Abstract: A mobile communication system includes an exchange, a plurality of radio base stations coupled to the exchange, and a plurality of mobile stations. The exchange includes a device for obtaining receivable periods of the mobile stations, which are continuously maintained in a call receivable state during the receivable periods, a device for determining whether the mobile stations are busy and a decision device which sequentially sends call signals to the mobile stations to determine whether or not each of the stations responds to a call signal. The results of determination by the decision device are recorded in a recording device.

Abstract: A radio communication adapter is used in a mobile communication system including a first mobile communication network, first mobile stations registered in the first mobile communication network, a second mobile communication network connected to the first mobile communication network, and second mobile stations registered in the second mobile communication network. The radio communication adapter is attached to one of the second mobile stations. The radio communication adapter includes a radio communicating part for communicating, via a radio channel, with the first mobile stations located outside a first service area of the first mobile communication network, and a requesting part for requesting that first identification data of the first mobile stations located outside the first service area is registered, in connection with the second mobile stations, in the first mobile communication network via the above-mentioned one of the second mobile stations and the second mobile communication network.

Abstract: A portable telephone system includes a first telephone network including a terminal station, a first telephone set connected to the first telephone network via the terminal station, a second telephone network including a communication center which is connected to the first telephone network at a connection point, and at least one second telephone set registered in the communication center and connected to the first telephone network via the terminal station. The second telephone set is a portable telephone set having a base station part and a portable station.

Abstract: A digital loop filter is used in a phase-tracking circuit which is adaptable to a phase-comparison radio navigation system such as the DECCA (registered trademark) system. The digital loop filter includes a feedback circuit which receives a digital input signal from the digital loop filter and outputs a digital output signal and which feeds back and adds the digital output signal to the digital input signal after a predetermined delay. It also includes a feed-forward circuit which multiplies the digital input signal by a predetermined constant (B) and adds the resultant signal to the digital output signal of the feedback circuit to generate a digital output signal. A multiplier for multiplying the digital output signal of the feed-forward circuit by a second predetermined constant (C) to generate a digital output signal of the digital loop filter is also provided.