation, the technology disclosed in patent literature 1 for example has been devised. Patent literature 1 discloses a technology in which, while coding signals are not transmitted to the first antenna, a disturbing signal is transmitted to the second antenna to turn the second antenna into no-signal conditions. 
     CITATION LIST 
     Patent Literature 
     PTL 1 Japanese Patent Unexamined Publication No. 2004-129228 
     SUMMARY OF THE INVENTION 
     In the technology disclosed in above patent literature 1, the left front door is provided with a first antenna, and the right front door is provided with a second antenna, where these antennas are placed apart from each other, and thus each coverage of signals from each antenna does not overlap with the other coverage. In a typical smart entry system, an antenna is provided inside the vehicle and the antenna (e.g., the first antenna) inside the vehicle and the antenna (e.g., a second antenna) on the front door are close to each other, and thus the coverages of signals from the antennas may overlap with each other. In such circumstances, the antenna on the front door in no-signal conditions induces the in-vehicle antenna to no-signal conditions in a region where signal coverages overlap, which disables detecting a smart key present in this region. 
     An object of the present disclosure is to provide a short-distance radio communication system for a vehicle that properly detects the position of a smart key even in a case where crosstalk occurs between antennas. 
     A short-distance radio communication system for a vehicle of the present disclosure includes an in-vehicle unit having more than one antenna and a portable unit performing radio communications with the in-vehicle unit. The system may have the following configuration. That is, the in-vehicle unit transmits a first burst together with a call signal for calling the portable unit through the first antenna, and then transmits a second burst subsequent to the first burst through the second antenna. The portable unit receives the signals transmitted from the in-vehicle unit; measures respective received signal strength indicators from the first and second bursts contained in the received signals; and responds to the in-vehicle unit according to results of comparing the respective indicators with given thresholds. 
     A short-distance radio communication system for a vehicle of the present disclosure includes more than one antenna provided in an enclosed space formed of an enclosed body; a first communication device provided in the enclosed space; and a second communication device that is portable and performs radio communications with the first communication device. The system may have the following configuration. That is, the first communication device transmits a first burst together with a call signal for calling the second communication device through the first antenna, and then transmits a second burst subsequent to the first burst through the second antenna. The second communication device receives the signals transmitted from the first communication device; measures respective received signal strength indicators from the first and second bursts contained in the received signals; and responds to the first communication device according to results of comparing the respective indicators with given thresholds. 
     According to the present disclosure, the position of a smart key can be accurately detected even in a case where crosstalk occurs between antennas. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates the positions of antennas for smart entry provided on a vehicle. 
         FIG. 2  illustrates an outline structure of a smart entry system according to the first exemplary embodiment of the present disclosure. 
         FIG. 3  illustrates signals and their timing transmitted from each antenna by the microprocessor on the in-vehicle unit shown in  FIG. 2 . 
         FIG. 4  is a flowchart showing the operation procedure of the microprocessor on the smart key shown in  FIG. 2 . 
         FIG. 5  illustrates possible areas where the smart key is present with respect to the vehicle viewed from the above. 
         FIG. 6  illustrates signals and their timing transmitted from each antenna by the microprocessor on the in-vehicle unit according to the second exemplary embodiment of the present disclosure. 
         FIG. 7  is a flowchart showing the operation procedure of the microprocessor on the smart key according to the second embodiment. 
         FIG. 8  illustrates possible areas where the smart key is present with respect to the vehicle viewed from the above. 
         FIG. 9  illustrates signals and their timing transmitted from each antenna by the microprocessor on the in-vehicle unit according to the third exemplary embodiment of the present disclosure. 
         FIG. 10  is a flowchart showing the operation procedure of the microprocessor on the smart key according to the third embodiment. 
         FIG. 11  illustrates possible areas where the smart key is present with respect to the vehicle viewed from the above. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a detailed description is made of some embodiments of the present disclosure in reference to the drawings. 
