1. Field of the Invention
The present invention relates to a selective calling communication system and a selective calling receiver, and is applicable to, for example, a paging system utilizing FM multiplex broadcasting.
2. Description of the Related Art
Conventionally, a selective calling communication system referred to as a "paging system" has been used to call a person who has a receiver terminal unit. When a receiver terminal unit is called in this paging system, a calling party first inputs the terminal number of a desired receiver terminal unit from a predetermined telephone and then inputs information such as a message or the like also from the telephone. The input information is once transmitted to a central base station, which is a provider of communication services, through a common telephone network. The central base station, upon receiving the information, instructs an associated local base station to call the receiver terminal unit. The local base station, in response to this instruction, transmits a paging signal and the message, for example, using radio waves in a VHF band to call the receiver terminal unit. The receiver terminal unit receives the paging signal, generates call receiving sound, and displays the information such as the received message or the like on a liquid crystal display. In this way, the person having the receiver terminal unit recognizes that he is being called and can know information such as the message or the like from the calling party.
In addition to the conventional paging system mentioned above, a paging system utilizing FM multiplex broadcasting has been proposed in recent years. This system multiplexes a paging signal for calling a receiver terminal unit and information such as a message or the like on radio waves for transmitting FM radio broadcasting. Thus, the system advantageously utilizes existing transmission facilities which have been built for the FM radio broadcasting without any modification. For multiplexing the paging signal and information such as a message or the like in this paging system, multiplexing scheme such as a DARC (Data Radio Channel) scheme, a receptor scheme, or the like, is employed by way of example.
It should be noted that this kind of paging system is basically identical to conventional paging systems utilizing dedicated radio waves except that radio waves for FM radio broadcasting are employed.
In these paging systems, certain countermeasures must be taken for improving the reliability of reception. Particularly, since the paging system utilizing FM multiplex broadcasting has a less number of base stations than the paging systems utilizing dedicated radio waves, such countermeasures are indispensable.
A method of improving the reliability of reception involves, for example, calling a receiver terminal unit twice within a predetermined time interval, continually calling a receiver terminal unit for three hours at intervals of 15 minutes, or the like.
However, the twice-call method has an inconvenience that if a receiver terminal unit is in a radio wave inaccessible place, such as in a tunnel, while a paging signal is transmitted twice, the receiver terminal unit cannot receive the paging signal. The three-hour call method, in turn, has an inconvenience that as a receiver terminal unit receives the same paging signal a plurality of times, a time stamp is sequentially updated in response to repetitively received messages, thereby missing the time stamp generated when the paging signal was first received.