1. Field of the Invention
The invention relates to a method of judging whether a radio receiver is situated in which area, a circuit for judging the same, and a radio selective-calling receiver including the circuit, and more particularly to such a method, a circuit, and a radio selective-calling receiver to be used in a radio selective-calling system including a plurality of radio frequency channels having different radio frequencies.
2. Description of the Related Art
In these days, there has been increased a demand for a radio selective-calling receiver (hereinafter, referred to as "scanning receiver") having a plurality of frequencies through which a calling number of the radio selective-calling receiver is transmitted. Such a scanning receiver is used as a multi-area type receiver which automatically switches a plurality of areas having different frequencies. In a scanning receiver, since a calling number of the receiver is transmitted through a plurality of frequencies, a transmitted signal to be received is searched as the frequencies are scanned, when the scanning receiver is without a wireless range.
When a synchronization signal has been received, it is stopped to scan frequencies. Then, a certain frequency is fixed to thereby put the scanning receiver in a condition for receiving its calling number, that is, in an in-range condition. Once the scanning receiver has been transferred into an in-range condition, the certain frequency is kept fixed until requirements for transferring to an out-range condition are satisfied. Namely, a scanning receiver has a fixed frequency in an in-range condition, and after transferring to an out-range condition, a scanning receiver ceases synchronization operation having been conducted with transmitted signals in an in-range condition, and detects a synchronization signal with frequencies being switched. Thus, there is accomplished automatic selection in multi-areas.
For instance, Japanese Unexamined Patent Publication No. 6-315001 having been published on Nov. 8, 1994 has suggested such automatic multi-area selection wherein frequencies are scanned in a predetermined pattern in an in-range condition.
An operation for an out-range condition usually starts when a synchronization signal is not received a predetermined number of times, or when a synchronization signal is not received by a certain period of time.
The above-mentioned scanning receiver cannot concurrently a plurality of frequencies at a time, an interval for searching a synchronization signal at a certain frequency cannot avoid becoming longer in proportion to the number of scanning frequencies. That is, as the number of frequencies to be searched increases, it would take much time to detect a synchronization signal, even if the scanning receiver is situated within an area in which transmitted signals can be received. This is accompanied with a problem that it takes much time to transfer an out-range operation to an in-range operation.
As a solution to this problem, Japanese Unexamined Patent Publication No. 6-244777 having been published on Sep. 2, 1994 and having been patented as Japanese Patent No. 2626504 on Apr. 11, 1997, has suggested a radio selective-calling receiver where an electric field is detected when a synchronization signal is searched, to thereby shorten time necessary for searching a lot of frequencies.
On the other hand, considering battery saving, there is another problem. If battery saving is taken precedence over detection of a synchronization signal, it is delayed to detect a synchronization signal, because battery saving would need much time. To the contrary, if synchronization is intended to promptly re-established, there would be caused a problem of poor battery saving.
When a radio selective-calling receiver is situated in an area to which radio is interrupted to enter, such as a tunnel, it would be frequently preferable to promptly re-establish synchronization rather than carry out battery saving. To the contrary, when a radio selective-calling receiver is situated in an area other than a service area where transmitted radio signal can be received, it would be preferable to carry out battery saving rather than re-establish synchronization.
Thus, if an out-range condition is defined to include both the above-mentioned area to which radio is interrupted to enter, such as a tunnel and an underground market, and the above-mentioned area other than an area where transmitted radio signal can be received, such as an area other than a service area, it would be quite difficult or almost impossible to concurrently satisfy prompt battery saving and establishment of synchronization at a time.
In view of such a problem, Japanese Patent Applications Nos. 8-203958 and 8-215697 have suggested a radio selective-calling receiver capable of promptly re-establishing synchronization with battery saving being taken precedence, taking the above-mentioned two areas into consideration.
The radio selective-calling receiver suggested in the above-mentioned Japanese Unexamined Patent Publication No. 6-244777 can surely shorten a period of time necessary for searching a lot of frequencies, but is accompanied with a problem that it is impossible to concurrently satisfy battery saving and prompt re-establishment of synchronization, if an out-range condition is defined to include both an area to which radio is interrupted to enter, such as a tunnel and an underground market, and an area other than an area where transmitted radio signal can be received, such as an area other than a service area.
The radio selective-calling receivers suggested in the above-mentioned Japanese Patent Applications Nos. 8-203958 and 8-215697 can concurrently satisfy battery saving and re-establishment of synchronization, if it is possible to accurately judge whether the radio selective-calling receiver is in an area to which radio is interrupted to enter, such as a tunnel and an underground market, or an area other than an area where transmitted radio signal can be received, such as an area other than a service area.
The above-mentioned radio selective-calling receivers can judge they are situated in an area to which radio is interrupted to enter, such as a tunnel and an underground market, if they promptly enter the area. However, if they gradually enter the area, it would be quite difficult for them to judge whether they are located in the area or an area other than an area where transmitted radio signal can be received, such as an area other than a service area. As a result, the above-mentioned radio selective-calling receivers frequently misjudge that they are situated in an area other than an area where transmitted radio signal can be received, such as an area other than a service area.