     First Exemplary Embodiment 
       FIG. 1  illustrates the positions of antennas for smart entry provided on a vehicle (enclosed body). In  FIG. 1 , the vehicle has three antennas: in-vehicle front antenna (F antenna hereinafter)  111 , in-vehicle middle antenna (M antenna hereinafter)  112 , and in-vehicle rear antenna (R antenna hereinafter)  113  inside the vehicle (an enclosed space). 
     Outside the vehicle, three antennas are provided: out-vehicle driver&#39;s seat door handle antenna (FRDR antenna hereinafter)  114 , out-vehicle passenger&#39;s seat door handle antenna (FRAS antenna hereinafter)  115 , and out-vehicle tail gate antenna (TG antenna hereinafter)  116 . 
     In the first embodiment, a description is made of a case where the system prevents the engine from starting when a smart key is present near FRDR antenna or FRAS antenna (i.e., outside the vehicle). In the following description, the smart key is assumed to be present near FRAS antenna. 
       FIG. 2  illustrates an outline structure of smart entry system  100 , which is a short-distance radio communication system for a vehicle according to the first exemplary embodiment. Smart entry system  100  includes in-vehicle unit  110  (the first communication device) provided on the vehicle, and smart key  130  (the second communication device) carried by a user. 
     In-vehicle unit  110  includes multiple transmitting antennas  111  through  116 , transmitting units  121  through  126 , RF receiving antenna  117 , RF receiving unit  127 , and in-vehicle unit microprocessor (also referred to as an antenna control unit)  128 . 
     Multiple transmitting antennas  111  through  116  correspond to F antenna  111 , M antenna  112 , R antenna  113 , FRDR antenna  114 , FRAS antenna  115 , and TG antenna  116 , where the first three are provided inside the vehicle; the last three, outside. 
     Transmitting units  121  to  126  are respectively connected to transmitting antennas  111  to  116 , perform transmission processes (e.g., modulation, amplification) on a signal output from in-vehicle unit microprocessor  128 , and transmit the resulting signal through one of the transmitting antennas. 
     RF receiving antenna  117  receives an RF (radio frequency) signal transmitted from smart key  130 . RF receiving unit  127  performs reception processes (e.g., demodulation) on the signal received by RF receiving antenna  117 , and outputs the signal that has undergone reception processes to in-vehicle unit microprocessor  128 . 
     In-vehicle unit microprocessor  128  controls to determine which one of transmitting antennas  111  to  116  is to be used, and controls operation such as locking/unlocking of vehicle doors and permits engine start according to results of detecting smart key  130 . 
     Meanwhile, smart key  130  includes receiving antenna  131 , receiving unit  132 , microprocessor on the smart key (smart key microprocessor)  133 , RF transmitting unit  134 , and RF transmitting antenna  135 . 
     Receiving antenna  131  receives a signal transmitted from each of antennas  111  through  116  of in-vehicle unit  110 . Receiving unit  132  performs reception processes (e.g., demodulation) on the signal received through receiving antenna  131 , and outputs the signal that has undergone reception processes to smart key microprocessor  133 . 
     Smart key microprocessor  133  measures an RSSI (received signal strength indicator) from a signal output from receiving unit  132 ; compares the RSSI measured with a given threshold (threshold decision); and outputs an RF response to RF transmitting unit  134  according to the decision results. Detailed operation of smart key microprocessor  133  is described later. 
     RF transmitting unit  134  performs transmission processes (e.g., modulation, amplification) on the RF response output from smart key microprocessor  133 , and transmits the RF response that has undergone transmission processes to in-vehicle unit  110  through RF transmitting antenna  135 . 
       FIG. 3  illustrates signals and their timing that in-vehicle unit microprocessor  128  shown in  FIG. 2  transmits through each of antennas  111  through  116 . In-vehicle unit microprocessor  128  successively transmits call signals and RSSI bursts (first burst) through F antenna  111 , M antenna  112 , and R antenna  113 , and at their each timing transmits RSSI bursts (second burst) for FRDR and FRAS.  FIG. 3  illustrates circumstances when signals are transmitted through F antenna  111  as an example. Here, a burst refers to a radio signal transmitted for measuring a received signal strength indicator. 
