Portable telephone set

The present invention relates to a portable telephone set and a portable telecommunication apparatus to be suitably used for a second generation cordless telephone system and, more particularly, it relates to a telephone set and a portable telecommunication apparatus having a function of automatically determining if it is in or out of the service area. Such a telephone set is known from US-A-5 127 042

With the rapid development of digital technologies in recent years, mobile telecommunication systems using digital communication scheme are becoming more and more popular instead of those using analog communication scheme. The mobile telecommunication system necessarily involves exchange of control signals between radio base stations connected to a telecommunication network and portable telecommunication apparatuses belonging to the system. A basic system for realizing high efficient and high quality radio communications can be constructed by using digitized control signals. With the application of digital technologies to the mobile telecommunication system, both the radio base station and the portable telecommunication apparatuses can be downsized so that the portable telecommunication apparatuses may provide an enhanced level of portability.

Thus, the number of subscribers of portable telephone sets which is a kind of the portable communication apparatus utilizing digital technologies are remarkably increasing. The digital communication system is free from the problems that conventional analog communication systems are hard to meet the increasing number of subscribers for mobile telecommunication systems because of restricted frequency allocation and has a deficiency of limited confidentiality. Recently, a second generation cordless telephone system utilizing widely such digital technologies has been intensively developed.

In the second generation cordless telephone system, a vast number of channels is created by limiting the effective area of each single portable telephone set to a small zone called micro cell and allocating repeatedly a single frequency band to different subscribers through the time division multiplex so that the limited number of frequency resources available for telecommunications can be effectively exploited. Since each zone (or cell) is made small, only a low output is required for the portable telephone set so that it may consume power at a reduced rate and can be constructed to be compact. Hence the telephone set can operate for a prolonged period and it can use a small size battery.

In the second generation cordless telephone system using portable telephone sets, telecommunication channels are established separately between portable telephone sets and a public radio base station connected to the telecommunication network and between the portable telephone set and a private radio base station set (a base station telephone set) which is the base station of the portable telephone set. A particular service area is assigned to the base station, where it can communicate with its portable telephone sets. The public radio base station can be called by any portable telephone sets whereas the private base station set can be called only by its portable telephone set.

In the second generation cordless telephone system, each portable telephone set can be used as an extension set (child set) of a base station set (parent set) of a conventional telephone system having an automatic answering function. The portable telephone set, on its part, is provided with a remote control function, with which, for example, it can obtain messages stored in the base station set during the subscribers' absence by operating remotely the automatic answering function of the base station set in the home from the portable telephone set.

On the other hand, the base station set can also be accessed from and controlled by a public telephone set. If such is the case, a particular identification number which is used to determine whether the remote operation from the public telephone set is allowered is normally registered in the base station set in advance. Then, the subscriber who owns the base station set and hence knows the identification number and the key number corresponding to a function to be operated by the remote operation can successfully obtain the stored message during the subscriber's absence, by calling the base station set through the public telephone set which can transmit the tone signal and then inputting the identification number to the base station set and operating the key number which corresponds to a function to be operated.

However, in case of a portable telephone set which is used in the second generation cordless telephone system and is also used at home as an extension set of a base station set of a conventional telephone system, when an automatic answering function etc. of the base station set is used by means of a remote control operation, two different modes of operation are required for users to use it, one for remotely controlling the base station set having an automatic answering function outside of home and the other for using it at home. Then, the subscriber have to memorize two different sets of procedures for remotely controlling the base station set in two different modes, demanding troublesome remote control procedures to the subscriber, which are hard for the subscriber to remember.

A portable telephone set is normally provided with a display screen that displays an "OUT-OF THE SERVICE AREA" message when it has moved out of the service area of the public radio base station, telling that no telecommunications channel can be established between them any more. When the portable telephone set returns to the service area, the display is cleared to tell that it can now call the base station or vice versa.

Thus, unless the subscriber carrying the portable telephone set always check the display screen, the subscriber cannot know if he or she is in the service area of the radio base station whenever the subscriber is moving with the portable telephone set.

As a kind of portable communication apparatus, there is a transceiver which can communicate by directly transmitting and receiving radio signals. However, when the destination transceiver is not in the neighborhood, the transceiver requires the user to call the destination transceiver frequently to determine whether or not the destination transceiver is located within its service area since the service area of the transceiver is normally very small. Thus a serious inconvenience is caused to operate the transceiver.

