Abstract:
A radio telephone system includes a base station and a plurality of mobile stations. The base station transmits echo-back request signals to the mobile stations in turn. When the mobile station fails to transmit an echo-back response signal, the base station determines that the non-responsive mobile station is outside a radio zone of the base station, and deletes the corresponding data from a storage section thereof. Further, the base station is capable of changing its transmission power level so as to change a size of the radio zone correspondingly. Specifically, the base station monitors a range of presence of the mobile stations within the radio zone, and changes its transmission power level depending on such a range. For example, when such a range spreads over the radio zone, the base station may control its transmission power level to be maximum. On the other hand, when such a range is only around the base station, the base station may control its transmission power level to be minimum.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a radio telephone system. 
   2. Description of the Prior Art 
   The recent development in mobile radio communication systems, particularly in radio telephone systems, has been remarkable. In the radio telephone system, a user&#39;s mobile station is connected by radio to a base station which is connected to the fixed telephone network. As a method of communication between the base station and the mobile station, the multi-zone system has been available, wherein the base stations are dispersed to allow each of them to use the same frequency band so as to increase the total number of the mobile stations to be accommodated. 
   In this multi-zone system, each of the base stations is set to have a relatively small radio zone so as to cover the service area by a number of the base stations. This can reduce output levels of the respective base and mobile stations, while capable of increasing the number of the mobile stations to be accommodated. 
   Hereinbelow, a conventional radio telephone system will be described. 
     FIG. 9  is a block diagram showing a structure of the conventional radio telephone system. 
   In  FIG. 9 , numeral  1  denotes a base station connected to a wire circuit  3  and numeral  2  denotes a mobile station which is connectable to the base station  1  by radio. The mobile station  2  performs signal transmission and reception relative to the wire circuit  3  via the base station  1 . 
   In the base station  1 , numeral  4  denotes a communication section for receiving sound and data signals sent through the wire circuit  3  and for sending sound and data signals into the wire circuit  3 . Numeral  5  denotes a B-M radio interface in the form of transmitter-receiver for call and control channels for communication with the mobile station  2 . Numeral  6  denotes a base station antenna connected to the B-M radio interface  5 . Numeral  7  denotes a base station ID storing section where an identification code of the base station for identifying classification, such as, service and owner company, of the base station is stored. Hereinafter, this identification code will be also referred to as “base station ID”. Numeral  8  denotes a mobile station ID storing section where an identification code/codes of the mobile station/stations  2  (hereinafter also referred to as “mobile station ID”) present in a radio zone of the base station  1  is/are registered. Numeral  9  denotes a location registration managing section for managing registration of the mobile station ID of the mobile station  2  present in the radio zone of the base station  1 . Numeral  9   a  denotes a managing table where data of the mobile station/stations  2  present in the radio zone of the base station  1  is/are registered. Numeral  10  denotes a base station control section for controlling communications performed by the base station  1  relative to the mobile station  2  and a circuit control office and other operations of the base station  1 . 
   On the other hand, in the mobile station  2 , numeral  11  denotes an M-B radio interface in the form of transmitter-receiver for call and control channels for communication with the base station  1 . Numeral  12  denotes a mobile station antenna connected to the M-B radio interface  11  for transmission and reception of radiowaves with the base station antenna  6 . Numeral  13  denotes a base station ID storing section where a base station ID of a base station  1  having a classification which is connectable to the present mobile station  2  is stored. Numeral  14  denotes a mobile station ID storing section where the mobile station ID of the present mobile station  2  is stored. Numeral  15  denotes an operating section for performing dial inputting and instruction inputting, such as, “on-hook” and “off-hook”. Numeral  16  denotes a display section in the form of, such as, an LED (light-emitting diode) display or an LCD (liquid crystal display) for displaying information, such as, a dialed number and a date. Numeral  17  denotes a telephone transmitter and number  18  denotes a telephone receiver. Numeral  19  denotes a sound interface for receiving a sound signal from the M-B radio interface  11  for an output through the telephone receiver  18 , for sending a sound signal from the telephone transmitter  17  to the M-B radio interface  11  and for performing signal gain adjustments in those instances. Numeral  20  denotes a mobile station control section for controlling operations of the mobile station  2 . 
   Now, operations of the radio telephone system as structured above will be described hereinbelow. 
   First, the location registration of the mobile station  2  will be explained. 
   For specifying one of the radio zones of the base stations  1  where the mobile station  2  concerned is present, the location registration is performed to register the mobile station ID of the mobile station  2  in the base station  1 . 
   The location registration is performed when a signal level received through the radio channel is lowered below a given value during communication and thus switching is necessary to another base station  1 , or when an operation for the location registration is performed via the operating section  15  in the mobile station  2 . On the other hand, there is another radio telephone system, wherein a location registration request is periodically transmitted from the mobile station  2  to cause the base station  1  to perform the location registration. 