     In  FIG. 3 , a call signal contains a signal for waking up smart key  130  in a sleep state, an ID for authenticating pairing of in-vehicle unit  110  and smart key  130 , and auxiliary bits. Here, a sleep state refers to a state where a smart key wakes up when receiving a call signal. Details about auxiliary bits are described later. An RSSI burst for F antenna is a continuous signal for smart key  130  to measure an RSSI (received signal strength indicator) through F antenna  111 . The signals of from a call signal to an RSSI burst for F antenna are in the existing format. 
     Similarly, RSSI bursts for FRDR and for FRAS are continuous signals for measuring RSSIs from each antenna. 
       FIG. 3  shows a case where a signal is transmitted through F antenna  111 ; the situation is the same for M antenna  112  and R antenna  113 . 
     Smart key  130  that has received such signals measures an RSSI from each RSSI burst; compares the RSSI measured with a given threshold; and detects the position of smart key  130  based on the results of the threshold decision. In this case, smart key  130  is assumed to be present near FRAS antenna  115 , and thus does not respond to the call through F antenna  111 . 
     Next, a description is made of auxiliary bits. The auxiliary bits are set to “0000” in the existing format, which directs decision of an RSSI burst for each antenna subsequent to the auxiliary bits. In this embodiment, when auxiliary bits are “1000”, the existing format is followed by an RSSI burst for FRDR and an RSSI burst for FRAS that are allocated direction of measuring RSSIs. That is, the auxiliary bits contained in the call signal shown in  FIG. 3  are set to “1000”. 
     Next, a description is made of detailed operation of smart key microprocessor  133  shown in  FIG. 2  using  FIGS. 4 and 5 .  FIG. 4  is a flowchart showing the operation procedure of smart key microprocessor  133 .  FIG. 5  illustrates possible areas where the smart key is present with respect to the vehicle viewed from the above. 
     In step ST 201  of  FIG. 4 , smart key microprocessor  133  receives a call signal through F antenna. In step ST 202 , determination is made whether or not the auxiliary bits contained in the call signal are “1000”. If not “1000” (ST 202 : NO), the process proceeds to step ST 203 ; otherwise (YES: step ST 202 ), to step ST 207 . 
     In step ST 203 , smart key microprocessor  133  determines whether or not the auxiliary bits contained in the call signal are “0100”. If not “0100” (step ST 203 : NO), the process proceeds to step ST 204 ; otherwise (step ST 203 : YES), to the flowchart of  FIG. 7 . 
     In step ST 204 , smart key microprocessor  133  determines whether or not the auxiliary bits contained in the call signal are “0000”. If not “0000” (step ST 204 : NO), the process proceeds to ST 219 , and smart key  130  shifts to a sleep state. Meanwhile, if the auxiliary bits are “0000” (step ST 204 : YES), the process proceeds to step ST 205 . 
     In step ST 205 , smart key microprocessor  133  measures an RSSI burst (first burst) for each in-vehicle antenna (here, F antenna for example). In step ST 206 , smart key microprocessor  133  determines whether or not the RSSI from in-vehicle F antenna exceeds given threshold F (a threshold indicating the effective coverage of F antenna). If exceeding (step ST 206 : YES), the process proceeds to step ST 218 ; otherwise (step ST 206 : NO), to ST 219 , and smart key  130  shifts to a sleep state. 
     In step ST 202 , if the auxiliary bits are “1000” (step ST 202 : YES), smart key microprocessor  133  measures an RSSI burst for each in-vehicle antenna (here, F antennae for example) in step ST 207 . At this moment, the AD converter (hereinafter, referred to as an ADC) is set to a 10-bit resolution and 64 averaging times. 
     In step ST 208 , smart key microprocessor  133  sets the ADC to a 10-bit resolution and 4 averaging times. In step ST 209 , smart key microprocessor  133  measures an RSSI burst (second burst) for FRDR. 