In the second generation cordless telephone system, each portable telephone set is required to have a transceiver function so that a communication channel may be established between any two portable telephone sets without the radio base station if they are located within their service areas. However, since the antenna power of a portable telephone set of the second generation cordless telephone system is limited as low as less than 10 milliwatts, the service areas of the portable telephone sets would be limited to a narrow range having a radius as small as 50 to 100 meters.

Any known portable telephone set that can operate as a transceiver has to have its transceiver function canceled to return to a normal portable telephone set or move around to enter the service area of the destination telephone set, if it has moved out of the service area of the destination portable telephone set and needs to contact with the destination portable telephone set immediately while it is operating as a transceiver.

It is therefore the object of the present invention to provide a portable telephone set to be used for a second generation cordless telephone system in which substantially identical remote operations for function added to the base station set of a first generation cordless telephone system can be utilized at home and outside home thus simplifying the remote operation of the base station set outside home.

According to an aspect of the present invention, there is provided a portable telephone set with which the user can easily find out if the user enters the service area of the radio base station to establish a telecommunications channel with the radio base station.

According to another aspect of the present invention, there is provided a portable telephone set equipped with a transceiver function with which the destination telephone set can be called without fail.

According to still another aspect of the present invention, there is provided a portable telephone set capable of establishing a telecommunications channel with a public radio base station connected to a telecommunications network and a base station telephone set, the portable telephone set comprising: an input means for remotely controlling the base station telephone set; a determination means for determining if the portable telephone set is within or out of the service area of the base station telephone set; and a control means for transmitting a remote control signal corresponding to the operation to be carried out by remote control to the base station telephone set via the public radio base station and the telecommunications network to remotely control the base station telephone set by the input means if it is determined by the determination means that the portable telephone set is out of the service area of the base station telephone set.

In the present invention, a portable telephone set can simply conduct a remote control of the base station telephone set out of the service area of the base station telephone set.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a schematic illustration of a first embodiment of portable telephone sets according to the present invention incorporated in a second generation cordless telephone system;</li><li>FIG. 2 is a schematic perspective view of a base station telephone set to be installed in a subscriber's home of the second generation cordless telephone system shown in FIG. 1;</li><li>FIG. 3 is a schematic perspective view of a portable telephone set to be used in the cordless telephone system of FIG. 1;</li><li>FIG. 4 is a schematic plan view of the key input section of the portable telephone set of FIG. 3;</li><li>FIG. 5 is a block diagram of the portable telephone set of FIG. 3;</li><li>FIG. 6 is a schematic illustration of the configuration of the control and telecommunications physical slots of a second generation cordless telephone system;</li><li>FIG. 7 is a flowchart of the operation for determining if the portable telephone set of FIG. 5 is located within the service area of the base station telephone set, that of the public radio base station or out of the public radio base station;</li><li>FIG. 8 is a flowchart of the operation for processing a stand-by process to be carried out in step S7 of FIG. 7;</li><li>FIG. 9 is a flowchart of an alternative operation that can be used for determining if the portable telephone set of FIG. 5 is located within the service area of the base station telephone set, that of the public radio base station or out of the public radio base station;</li><li>FIG. 10 is a flowchart of the operation for processing a stand-by process to be carried out in step S35 of FIG. 9;</li><li>FIG. 11 a flowchart of the basic operation of the portable telephone set of FIG. 5 when the key input section of FIG. 4 is used to remotely control the base station telephone set;</li><li>FIGS. 12A and 12B are schematic illustrations of the configuration of a signal to be transmitted when the portable telephone set of FIG. 5 is used to remotely control the base station telephone set within the service area of the public telecommunications network;</li></ul>

A preferred embodiment of the present invention will be described in greater detail by referring to the accompanying drawings.

FIGS. 1 through 12 illustrate a preferred embodiment of portable telephone set according to the present invention to be suitably used for a second generation cordless telephone system.