   Hereinbelow, the location registration sequence will be described with reference to  FIG. 10  which shows a location registration sequence diagram. 
   When the location registration becomes necessary due to the foregoing situation, the mobile station control section  20  causes the M-B radio interface  11  to transmit an location registration request signal and a mobile station ID via the mobile station antenna  12  (step  101 ). 
   When these signals are received at the B-M radio interface  5  of the base station  1  via the base station antenna  6 , the base station control section  10  causes the B-M radio interface  5  to send a base station ID authentication request signal for requesting the mobile station  2  to transmit a base station ID stored in the mobile station  2  (step  102 ). 
   When the M-B radio interface  11  of the mobile station  2  receives this base station ID authentication request signal, the mobile station control section  20  reads out the base station ID from the base station ID storing section  13  and causes the M-B radio interface  11  to send a base station ID authentication response signal including the read-out base station ID (step  103 ). 
   When the B-M radio interface  5  of the base station  1  receives this authentication response signal, the base station control section  10  checks the base station ID included in the received authentication response signal with a base station ID stored in the base station ID storing section  7 . When both identification codes are matched with each other, the base station control section  10  causes the mobile station ID storing section  8  to store the mobile station ID transmitted in advance from the mobile station  2 . Simultaneously, the base station control section  10  sends this mobile station ID to the location registration managing section  9 . 
   The location registration managing section  9  registers data of the mobile station  2  in the managing table  9   a , and controls the communication section  4  to send a signal indicative of completion of the location registration of the mobile station  2  to a managing center (not shown) which is connected to the wire circuit  3  and performs the overall management of the base stations. Subsequently, the base station control section  10  transmits to the mobile station  2  via the B-M radio interface  5  a signal representing that the foregoing location registration processes have been acknowledged (step  104 ). Thereafter, the base station control section  10  further transmits to the mobile station  2  via the B-M radio interface  5  a signal requesting the mobile station  2  to disconnect the radio channel now used (step  105 ). 
   In response to this disconnect request signal, the mobile station  2  sends a signal indicative of disconnection of the radio channel to the base station  1  so as to release the current radio channel (step  106 ). 
   The location registration of the mobile station  2  is completed by the foregoing operation sequence. 
   Now, explanation will be made to a case where a call is made from the mobile station  2 . 
   The mobile station control section  20  transmits the base station ID stored in the base station ID storing section  13  and other given signals, such as, a sound signal, via the M-B radio interface  11 . 
   When the B-M radio interface  5  of the base station  1  receives these signals, the base station control section  10  compares the received base station ID with the station ID stored in the base station ID storing section  7 . When both identification codes are matched with each other, the base station control section  10  connects between the B-M radio interface  5  and the communication section  4  to allow communication between the wire circuit  3  and the mobile station  2 . 
   Now, explanation will be made to a case where the mobile station  2  receives a call. 
   When the communication section  4  of the base station  1  receives from the wire circuit  3  a reception request signal including date of the mobile station  2  to be called, the location registration managing section  9  checks whether the data of the mobile station  2  included in the received signal are registered in the managing table  9   a . If registered, the location registration managing section  9  notifies the base station control section  10  accordingly. In response to this, the base station control section  10  causes the B-M radio interface  5  to transmit the corresponding mobile station ID stored in the mobile station ID storing section  8  and a sound signal. When the M-B radio interface  11  of the mobile station  2  receives the transmitted signal, the mobile station control section  20  compares the received mobile station ID with the mobile station ID stored in the mobile station ID storing section  14 . When both identification codes are matched with each other, the mobile station control section  20  transmits a connection request signal via the M-B radio interface  11 . When this connection request signal is received by the B-M radio interface  5  of the base station  1 , the base station control section  10  connects between the mobile station  2  and the wire circuit  3 . 
   In the foregoing conventional radio telephone system, however, in case the location registration is performed based on the operation of the operating section  15  of the mobile station  2 , the data and the mobile station ID of the mobile station  2  continue to remain in the managing table  9   a  and the mobile station ID storing section  8  of the basic station  1  until a location registration of the mobile station  2  is performed in another radio zone. Accordingly, even when the mobile station  2  goes away from the radio zone of the basic station  1  concerned, the data and the mobile station ID of the mobile station  2  still remain in that base station  1 . This requires the base station  1  to make a transmission for call reception to the mobile station  2  which is now outside the radio zone concerned. This further requires unnecessarily increased storage capacities of the mobile station ID storing section  8  and the managing table  9 a. Particularly, in the base station  1  having a radio zone where the coming-in and coming-out of the mobile stations  2  are frequent, a great amount of data which are actually not necessary remains stored. 