     In step ST 210 , smart key microprocessor  133  determines whether or not the RSSI from in-vehicle F antenna exceeds given threshold F. If exceeding (step ST 210 : YES), the process proceeds to step ST 211 ; otherwise (step ST 210 : NO), smart key  130  being assumed to be present away from the vehicle (case  1  shown in  FIG. 5 ), the process proceeds to step ST 219 , and smart key  130  shifts to a sleep state. 
     In step ST 211 , smart key microprocessor  133  determines whether or not the RSSI from out-vehicle FRDR antenna exceeds given threshold b. If exceeding (step ST 211 : YES), the process proceeds to step ST 212 ; otherwise, to step ST 213 . 
     In step ST 212 , smart key microprocessor  133  determines that smart key  130  is present near FRDR antenna (case  2  shown in  FIG. 5 ). The process proceeds to step ST 219 , and smart key  130  shifts to a sleep state. 
     In step ST 213 , smart key microprocessor  133  sets the ADC to a 10-bit resolution and 4 averaging times. In step ST 214 , smart key microprocessor  133  measures an RSSI burst (third burst) for FRAS. 
     In step ST 215 , smart key microprocessor  133  determines whether or not the RSSI from in-vehicle F antenna exceeds given threshold F. If exceeding (step ST 215 : YES), the process proceeds to step ST 216 ; otherwise (step ST 215 : NO), smart key  130  being assumed to be present away from the vehicle (case  1  in  FIG. 5 ), the process proceeds to step ST 219 , and smart key  130  shifts to a sleep state. 
     In step ST 216 , smart key microprocessor  133  determines whether or not the RSSI from out-vehicle FRAS antenna exceeds given threshold b (the same value as the crosstalk threshold). If exceeding (step ST 216 : YES), the process proceeds to step ST 217 ; otherwise (step ST 216 : NO), to step ST 218 . 
     In step ST 217 , smart key microprocessor  133  determines that smart key  130  is present near FRAS antenna (case  2 ′ shown in  FIG. 5 ), the process proceeds to step ST 219 , and smart key  130  shifts to a sleep state. 
     In step ST 218 , smart key  130  is assumed to be present inside the vehicle (case  4  shown in  FIG. 5 ) and smart key microprocessor  133  transmits an RF response to the vehicle. 
     In this way, smart key microprocessor  133  compares an RSSI measured from an RSSI burst for each in-vehicle antenna with given threshold F. If this RSSI exceeds threshold F, smart key  130  is present inside the vehicle with a high possibility. Smart key  130 , however, can be present outside the vehicle under the influence of crosstalk. Thus, smart key microprocessor  133  compares RSSIs measured from RSSI bursts for FRDR and for FRAS with given threshold b. If determination has been made that an RSSI is larger than threshold b, smart key  130  proves present near the relevant antenna outside the vehicle. Hence, the position of a smart key can be detected without the need of an additional hardware device. 
     Thus according to the first embodiment, every time the system transmits a call signal and an RSSI burst successively through each in-vehicle antenna, the system transmits RSSI bursts for FRDR and for FRAS, measures an RSSI from each RSSI burst received by the smart key, and compares the RSSI measured with a given threshold. This allows the position of the smart key to be accurately detected based on results of the threshold decision. This prevents the engine from starting when the smart key is present near FRDR antenna or FRAS antenna (i.e., outside the vehicle). Here, the first and second bursts may be those transmitted from three antennas freely chosen from six antennas: F antenna  111 , M antenna  112 , R antenna  113 , FRDR antenna  114 , FRAS antenna  115 , and TG antenna  116 . 
     Second Exemplary Embodiment 
     In the second exemplary embodiment, a description is made of a case where, when a smart key as a portable unit is present near one door antenna (FRDR or FRAS antenna), the system prevents entry from another door. In the following description, a smart key is assumed to be present near FRAS antenna. The smart entry system, which is a short-distance radio communication system for a vehicle of the second embodiment, has a configuration similar to that of the first embodiment shown in  FIG. 2 , and thus a description is made referring to  FIG. 2  as required. 
