Abstract:
A selective calling receiver enabling a user to know exactly whether the receiver used by the user is placed within an area where a transmitted radio wave can be received well. The selective calling receiver collates a frame synchronization signal in the transmitted signal at predetermined intervals and as far as the collation is successful, the synchronization is kept and the receiver is operated in an intermittent reception mode. When the collation is not successful, an alarm is issued after a predetermined time to inform the user that the receiver has been placed outside of the area where the transmitted radio wave can be received well.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an individually selective calling receiver for use in a mobile communication system or the like. 
     An individually selective calling receiver is carried by a person who is outdoors or away from his desk and receives a calling signal sent by means of an electric wave to respond to it. In such a receiver, different identification numbers are assigned to a plurality of receivers, respectively, and a transmitter sends the identification number assigned to a specific receiver to selectively call the receiver. Accordingly, in the individually selective calling receiver, reception sensitivity is most important and even when a user of the receiver is present in any area, it is desirable to be able to receive a call. 
     In the conventional individually selective calling receiver, however, an interval of transmission time is about five minutes with transmission for two minutes and a pause of transmission for three minutes repeated. However, there is a case where an electric wave is attenuated behind a building or within a building and a field intensity of the electric wave does not reach reception sensitivity. Thus, there is a problem that the user does not understand whether it is possible to receive a call in his place during a time domain for the pause of transmission. 
     A technique to solve the above problem is disclosed in Japanese Patent Unexamined Publication No. 2-266617. In this publication, the reception intensity of an electric wave sent from a base station is detected and when the reception intensity is smaller than a first threshold it is judged that the receiver is placed outside of an area where the electric wave can be received while when the reception intensity is larger than the first threshold but smaller than a second threshold it is judged that the receiver is placed within the area but can not receive the electric wave exactly with reliability. 
     In the above method, since judgment is made on the basis of the intensity of the received electric wave, the interference condition or the reception condition of the electric wave is different depending on circumstances of a building or the like even if the receiver is placed within the area and accordingly it is difficult to set the threshold. 
     SUMMARY OF THE INVENTION 
     The present invention is to solve the above problems in the prior art by providing an apparatus for selectively receiving a calling signal having the function of informing a user of an alarm when the field intensity of an electric wave is small or when the receiver is moved outside of an area where the electric wave can be received and capable of exactly informing the user that the receiver used by the user is moved outside of the area. 
     According to the present invention, a frame synchronization signal in a transmission signal is collated at predetermined intervals to keep synchronization and an alarm is generated after an elapse of a predetermined time when the collation of the frame synchronization signal has not been successful, so that conditions within and outside the area where the receiver can receive the electric wave can be judged relatively to exactly inform the user that the receiver is moved outside of the area. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram schematically illustrating a selective calling receiver according to an embodiment of the present invention; 
     FIG. 2 is a flow chart showing operation of the selective calling receiver of FIG. 1; and 
     FIG. 3 is a timing chart for explaining operation of the selective calling receiver of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a block diagram schematically illustrating a selective calling receiver according to the present invention. In FIG. 1, numeral 11 denotes an electric wave receiving unit which receives an RF electric wave sent from a transmitting station through an antenna 10 and amplifies the received signal to supply it to a demodulation unit 12. The demodulation unit 12 demodulates the received signal and supplies the demodulated signal to a synchronization keeping (maintaining) circuit 14. Numeral 13 denotes an IDROM constituting read-only memory means in which an individual selective calling number for each receiver is stored and is supplied to the demodulation unit 12. 
     The synchronization keeping circuit 14 is supplied with an output of the demodulation unit 12 and performs synchronization collation or detection (hereinafter referred to as SC). The circuit 14 keeps a frame synchronization signal in a preamble. Numeral 15 denotes a control circuit (hereinafter referred to as a CPU). When the CPU 15 receives the frame synchronization signal 14a held in the synchronization keeping circuit 14, that is, a detection signal of the frame synchronization signal, the CPU is adapted to start an intermittent reception timer 16, and when synchronization keeping operation is released, the CPU is adapted to start a synchronization keeping (maintaining) watch timer 17. The synchronization keeping watch timer 17 and the intermittent reception timer 16 constitute a timer circuit. 