   Further, in the foregoing conventional radio telephone system, the base station  1  always transmits radiowaves in the maximum output power so as to make it possible to communicate with the mobile station  2  anywhere in the radio zone concerned. Accordingly, even when no mobile station  2  is present in the radio zone concerned or the mobile stations  2  are concentrated around the base station  1 , the radiowaves are transmitted in the maximum power. This consumes the electric power more than necessary. 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the present invention to provide an improved radio telephone system. 
   According to one aspect of the present invention, a radio telephone system including a base station connected to a wire circuit and a plurality of mobile stations which perform radio communications with the base station, respectively, comprises a base station echo-back transmit-receive section provided in the base station for transmitting echo-back request signals to the mobile stations, respectively, and for receiving echo-back response signals transmitted from the mobile stations; and a mobile station echo-back transmit-receive section provided in each of the mobile stations for transmitting the echo-back response signal in response to the echo-back request signal transmitted from the base station. 
   According to another aspect of the present invention, a radio telephone system including a base station connected to a wire circuit and a plurality of mobile stations which perform radio communications with the base station, respectively, comprises a mobile station echo-back transmit-receive section provided in each of the mobile stations for transmitting an echo-back response signal in response to an echo-back request signal transmitted from the base station; a managing table provided in the base station for storing date of the mobile stations: a base station echo-back transmit-receive section provided in the base station for transmitting the echo-back request signals to the mobile stations stored in the managing table and for receiving the echo-back response signals transmitted from the mobile stations; and a control section provided in the base station for deleting from the managing table the data of the mobile station to which the echo-back request signal was transmitted while no echo-back response signal was received therefrom. 
   According to another aspect of the present invention, a base station of a radio telephone system, connected to a wire circuit, comprises a first interface for performing radio communication with a mobile station; and a control section provided in the base station for controlling a transmission power of the first interface depending on a signal transmitted from the mobile station. 
   According to another aspect of the present invention, a radio telephone system including a base station connected to a wire circuit and a plurality of mobile stations which perform radio communications with the base station, respectively, comprises a first interface provided in each of the mobile stations for performing communication with the base station; a second interface provided in the base station, the second interface adapted to change its transmission power for performing radio communications with the mobile stations, respectively; and a control section provided in the base station for controlling the transmission power of the second interface depending on positions of the mobile stations. 
   According to another aspect of the present invention, a base station of a radio telephone system, connected to a wire circuit, comprises a first interface for performing radio communications with a plurality of mobile stations; a transmit-receive section for receiving signals from the mobile stations via the first interface; and a control section for controlling an output level of the first interface depending on an operation of the transmit-receive section. 
   According to another aspect of the present invention, a radio telephone system including a base station connected to a wire circuit and a plurality of mobile stations which perform radio communications with the base station, respectively, comprises a first interface provided in each of the mobile stations for performing communication with the base station; a second interface provided in the base station, the second interface being variable in its transmission power for performing radio communications with the mobile stations; a base station echo-back transmit-receive section provided in the base station for transmitting echo-back request signals to the mobile stations via the second interface and for receiving echo-back response signals transmitted from the mobile stations via the second interface; a mobile station echo-back transmit-receive section provided in each of the mobile stations for transmitting the echo-back response signal in response to the echo-back request the transmitted from the base station; and a control section for controlling a transmission power of the second interface depending on an operation of the base station echo-back transmit-receive section. 
   According to another aspect of the present invention, a radio telephone system including a base station connected to a wire circuit and a plurality of mobile stations which perform radio communications with the base station, respectively, comprises a first interface provided in each of the mobile stations for performing communication with the base station; a second interface provided in the base station for performing radio communications with the mobile stations; a base station echo-back transmit-receive section provided in the base station for transmitting echo-back request signals to the mobile stations via the second interface and for receiving echo-back response signals transmitted from the mobile stations via the second interface; a mobile station echo-back transmit-receive section provided in each of the mobile stations for transmitting the echo-back response signal in response to the echo-back request signal transmitted from the base station; a readable/writable storing section; a first control section for controlling an output level of the second interface to be maximum, for controlling the base station echo-back transmit-receive section to transmit the echo-back request signals to the mobile stations so as to count the number of the echo-back response signals in response to the echo-back request signals, and for writing the counted value in the storing section; a second control section for controlling an output level of the second interface to a given value and for controlling the base station echo-back transmit-receive section to transmit the echo-back request signals to the mobile stations so as to count the number of the echo-back response signals in response to the echo-back request signals; a comparing section for comparing the counted values stored in the second control section and the storing section; and a third control section for fixing the output level of the second interface to a given value when the counted values are equal to each other based on the comparison of the comparing section. 