       FIG. 6  illustrates signals and their timing that in-vehicle unit microprocessor  128  according to the second embodiment transmits through each antenna. In-vehicle unit  110  successively transmits a silent direction signal and an RSSI burst from each antenna: F antenna  111 , M antenna  112 , and R antenna  113 , then transmits a call signal and an RSSI burst for FRDR through FRDR antenna  114 , and further transmits an RSSI burst for FRAS through FRAS antenna  115 . The signals of from the silent direction signal of F antenna  111  to the RSSI burst for FRDR are in the existing format. 
     Smart key  130  that has received such signals, when receiving a silent direction signal from each in-vehicle antenna, maintains a silent state. A silent state refers to a state where a call signal received is ignored. Then, smart key  130 , when receiving a call signal through FRDR antenna  114 , measures respective RSSIs from RSSI bursts for FRDR and for FRAS, and compares the RSSIs measured with given thresholds to detect the position of smart key  130 . Here, smart key  130  is assumed to be present near FRAS antenna  115 , and thus smart key  130  does not respond to the call form FRDR antenna  114 . 
     Next, a description is made of auxiliary bits contained in the call signal shown in  FIG. 6 . In this embodiment, when the auxiliary bits are “0100”, the existing format is followed by an RSSI burst for FRAS that are allocated direction of measuring an RSSI. That is, the auxiliary bits contained in the call signal shown in  FIG. 6  are set to “0100”. 
     Next, a description is made of detailed operation of smart key microprocessor  133  according to the second embodiment using  FIGS. 7 and 8 .  FIG. 7  is a flowchart showing the operation procedure of smart key microprocessor  133 .  FIG. 8  illustrates possible areas where the smart key is present with respect to the vehicle viewed from the above. 
     In step ST 301  of  FIG. 7 , smart key microprocessor  133  receives a call signal through FRDR antenna  114 . In step ST 302 , smart key microprocessor  133  determines whether or not the auxiliary bits contained in the call signal are “0100”. If not “0100” (step ST 302 : NO), the process proceeds to step ST 303 ; otherwise (step ST 302 : YES), to step ST 306 . 
     In step ST 303 , smart key microprocessor  133  determines whether or not the auxiliary bits contained in the call signal are “0000”. If not “0000” (step ST 303 : NO), the process proceeds to ST 317 , and smart key  130  shifts to a sleep state. Meanwhile, if the auxiliary bits are “0000” (step ST 303 : YES), the process proceeds to step ST 304 . 
     In step ST 304 , smart key microprocessor  133  measures an RSSI burst for each in-vehicle antenna (here, F antennae for example). In step ST 305 , smart key microprocessor  133  determines whether or not the RSSI from in-vehicle F antenna exceeds given threshold F (a threshold indicating the effective coverage of F antenna). If exceeding (step ST 305 : YES), the process proceeds to step ST 314 ; otherwise (step ST 305 : NO), to ST 317 , and smart key  130  shifts to a sleep state. 
     If the auxiliary bits are “0100” (step ST 302 : YES) in step ST 302 , smart key microprocessor  133  measures an RSSI burst for FRDR in step ST 306 . At this moment, the ADC is set to a 10-bit resolution and 64 averaging times. 
     In step ST 307 , smart key microprocessor  133  determines whether or not the smart key is in silent. If in silent (step ST 307 : YES), the process proceeds to step ST 308 ; otherwise (step ST 307 : NO), to step ST 310 . 
     In step ST 308 , smart key microprocessor  133  determines whether or not an RSSI for FRDR exceeds a given crosstalk threshold. If exceeding (step ST 308 : YES), smart key microprocessor  133  releases the silent state in ST 309 ; otherwise (step ST 308 : NO), the process proceeds to step ST 316 . In step ST 309 , smart key  130  is assumed to be present near FRDR antenna  114  (case  3  shown in  FIG. 8 ). 