     Numeral 18 denotes an alarm drive informing (generating) circuit which is started by an output of the CPU 15 and drives a sound generator 19 such as a loud speaker. The alarm drive informing circuit 18 and the bound generator 19 constitute informing means. 
     Numeral 20 denotes a battery power supply which applies an operation voltage to the CPU 15. Numeral 21 denotes an intermittent reception control circuit which is supplied with an output of the intermittent reception timer 16 and controls the electric wave receiving unit 11 and the demodulation unit 12 to be in synchronous intermittent reception condition. The intermittent reception control circuit 21 always controls to save electric power of the battery. 
     Operation of the embodiment is now described with reference to the timing chart of FIG. 3. The electric wave received by the electric wave receiving unit 11 is demodulated and a reception signal is obtained in response to a transmission signal 31 shown in FIG. 3(A). 
     The collation and the keeping of SC are made in the synchronization keeping circuit 14. When the frame synchronization signal is detected subsequently to the preamble of the transmission signal 31, the synchronization keeping circuit 14 keeps the synchronization as shown by signal 32 in FIG. 3(B) until the transmission is cut off (FIG. 3(A)). The transmission signal 31 shown in FIG. 3(A) has a signal format conforming with the CCIR No. 1 system, for example. 
     The keeping of the frame synchronization signal is such that, in the operation in which a timing of the frame synchronization signals, inserted at regular intervals by the CCIR No. 1 system after the frame, synchronization collation is measured to make collation of the SC and it is confirmed that the frame synchronization signals are inserted at one Batch (equal to 1.0625 seconds) of a predetermined time sequence, the frame synchronization signal is collated only in the predetermined time sequence in a period of time that the frame synchronization signals are successfully confirmed n (n is a natural number, for example 2) times continuously in the predetermined time sequence. A period of time that the frame synchronization is kept, that is, a period of time that frame synchronization signals are detected in the predetermined time sequence means that a group of receivers to which the own receiver of interest belongs is called. 
     At the same time as the CPU 15 receives a detection signal of the frame synchronization signal 14a, the CPU 15 starts the intermittent reception timer 16 and sets the intermittent reception control circuit 21 to the synchronous intermittent reception condition to start the electric wave receiving unit 11 and the demodulation unit 12 to perform the synchronous intermittent reception operation. 
     In the synchronous intermittent reception operation, effected are the SC collation in the predetermined time sequence and the collation operation of the individual selective calling number in a predetermined time sequence subsequent to the frame synchronization signal named the own frame. More particularly, the individual selective calling number stored in the IDROM 13 is read out and supplied to the demodulation unit 12 to demodulate this individual selective calling number and the received data, and the demodulated individual selective calling number is collated in the CPU 15. Successful collation result represents that its user is called, and the alarm drive informing circuit 18 drives the sound generator 19 to produce a calling sound. 
     Even if the SC collation is not successful, the synchronization keeping operation is not released immediately and the synchronization keeping operation is continued by the synchronization recovery function as a signal 33a shown in FIG. 3(B) until the state that the SC collation is not successful is repeated two times or less. When the state that the SC collation is not successful is continued two times and the synchronization keeping operation by the signal 33a is released, the CPU 15 then starts the synchronization keeping watch timer 17 newly as shown by a signal 35 of FIG. 3(C) and starts a counting operation for a time (TA) for judging that the receiver is placed outside of the predetermined receivable area. 
     If the frame synchronization is acquired before the time TA expires, it is judged that the receiver is placed within the receivable area and the synchronization keeping watch timer 17 is reset as shown by signal 35 of FIG. 3(C) so that the intermittent reception control circuit 21 is set to the synchronization intermittent reception condition by means of the intermittent reception timer 16. 