   According to another aspect of the present invention, a radio telephone system including a base station connected to a wire circuit and a plurality of mobile stations which perform radio communications with the base station, respectively, comprises a first interface provided in the mobile station for performing communication with the base station; a second interface provided in the base station for performing radio communication with the mobile stations; a base station echo-back transmit-receive section provided in the base station for transmitting echo-back request signals to the mobile stations via the second interface and for receiving echo-back response signals transmitted from the mobile stations via the second interface; a mobile station echo-back transmit-receive section provided in each of the mobile stations for transmitting the echo-back response signal in response to the echo-back request signal transmitted from the base station; a readable/writable storing section; a first control section for controlling an output level of the second interface to be maximum, for controlling the base station echo-back transmit-receive section to transmit the echo-back request signals to the mobile stations so as to count the number of the echo-back response signals in response to the echo-back request signals, and for writing the counted value in the storing section: a second control section for controlling an output level of the second interface to a given value and for controlling the base station echo-back transmit-receive section to transmit the echo-back request signals to the mobile stations so as to count the number of the echo-back response signals in response to the echo-back request signals; a comparing section for comparing the counted values stored in the second control section and the storing section; a third control section for fixing the output level of the second interface to a given value when the counted values are equal to each other based on the comparison of the comparing section; and a fourth control section for activating the second control section when the counted values are different from each other based on the comparison of the comparing section. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which are given by way of example only, and are not intended to limit the present invention. 
     In the drawings: 
       FIG. 1  is a block diagram showing a structure of a radio telephone system according to a first preferred embodiment of the present invention; 
       FIG. 2  is a location registration sequence diagram according to the first preferred embodiment; 
       FIG. 3  is a flowchart showing an echo-back routine to be executed in a base station of the radio telephone system according to the first preferred embodiment; 
       FIG. 4  is a flowchart showing an echo-back routine to be executed in a mobile station of the radio telephone system according to the first preferred embodiment; 
       FIG. 5  is a block diagram showing a structure of a radio telephone system according to a second preferred embodiment of the present invention; 
       FIG. 6  is a flowchart showing an echo-back routine to be executed in a mobile station of the radio telephone system according to the second preferred embodiment; 
       FIG. 7  is a flowchart showing a routine to be executed in a base station of the radio telephone system according to the second preferred embodiment; 
       FIG. 8A  is a diagram showing a state where a radio zone of the radio telephone system is set maximum according to the second preferred embodiment; 
       FIG. 8B  is a diagram showing a state where the radio zone of the radio telephone system is reduced according to the second preferred embodiment; 
       FIG. 9  is a block diagram showing a structure of a conventional radio telephone system; and 
       FIG. 10  is a conventional location registration sequence diagram. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Now, preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1  is a block diagram showing a structure of a radio telephone system according to a first preferred embodiment of the present invention. 
   In  FIG. 1 , numeral  201  denotes a base station connected to a wire circuit  203  and numeral  202  denotes a mobile station which is connectable to the base station  201  by radio. The mobile station  202  performs signal transmission and reception relative to the wire circuit  203  via the base station  201 . 
   In the base station  201 , numeral  204  denotes a communication section for receiving sound and data signals sent through the wire circuit  203  and for sending sound and data signals into the wire circuit  203 . Numeral  205  denotes a B-M radio interface in the form of transmitter-receiver for call and control channels for communication with the mobile station  202 . The B-M radio interface  205  has a plurality of radio channels and is capable of communicating with a plurality of the mobile stations  202 . Numeral  206  denotes a base station antenna connected to the B-M radio interface  205 . Numeral  207  denotes a base station ID storing section where an identification code of the base station for identifying classification, such as, service and owner company, of the base station is stored. Hereinafter, this identification code will be also referred to as “base station ID”. Numeral  208  denotes a mobile station ID storing section where an identification code/codes of the mobile station/stations  202  (hereinafter also referred to as “mobile station ID”) present in a radio zone of the base station  201  is/are registered. Numeral  209  denotes a location registration managing section for managing registration of the mobile station ID of the mobile station  202  present in the radio zone of the base station  201 . Numeral  209   a  denotes a managing table where data of the mobile station/stations  202  present in the radio zone of the base station  201  are registered. Numeral  210  denotes a base station control section for controlling communications performed by the base station  201  relative to the mobile station  202  and a circuit control office and other operations of the base station  201 . Numeral  221   a  denotes a base station echo-back transmit-receive section for controlling the B-M radio interface  205  to transmit echo-back request signals and for receiving echo-back response signals from the mobile stations  202  via the B-M radio interface  205 . Numeral  221   b  denotes an echo-back timer control section for regulating an interval for transmitting the echo-back request signals from the base station  201  to the mobile stations  202 . The echo-back timer control section  221   b  includes a timer which is counted up after lapse of a predetermined time from the start-up thereof. 