     In step ST 310 , smart key microprocessor  133  measures an RSSI burst for FRAS. In step ST 311 , smart key microprocessor  133  determines whether or not an RSSI for FRDR exceeds a given threshold. If exceeding (step ST 311 : YES), the process proceeds to step ST 312 ; otherwise (step ST 311 : NO), smart key  130  is assumed to be present away from the vehicle (case  1  shown in  FIG. 8 ) and the process proceeds to step ST 317 . 
     In step ST 312 , smart key microprocessor  133  determines whether or not the RSSI for FRDR exceeds the RSSI for FRAS. If exceeding (step ST 312 : YES), the process proceeds to step ST 313 ; otherwise (step ST 312 : NO), to ST 315 , where smart key  130  is assumed to be present near FRAS antenna  115  (case  5  shown in  FIG. 8 ) and the process proceeds to step ST 317 . 
     In step ST 313 , smart key microprocessor  133  determines that smart key  130  is present near the FRDR antenna. In step ST 314 , smart key  130  is assumed to be present near FRDR antenna  114  (case  3  shown in  FIG. 8 ), and smart key microprocessor  133  transmits an RF response to the vehicle. 
     In step ST 315 , smart key microprocessor  133  determines that smart key  130  is near FRAS antenna, the process proceeds to step ST 317 , and smart key  130  shifts to a sleep state. 
     In step ST 316 , smart key  130  is assumed to be present inside the vehicle (case  4  or case  4 ′ shown in  FIG. 8 ), and smart key microprocessor  133  continues silent until the end of silent. In step ST 317 , smart key  130  shifts to a sleep state. 
     In this way, smart key microprocessor  133  compares an RSSI measured from an RSSI burst for FRDR with a given threshold. If this RSSI exceeds the threshold, smart key  130  is present near FRDR antenna  114  with a high possibility. Smart key  130 , however, can be present near FRAS antenna  115  under the influence of crosstalk. Thus, smart key microprocessor  133  compares RSSIs measured from RSSI bursts for FRDR and for FRAS with each other. This determination results prove that smart key  130  is present near the antenna the RSSI of which has been determined as larger. 
     Thus according to the second embodiment, a call signal and an RSSI burst for FRDR are transmitted through FRDR antenna; further an RSSI burst for FRAS is transmitted through FRAS antenna; and an RSSI is measured from each RSSI burst received by a smart key for comparison between RSSIs. This allows the position of the smart key to be accurately detected according to the comparison results, which, when a smart key is present near one door antenna, prevents entry from another door. 
     Third Exemplary Embodiment 
     In a short-distance radio communication system for a vehicle according to the third exemplary embodiment, a description is made of a case where, when a smart key as a portable unit is present near one door antenna (FRDR or FRAS antenna), the system prevents unlocking of the tail gate. In the following description, a smart key is assumed to be present near FRAS antenna. The smart entry system, which is a short-distance radio communication system for a vehicle of the third embodiment, has a configuration similar to that of the first embodiment shown in  FIG. 2 , and thus a description is made referring to  FIG. 2  as required. 
       FIG. 9  illustrates signals and their timing that in-vehicle unit microprocessor  128  according to the third exemplary embodiment transmits through each antenna. In-vehicle unit  110  successively transmits a silent direction signal and an RSSI burst from each antenna: F antenna  111 , M antenna  112 , and R antenna  113 , then transmits a call signal and an RSSI burst for TG through TG antenna  116 , and further transmits an RSSI burst for FRDR through FRDR antenna  114  and an RSSI burst for FRAS through FRAS antenna  115 . The signals of from the silent direction signal of F antenna  111  to the RSSI burst for TG are in the existing format. 
     Smart key  130  that has received such signals, when receiving a silent direction signal from each in-vehicle antenna, maintains a silent state. Then, smart key  130 , when receiving a call signal through TG antenna  116 , measures respective RSSIs from RSSI bursts for TG, FRDR, and FRAS, and compares the RSSIs measured with given thresholds to detect the position of smart key  130 . Here, smart key  130  is assumed to be present near FRAS antenna  115 , and thus smart key  130  does not respond to the call from TG antenna  116 . 
