The block diagram of FIG. 1 shows one example of a prior art cordless telephone. A battery 1, which is chargeable through a charging terminal 2, supplies power to a power source circuit 3 by closing a power switch 4. Power source circuit 3, under control of a control circuit 5, converts the voltage from battery 1 to an appropriate voltage, and supplies this voltage to control circuit 5, to a transmitter 6 and to a receiver 7.
In a waiting state, i.e., awaiting an incoming call, the converted voltage from power source circuit 3, controlled by circuit 5, is intermittently applied to receiver 7. Upon receipt of a group signal, the receiver is synchronized with the transmitted signal from a base station for receipt of further intermittent signals. Consequently, receiver 7 carries out reception operation intermittently as shown in FIG. 2, thereby saving power consumption of the cordless telephone.
A significant problem in these prior art systems is the inability of the user of the battery powered radio to detect whether the system is operating properly. In particular, it has become necessary for the user to determine whether the communication link from the base station to the radio receiver is operating properly. Any defect in the base transmitter, or in the radio receiver, or due to interference could prevent the reception and processing of the data at the receiver. Not only is it important to identify the existence of a problem but to indicate it to the user in a manner which will avoid a large consumption of power.
The synchronized operation of several battery powered radio devices incorporating the prior art battery saving function is shown in FIGS. 3(a)-(c).
Referring to FIG. 3(b), all devices in a system are divided into M groups, for example, 3 groups. A base unit repeatedly broadcasts a frame signal consisting of M group signals. Each group signal includes preamble words and N calling words (FIG. 3(a)). The preamble word includes a predetermined synchronization signal for synchronization of received signals and a group identification signal for showing that the following paging words are addressed to pagers belonging to the group designated by the group identification signal. In response to a calling request, an identification signal of the device to be called is assigned to one of calling words 1 to N.
Referring to FIG. 3(c), if a device is turned on, the device examines the received signals to detect the synchronization signal. Once detecting the synchronization signal, the device performs intermittent reception so that only a group signal to the device is received.
In fact, prior art radio devices such as the cordless devices mentioned above have not even provided any indication whether the power source switch was closed. Apparently, if such an indicator was used to display the activation of the source, the display would emit light continuously thereby increasing power consumption and shortening the active life of the radio devices. It would be possible to increase the capacity of the battery, but this would necessarily result in high cost, increased weight and longer charging times.