     On the other hand, as shown by the operation subsequent to the signal 33b of FIG. 3(B), when the frame synchronization is not acquired until the synchronization keeping watch timer 17 reaches the time TA, the timer is timed out as shown by a signal 36 of FIG. 3(C) and it is judged that the receiver is placed outside of the receivable area. Thus, the alarm drive informing circuit 18 is started at the timing shown by a signal 37 of FIG. 3(D) to drive the sound generator 19 so that the alarm indicating that the receiver is placed outside of the receivable area is informed to the user. It is needless to say that the alarm sound generated at this time is different from the calling sound in modulation and rhythm. 
     FIG. 2 is a flow chart showing operation until the alarm is issued when the receiver is placed outside of the receivable area as described above. When the synchronization is acquired at steps 21a and 22a, the intermittent reception timer 16 is started at step 23 and an SC non-collation counter (hereinafter referred to as an SC counter) of the synchronization keeping watch timer 17 is reset (initialized) to zero at step 24. 
     When the intermittent reception operation is started by the intermittent reception timer 16, the SC collation in the predetermined time series at step 25a and the collation operation of the individual selective calling number in the predetermined time series subsequent to the own frame synchronization signal at step 25a are made. 
     While the synchronization keeping operation is continued (corresponding to the signals 32-34 of FIG. 3(B)), a count of the TS counter is less than 2 and the synchronous intermittent reception condition is maintained. 
     When the SC collation for keeping the synchronization at step 25b is not successful (out of synchronization), the process proceeds to step 26a and an out-of-synchronization counter TSC 1 is incremented by one. 
     Further, at step 26b, if the out-of-synchronization condition is continued until the TSC is equal to two in the process of step 26a, the intermittent reception timer 16 is released and the synchronization keeping operation is released at step 26c. When the synchronization is collated again, the process is returned from step 26b to step 24 to reset the counter TSC to 0. 
     When the intermittent reception timer 16 is released, the synchronization keeping watch timer 17 is initialized (T2=0) at step 27a and is then started at step 27b, so that the asynchronous reception operation is started at step 21b. The frame synchronization acquiring operation is continued at step 21b until the predetermined value (time TA) is exceeded or until the synchronization is acquired again in the asynchronous reception operation. The synchronization acquiring operation adds one to the contents T2 of a counter of the synchronization keeping watch timer 17 if there is no SC at step 22b, and whether the contents reaches the predetermined value TA or not is judged at step 28b. When the contents does not reach the predetermined value TA, the process is returned to step 21b and the synchronization acquiring operation is made again. Then, the SC collation is made at step 22b and the same operation is repeated. Thus, if the synchronization is acquired again at step 21b before the count of T2 reaches the time TA, the process is returned to step 23 in accordance with judgment at step 22b and the receiver enter to the normal intermittent reception state. 
     On the other hand, at step 28b, when the count T2 of the synchronization keeping watch timer 17 reaches or exceeds the time TA, the alarm informing function is started at step 29 and the user is informed that the receiver is placed outside of the reception area. Then, the process is returned to step 21a and the asynchronous reception operation which is the synchronization acquiring operation is kept. 
     As described above, according to the embodiment, when the frame synchronization signal received by the electric wave receiving unit 11 is collated at predetermined intervals to keep synchronization, the intermittent reception timer 16 is started by the CPU 15 to set the intermittent reception control circuit 21 to the synchronous intermittent reception condition and the electric wave receiving unit 11 and the demodulation unit 12 are operated. Then, data from the IDROM 13 is supplied to the demodulation unit 12 and collation of the individually selective calling number is made in the CPU 15. When the collation of the frame synchronization signal is not successful, the alarm drive informing circuit 18 is driven by an output of the synchronization keeping watch timer 17 after elapse of the predetermined time so that the sound generator 19 is operated. Accordingly, when the field intensity is lowered or the receiver is moved outside of the receivable area, the alarm can be informed and the user can understand the current position. Thus, even if the collation of the frame synchronization is not successful, the synchronization is kept by the synchronization recovery function within a fixed time. Further, when the state that the synchronization is not collated is continued, it is judged that the receiver is placed outside of the receivable area after the predetermined counting operation by the timer circuit and the informing means is started to inform the alarm. Accordingly, when the field intensity is small or the receiver is moved outside of the receivable area, the alarm is issued to inform it and the position of the user can be detected by collation of the synchronization signal instead of the individually selective signal.