     Note that the auxiliary bits contained in the call signal shown in  FIG. 9  are the same as those of the first embodiment. When the auxiliary bits are “1000”, the existing format is followed by RSSI bursts for FRDR and FRAS that are allocated direction of measuring RSSIs. 
     Next, a description is made of detailed operation of smart key microprocessor  133  according to the third embodiment using  FIGS. 10 and 11 .  FIG. 10  is a flowchart showing the operation procedure of smart key microprocessor  133 .  FIG. 11  illustrates possible areas where the smart key is present with respect to the vehicle viewed from the above. 
     In step ST 401  in  FIG. 10 , smart key microprocessor  133  receives a call signal from the TG antenna. In step ST 402 , smart key microprocessor  133  determines whether or not the auxiliary bits contained in the call signal are “1000”. If not “1000” (step ST 402 : NO), the process proceeds to step ST 403 . 
     In step ST 403 , smart key microprocessor  133  determines whether or not the auxiliary bits contained in the call signal are “0100”. If not “0100” (step ST 403 : NO), the process proceeds to ST 404 ; otherwise (step ST 403 : YES), to the flowchart of  FIG. 7 . 
     In step ST 404 , smart key microprocessor  133  determines whether or not the auxiliary bits contained in the call signal are “0000”. If not “0000” (step ST 404 : NO), the process proceeds to ST 423 , and smart key  130  shifts to a sleep state. Meanwhile, if the auxiliary bits are “0000” (step ST 404 : YES), the process proceeds to step ST 405 . 
     In step ST 405 , smart key microprocessor  133  measures an RSSI burst for TG. In step ST 406 , smart key microprocessor  133  determines whether or not the RSSI from TG antenna exceeds given threshold TG (a threshold indicating the effective coverage of TG antenna). If exceeding (step ST 406 : YES), the process proceeds to step ST 421 ; otherwise (step ST 406 : NO), to step ST 423 , and smart key  130  shifts to a sleep state. 
     In step ST 402 , if the auxiliary bits are “1000” (step ST 402 : YES), smart key microprocessor  133  measures an RSSI burst for TG in step ST 407 . At this moment, the ADC is set to a 10-bit resolution and 64 averaging times. 
     In step ST 408 , smart key microprocessor  133  determines whether or not the smart key is in silent. If in silent (step ST 408 : YES), the process proceeds to step ST 409 ; otherwise (step ST 408 : NO), to step ST 411 . 
     In step ST 409 , smart key microprocessor  133  determines whether or not the RSSI for TG exceeds a given crosstalk threshold. If exceeding (step ST 409 : YES), smart key microprocessor  133  releases the silent state in ST 410 ; otherwise (step ST 409 : NO), the process proceeds to step ST 422 . In ST 410 , smart key  130  is assumed to be present near the TG antenna (case  5  shown in  FIG. 11 ), near the FRDR antenna (case  2 ), or near FRAS antenna (case  2 ′). 
     In step ST 411 , smart key microprocessor  133  sets the ADC to a 10-bit resolution and 4 averaging times. In step ST 412 , smart key microprocessor  133  measures an RSSI burst for FRDR. 
     In step ST 413 , smart key microprocessor  133  determines whether or not the RSSI for TG exceeds given threshold TG. If exceeding (step ST 413 : YES), the process proceeds to step ST 414 ; otherwise (step ST 413 : NO), to step ST 423 , where smart key  130  is assumed to be present away from the vehicle (case  1  shown in  FIG. 11 ). 
     In step ST 414 , smart key microprocessor  133  determines whether or not the RSSI for FRDR exceeds given threshold b. If exceeding (step ST 414 : YES), the process proceeds to step ST 415 ; otherwise (step ST 414 : NO), to step ST 416 . 
     In step ST 415 , smart key microprocessor  133  determines that smart key  130  is present near FRDR antenna (case  2 ), the process proceeds to step ST 423 , and smart key  130  shifts to a sleep state. 