   On the other hand, in the mobile station  202 , numeral  211  denotes an M-B radio interface in the form of transmitter-receiver for call and control channels for communication with the base station  201 . Numeral  212  denotes a mobile station antenna connected to the M-B radio interface  211  for transmission and reception of radiowaves with the base station antenna  206 . Numeral  213  denotes a base station ID storing section where a base station ID of a base station  201  having a classification which is connectable to the present mobile station  202  is stored. Numeral  214  denotes a mobile station ID storing section where the mobile station ID of the present mobile station  202  is stored. Numeral  215  denotes an operating section for performing dial inputting and instruction inputting, such as, “on-hook” and “off-hook”. Numeral  216  denotes a display section in the form of, such as, an LED (light-emitting diode) display or an LCD (liquid crystal display) for displaying information, such as, a dialed number and a date. Numeral  217  denotes a telephone transmitter and number  218  denotes a telephone receiver. Numeral  219  denotes a sound interface for receiving a sound signal from the M-B radio interface  211  for an output through the telephone receiver  218 , for sending a sound signal from the telephone transmitter  217  to the M-B radio interface  211  and for performing signal gain adjustments in those instances. Numeral  220  denotes a mobile station control section for controlling operations of the mobile station  202 . Numeral  222  denotes a mobile station echo-back transmit-receive section for transmitting an echo-back response signal via the M-B radio interface  211  when the echo-back request signal is received at the M-B radio interface  211 . 
   Now, operations of the radio telephone system as structured above will be described hereinbelow. 
   First, the location registration of the mobile station  202  will be explained. 
   Hereinbelow, the location registration sequence will be described with reference to  FIG. 2  which shows a location registration sequence diagram. 
   When the location registration becomes necessary due to the foregoing situation, the mobile station control section  220  causes the M-B radio interface  211  to transmit an location registration request signal and a mobile station ID via the mobile station antenna  212  (step  301 ). 
   When these signals are received at the B-M radio interface  205  of the base station  201  via the base station antenna  206 , the base station control section  210  causes the B-M radio interface  205  to send a base station ID authentication request signal for requesting the mobile station  202  to transmit a base station ID stored in the mobile station  220  (step  302 ). 
   When the M-B radio interface  211  of the mobile station  202  receives this base station ID authentication request signal, the mobile station control section  220  reads out the base station ID from the base station ID storing section  213  and causes the M-B radio interface  211  to send a base station ID authentication response signal including the read-out base station ID (step  303 ). 
   When the B-M radio interface  205  of the base station  201  receives this authentication response signal, the base station control section  210  checks the base station ID included in the received authentication response signal with a base station ID stored in the base station ID storing section  207 . When both identification codes are matched with each other, the base station control section  210  causes the mobile station ID storing section  208  to store the mobile station ID transmitted in advance from the mobile station  202 . Simultaneously, the base station control section  210  sends this mobile station ID to the location registration managing section  209 . 
   The location registration managing section  209  registers data of the mobile station  202  in the managing table  209   a , and controls the communication section  204  to send a signal indicative of completion of the location registration of the mobile station  202  to a managing center (not shown) which is connected to the wire circuit  203  and performs the overall management of the base stations. Subsequently, the base station control section  210  transmits to the mobile station  202  via the B-M radio interface  205  a signal representing that the foregoing location registration processes have been acknowledged (step  304 ). Thereafter, the base station control section  210  further transmits to the mobile station  202  via the B-M radio interface  205  a signal requesting the mobile station  202  to disconnect the radio channel now used (step  305 ). 
   In response to this disconnect request signal, the mobile station  202  sends a signal indicative of disconnection of the radio channel to the base station  201  so as to release the current radio channel (step  306 ). 
   The location registration of the mobile station  202  is completed by the foregoing operation sequence. 
   Now, explanation will be made to a case where a call is made from the mobile station  202 . 
   The mobile station control section  220  transmits the base station ID stored in the base station ID storing section  213  and other given signals, such as, a sound signal, via the M-B radio interface  211 . 
   When the B-M radio interface  205  of the base station  201  receives these signals, the base station control section  210  compares the received base station ID with the station ID stored in the base station ID storing section  207 . When both identification codes are matched with each other, the base station control section  210  connects between the B-M radio interface  205  and the communication section  204  to allow communication between the wire circuit  203  and the mobile station  202 . 
   Now, explanation will be made to a case where the mobile station  202  receives a call. 
   When the communication section  204  of the base station  201  receives from the wire circuit  203  a reception request signal including date of the mobile station  202  to be called, the location registration managing section  209  checks whether the data of the mobile station  202  included in the received signal are registered in the managing table  209   a . If registered, the location registration managing section  209  notifies the base station control section  210  accordingly. In response to this, the base station control section  210  causes the B-M radio interface  205  to transmit the corresponding mobile station ID stored in the mobile station ID storing section  208  and a sound signal. When the M-B radio interface  211  of the mobile station  202  receives the transmitted signal, the mobile station control section  220  compares the received mobile station ID with the mobile station ID stored in the mobile station ID storing section  214 . When both identification codes are matched with each other, the mobile station control section  220  transmits a connection request signal via the M-B radio interface  211 . When this connection request signal is received by the B-M radio interface  205  of the base station  201 , the base station control section  10  connects between the mobile station  202  and the wire circuit  203 . 
   Now, the echo-back routine will be described hereinbelow. 
   First, the echo-back routine to be executed in the base station  201  will be described with reference to a flowchart shown in FIG.  3 . 
   At first step  401 , the base station control section  210  starts the timer  221   c  of the echo-back timer control section  221   b . Subsequently, step  402  checks whether the timer  221   c  counts up the preset time. If answer at step  402  is negative, the routine proceeds to step  412 . On the other hand, if answer at step  402  is positive, the routine proceeds to step  403  which checks whether there is any radio channel, among radio channels of the B-M radio interface  205 , which is not used for communication, that is, an empty radio channel. If answer at step  403  is negative, step  404  starts the timer  221   c  again, and the routine returns to step  402 . On the other hand, if answer at step  403  is positive, the routine proceeds to step  405  where the base station control section  210  reads out data for one of the mobile stations  202  stored in the managing table  209   a , via the location registration managing section  209 . The base station control section  210  further reads out from the mobile station ID storing section  208  a mobile station ID of the mobile station  202  corresponding to the data read out from the managing table  209   a . Subsequently, at step  406 , the base station echo-back transmit-receive section  221   a  transmits an echo-back request signal via the B-M radio interface  205  to the mobile station  202  having the read-out mobile station ID. Thereafter, at step  407 , the base station echo-back transmit-receive section  221   a  checks whether the B-M radio interface  205  has received an echo-back response signal from the mobile station  202 . If answer at step  407  is negative, the routine proceeds to step  408  where the base station echo-back transmit-receive section  221   a  determines that the mobile station  202  concerned is not present in the radio zone of the base station  201 , and thus causes the location registration managing section  209  to delete the data of the mobile station  202  concerned from the managing table  209   a  and further causes the base station control section  210  to delete the corresponding mobile station ID from the mobile station ID storing section  208 . From step  408 , the routine proceeds to step  409 . On the other hand, if answer at step  407  is positive, that is, the echo-back response signal has been received from the mobile station  202 , the routine proceeds to step  409  where the location registration managing section  209  looks through the managing table  209   a , and to step  410  where the location registration managing section  209  determines whether the echo-back process has been performed for all the mobile stations which are registered in the managing table  209   a . If answer at step  410  is negative, the routine returns to step  403  until answer at step  410  becomes positive. On the other hand, if answer at step  410  is positive, step  411  starts the timer again, and the routine returns to step  402  to repeat the foregoing processes. 
   Referring back to step  402 , if answer at step  402  is negative, that is, the timer is not counted up, the routine proceeds to step  412  where the base station control section  210  checks whether a transmission event, that is, a call reception request from the wire circuit  3 , is present for any of the mobile stations  2  in the radio zone covered by the base station  201 . If answer at step  412  is negative, the routine returns to step  402 . On the other hand, if answer at step  412  is positive, a process is performed to enable transmission of the transmission event signal along with the echo-back request signal, and then the routine proceeds to step  403  to perform the foregoing echo-back process. 
   Now, the echo-back routine to be executed in the mobile station  202  will be described with reference to a flowchart shown in FIG.  4 . 
   At first step  501 , the mobile station control section  220  checks whether a reception event is present, that is, whether a signal received from the base station  201  at the M-B radio interface  211  is present. If answer at step  501  is negative, the routine repeats step  501  until answer at step  501  becomes positive. On the other hand, if answer at step  501  is positive, step  502  checks whether the echo-back request signal is present in the reception event. If answer at step  502  is negative, the routine proceeds to step  503  where a process is performed depending on the reception event in the known manner, and then to step  501 . On the other hand, if answer at step  502  is positive, step  504  checks whether the reception event includes only the echo-back request signal. If answer at step  504  is positive, the routine proceeds to step  505  where the mobile station echo-back transmit-receive section  222  transmits the echo-back response signal via the M-B radio interface  211 , and then to step  501 . On the other hand, if answer at step  504  is negative, the routine proceeds to step  506  where a process is performed depending on the reception event other than the echo-back request signal in the known manner, and then to step  507  where the echo-back response signal is transmitted via the M-B radio interface  211  along with a response signal for the reception event other than the echo-back request signal. Subsequently, the routine returns to step  501 . 