     In step ST 416 , smart key microprocessor  133  sets the ADC to a 10-bit resolution and 4 averaging times. In step ST 417 , smart key microprocessor  133  measures an RSSI burst for FRAS. 
     In step ST 418 , smart key microprocessor  133  determines whether or not the RSSI for TG exceeds given threshold TG. If exceeding (step ST 418 : YES), the process proceeds to step ST 419 ; otherwise (step ST 418 : NO), to step ST 423 , and smart key  130  shifts to a sleep state. 
     In step ST 419 , smart key microprocessor  133  determines whether or not the RSSI for FRAS exceeds given threshold b. If exceeding (step ST 419 : YES), the process proceeds to step ST 420 ; otherwise (step ST 419 : NO), to step ST 421 . 
     In step ST 420 , smart key microprocessor  133  determines that smart key  130  is present near FRAS antenna  115  (case  2 ′), the process proceeds to step ST 423 , and smart key  130  shifts to a sleep state. 
     In step ST 421 , smart key  130  is assumed to be present near TG antenna (case  4  or case  5  shown in  FIG. 11 ), and smart key microprocessor  133  transmits an RF response to the vehicle. 
     In step ST 422 , smart key  130  is assumed to be present inside the vehicle (case  3  or case  3 ′ shown in  FIG. 11 ), and smart key microprocessor  133  continues silent until the end of silent. In step ST 423 , smart key  130  shifts to a sleep state. 
     In this way, smart key microprocessor  133  compares an RSSI measured from an RSSI burst for TG with given threshold TG. If this RSSI exceeds threshold TG, smart key  130  is present near the tail gate with a high possibility. Smart key  130 , however, can be present at another position under the influence of crosstalk. Thus, smart key microprocessor  133  compares RSSIs measured from RSSI bursts for FRDR and for FRAS with given threshold b. If determination has been made that an RSSI is larger than threshold b, smart key  130  is present near the relevant antenna outside the vehicle. 
     Thus according to the third embodiment, a call signal and an RSSI burst for TG are transmitted through TG antenna, RSSI bursts for FRDR and FRAS are further transmitted, and an RSSI is measured from each RSSI burst received by a smart key for comparison between RSSIs and given thresholds. This allows the position of the smart key to be accurately detected according to the comparison results, which, when the smart key is present near one door antenna (FRDR or FRAS antenna), prevents unlocking of the tail gate. 
     Note that this embodiment can be applied even in the following case. That is, a silent direction signal from each in-vehicle antenna leaks from FRDR antenna  114  or FRAS antenna  115 , and smart key  130  positioned near FRAS antenna  115  shifts to a silent state. Further, a call signal from TG antenna  116  leaks from FRDR antenna  114  or FRAS antenna  115 , and the silent state of smart key  130  is released. 
     All of the above completes the description of the embodiments. 
     In the above embodiments, the name smart key is used; a smart key is called otherwise, such as a fob key, electronic key, mobile key, and badge. 
     In the above embodiments, the description is made assuming that smart key microprocessor  133  performs RSSI measurement, threshold comparison, for example. However, the following operation may be performed. That is, smart key microprocessor  133  measures an RSSI and transmits the RSSI to in-vehicle unit microprocessor  128 , which performs threshold comparison. 
     INDUSTRIAL APPLICABILITY 
     A short-distance radio communication system for a vehicle according to the present disclosure is useful for detecting the position of a smart key. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               100  smart entry system (short-distance radio communication system for a vehicle) 
               110  in-vehicle unit (first communication device) 
               111  F antenna 
               112  M antenna 
               113  R antenna 
               114  FRDR antenna 
               115  FRAS antenna 
               116  TG antenna 
               117  RF receiving antenna 
               121  through  126  transmitting unit 
               127  RF receiving unit 
               128  in-vehicle unit microprocessor 
               130  smart key (portable unit, second communication device) 
               131  receiving antenna 
               132  receiving unit 
               133  microprocessor on the smart key 
               134  RF transmitting unit 
               135  RF transmitting antenna