   As described above, according to the first preferred embodiment, the base station  201  executes the echo-back process against the mobile stations  202  at every predetermined time interval or at every occurrence of the transmission event, and deletes the data and the mobile station ID of the non-responsive mobile station  202  from the managing table  209   a  and the mobile station ID storing section  208 , respectively. Accordingly, even when the once-registered mobile station  202  goes away from the radio zone of the base station  201  without contacting the base station  201 , since the corresponding data are automatically deleted from the managing table  209   a  and the mobile station ID storing section  208 , the storage capacities of the managing table  209   a  and the mobile station ID storing section  208  can be effectively used and thus prevented from being increased unnecessarily. 
   Now, a second preferred embodiment of the present invention will be described hereinbelow with reference to FIG.  5 . 
     FIG. 5  is a block diagram showing a structure of a radio telephone system according to the second preferred embodiment. 
   In  FIG. 5 , numeral  601  denotes a base station, numeral  602  a mobile station, numeral  603  a wire circuit, numeral  604  a communication section, numeral  606  a base station antenna, numeral  607  a base station ID storing section, numeral  608  a mobile station ID storing section, numeral  609  a location registration control section, numeral  609   a  a managing table, numeral  610  a base station control section, numeral  621   a  a base station echo-back transmit-receive section, numeral  621   b  an echo-back timer control section, numeral  612   c  a timer, numeral  611  an M-B radio interface, numeral  612  a mobile station antenna, numeral  613  a base station ID storing section, numeral  614  a mobile station ID storing section, numeral  615  an operating section, numeral  616  a display section, numeral  617  a telephone transmitter, numeral  618  a telephone receiver, numeral  619  a sound interface, numeral  620  a mobile station control section, and numeral  622  a mobile station echo-back transmit-receive section. Since these elements are the same as those in the first preferred embodiment, explanation thereof will be omitted. On the other hand, the transmission power of a B-M radio interface  605  can be switched between three levels. As shown in  FIG. 8A , a boundary of the radio zone of the base station  601  changes between boundaries  626 ,  627  and  628  depending on the output levels of the B-M radio interface  605 . 
   Numeral  623  denotes a mobility history managing section provided in the base station  601  for managing a past mobility history of the mobile stations  602 . Numeral  624  denotes a radio managing section for storing/managing the output levels of the B-M radio interface  605 . Numeral  625  denotes a radio zone timer control section for outputting an instruction signal at every predetermined time interval to the base station control section  610  for confirming presence or absence of the mobile stations  602  in the radio zone. The radio zone timer control section  625  includes a timer  625   a  which is counted up after lapse of a predetermined time from the start-up thereof. 
   Now, operations of the radio telephone system as structured above will be described hereinbelow. 
   Since the process of the location registration of the mobile station  602  and the process of the mobile station  602  making and receiving a call are the same as those in the first preferred embodiment, explanation thereof will be omitted. 
   First, the echo-back routine to be executed in the mobile station  602  will be described with reference to a flowchart shown in FIG.  6 . 
   At first step  701 , the mobile station control section  620  checks whether a reception event is present, that is, whether a signal received from the base station  601  at the M-B radio interface  611  is present. If answer at step  701  is negative, the routine repeats step  701  until answer at step  701  becomes positive. On the other hand, if answer at step  701  is positive, step  702  checks whether the echo-back request signal is present in the reception event. If answer at step  702  is negative, the routine proceeds to step  703  where a process is performed depending on the reception event in the known manner, and then to step  701 . On the other hand, if answer at step  702  is positive, step  704  checks whether the reception event includes only the echo-back request signal. If answer at step  704  is positive, the routine proceeds to step  705  where the mobile station echo-back transmit-receive section  622  transmits the echo-back response signal via the M-B radio interface  611 , and then to step  701 . On the other hand, if answer at step  704  is negative, the routine proceeds to step  706  where a process is performed depending on the reception event other than the echo-back request signal in the known manner, and then to step  707  where the echo-back response signal is transmitted via the M-B radio interface  611  along with a response signal for the reception event other than the echo-back request signal. Subsequently, the routine returns to step  701 . 
   Now, a control of radio transmission power of the base station  601  will be described hereinbelow with reference to  FIGS. 7 ,  8 A and  8 B. 
     FIG. 7  is a flowchart showing operations of the base station  601  in the radio telephone system according to the second preferred embodiment,  FIG. 8A  is a diagram showing the state where the radio zone of the radio telephone system is set maximum according to the second preferred embodiment, and  FIG. 8B  is a diagram showing the state where the radio zone of the radio telephone system is reduced according to the second preferred embodiment. In  FIGS. 8A and 8B , numerals  626 ,  627  and  628  represent the radio zone boundaries determined by the radio managing section  624  of the base station  601 , and numerals  629 ,  630  and  631  represent the mobile stations. 
   At first step  801 , the base station control section  610  starts the timer  625   a  of the radio zone timer control section  625 . Subsequently, at step  802 , the base station control section  610  checks whether the timer  625   a  is counted up. If answer at step  802  is negative, the routine repeats step  802  until answer at step  802  becomes positive. On the other hand, if answer at step  802  is positive, the routine proceeds to step  803  where the B-M radio interface  605  is controlled to have the maximum transmission power and the fact that the transmission power of the B-M radio interface  605  has been set to the maximum level is stored in the radio managing section  624 . This causes the radio zone of the base station  601  to extend to the boundary  628  as indicated by dotted area in  FIG. 8A , that is, to be the maximum. Subsequently, the routine proceeds to step  804  where the base station control section  610  controls the base station echo-back transmit-receive section  621   a  and the echo-back timer control section  621   b  so as to execute the echo-back process to all the mobile stations identified by the mobile station ID storing section  608  and the location registration managing section  609 . This echo-back process is executed essentially in the same manner as in the first preferred embodiment. 
   Subsequently, the routine proceeds to step  805  where the number of the mobile stations  602  data of which are stored in the managing table after the echo-back process at step  804 , is stored in the mobility history managing section  623 . 
   Thereafter, at step  806 , the base station control section  610  checks whether the number of the mobile stations stored in the mobility history managing section  623  is 0 (zero). If answer at step  806  is positive, step  807  stops the transmission from the B-M radio interface  605 , and then step  808  starts the timer  625   a  again. The routine then returns to step  802 . On the other hand, if answer at step  806  is negative, the routine proceeds to step  809  where the base station control section  610  controls the B-M radio interface  605  to reduce the transmission power by one level. This causes the radio zone to be reduced to the boundary  627  as shown by dotted area in FIG.  8 B. Subsequently, the fact that the transmission power of the B-M radio interface  605  has been reduced by one level is stored in the radio managing section  624 . The routine then proceeds to step  810  where the base station control section  610  controls the base station echo-back transmit-receive section  621   a  and the B-M radio interface  605  so as to execute the echo-back process again. However, in the echo-back process executed at step  810 , the deletion of the data and the mobile station ID of the non-responsive mobile station  602  from the managing table  609   a  and the mobile station ID storing section  608  is not performed, but the number of the responsive mobile stations  602  is only counted. Subsequently, step  811  checks whether the number of the mobile stations counted at step  810  and the number of the mobile stations stored in the mobility history managing section  623  before reducing the radio zone at step  809  are equal to each other. If answer at step  811  is positive, the routine returns to step  809  so as to further reduce the transmission power of the B-M radio interface  605  by one level, and proceeds to step  810  so as to execute the echo-back process. On the other hand, if answer at step  811  is negative, the routine proceeds to step  812  where the transmission power of the B-M radio interface  605  is increased by one level, and then to step  813  where the echo-back process is executed in the same manner as at step  810  so as to derive the number of the responsive mobile stations. 
   Thereafter, step  814  checks whether the number of the mobile stations stored in the mobility history managing section  623  at step  805  and the number of the mobile stations derived at step  813  are equal to each other. If answer at step  814  is positive, the routine proceeds to step  815  where the transmission power of the B-M radio interface  605  is decided to be the level as set at step  812 . On the other hand, if answer at step  814  is negative, step  816  checks whether the transmission power of the B-M radio interface  605  is set maximum. If answer at step  816  is positive, the routine proceeds to step  815  where the transmission power of the B-M radio interface  605  is decided to be the maximum. On the other hand, if answer at step  816  is negative, the routine proceeds to step  812  where the transmission power of the B-M radio interface  605  is increased by one level. 
   When the transmission power of the B-M radio interface  605  is decided, the base station control section  610  controls the B-M radio interface  605  to perform transmission in the decided power level. Subsequently, at step  817 , the decided power level is stored in the radio managing section  624 . The routine then proceeds to step  818  which starts the timer  625   a  again, and then returns to step  802 . 
   As described above, according to the second preferred embodiment of the present invention, the output level of the B-M radio interface  605  is changed depending on the situation where the mobile stations are concentrated around the base station  601  or no mobile station is present in the radio zone of the base station  601 . Accordingly, a power consumption of the B-M radio interface  605  can be reduced as compared with the conventional case where the transmission power of the B-M radio interface  605  is always set to the maximum value. 
   In the second preferred embodiment, the timer values of the echo-back timer control section  621   b  and the radio zone timer control section  625  may be fixed values, or may be dynamically changed depending on the number of the mobile stations  602  in the radio zone. When, for example, variation in the number of the mobile stations  602  of the mobility history managing section  623  is significant, the more flexible zone control can be achieved by dynamically changing those timer values. 
   Further, in the second preferred embodiment, the transmission power of the B-M radio interface  605  can be switched between three levels. However, as appreciated, the number of the switchable levels is not limited to three, but may be set to another value. 
   It is to be understood that this invention is not to be limited to the preferred embodiments and modifications described above, and that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.