FIG. 1 illustrates a schematic view of a second generation cordless telephone system. First the overall configuration of the system will be described. Referring to FIG. 1, a second generation cordless telephone system typically comprises a public telephone (telecommunication) network 1 and a large number of public radio base stations installed in public facilities such as public telephone booth 2 and railway stations 4, and base station telephone sets installed in subscribers' homes 3. The public radio base stations 2a and 4a of public telephone booths 2 and railway stations 4 and the base station sets 3a of subscribers' homes 3 are connected to the network 1. Each of the public base stations 2a and 4a of public telephone booths 2 and railway stations 4 is accessible to a number of portable telephone sets 2b, 2c or 4b, 4c that are affiliated to it while each of the base station telephone sets 3a is accessible only to its portable telephone set 3b. Additionally, public radio base stations 2a and 4b are accessible to portable telephone sets 3b, as well as the portable telephone sets 2b, 2c, 4b and 4c, when the portable sets 3b are out of their respective homes. The public telephone network 1 may be a PSTN (Public Services Telephone Network) or an ISDN (Integrated Services Digital Network).

FIG. 2 schematically illustrates a base station telephone set 3a that can be used for the system of FIG. 1. Referring to FIG. 2, the base station set 3a comprises a telephone receiver (handset) 11, a key input section 12 provided with push buttons for keying in numbers and signs and mode selection keys for operating a voice recorder of an automatic answering system 13, an automatic answering system 13 records the callers message when the recording mode is selected, a liquid crystal display section 14 for displaying the keys being pushed, the selected mode of operation of the voice recorder 13 or one of the functions of the telephone set currently selected by the user, and an antenna 15 for transmitting and receiving radio signals to/from the portable telephone set 3b.

FIG. 3 is a schematic perspective view of a portable telephone set 3b. Referring to FIG. 3, it comprises an antenna 21 for radio signal transmission and reception with the base station telephone set 3a or a public radio base station 2a or 4a, a liquid crystal display section 22 for displaying the keys being pushed or one of the functions of the telephone set currently selected by the user, a key input section 23 provided as shown in FIG. 4 with push buttons 23a for keying in numbers and signals and remote control buttons 23b for remotely operating the voice recorder of the base station telephone set including replaying, erasing and recording keys, a loud speaker 24 for reproducing the received voice, a microphone 25 for picking up the user's voice, and a loud speaker 26 for the ringer of the set.

Referring to FIG. 4, the push buttons 23a are used to key in the telephone number of the called party and/or the numerical data necessary to remotely control the base station set 3a, whereas the remote control buttons 23b are used to remotely control the voice recorder of the base station set 3a for recording, reproduction and erasure of speech.

FIG. 5 illustrates a block diagram of the portable telephone set of FIG. 3. As shown in FIG. 5, it comprises an antenna 21, a liquid crystal display (LCD) 22, a key input section 23, a loud speaker 24, a microphone 25, a loud speaker for ringer 26, a switch 27, a receiver 28, a transmitter 29, a synthesizer 30, a digital modulator/demodulator (MODEM) section 31, a TDMA processor 32, a speech coder/decoder (CODEC) section 33, an voice conversion circuit section 34, a controller 35, a ROM 36 and a RAM 37. The antenna 21 is used for to transmission and reception of signals in a predetermined frequency band including call signals and speech signals between the portable telephone set and the base station telephone set 3a located in the home 3 or a public radio base station 2a or 4a located in a public telephone booth 2 or a railway station 4. Signals received by the antenna 21 are then forwarded to the receiver 28 via the switch 27, whereas signals to be transmitted by the antenna 21 comes from the transmitter 29 via the switch 27. The switch (SW) 27 is controlled by the control section 35 for its switching operation to alternatively connect the antenna 21 to the receiver 28 or the transmitter 29. Normally, the antenna 21 is kept connected to the receiver 28 unless it is specifically connected to the transmitter 29 for signal transmission.

Upon receiving signals in a reception frequency band specified by the synthesizer 30 from the antenna 21 via the switch 27, the receiver 28 amplifies them with a given amplification factor and sends the amplified signals to the digital MODEM section 31. Upon receiving signals from the digital MODEM section 31 for transmission, on the other hand, the transmitter 29 transmits them out of the antenna 21 via the switch 27 in a transmission frequency band specified by the synthesizer 30.

As a particular oscillation frequency band is specified by the controller 35, the synthesizer 30 sets it up in the receiver 28 and the transmitter 29. The digital MODEM section 31 digitally demodulates digitally modulated signals received by the portable telephone set and coming from the receiver 28 and also digitally modulates signals for transmission coming from the TDMA processor 32 before sending them to the transmitter 29. The TDMA processor 32 exchanges speech signals with the digital MODEM section 31 and the speech CODEC section 33 for transmission and reception and telecommunications control signals with the control section 35 so that it may extract actual data from the digital data it receives in predetermined time slots and convert each set of digital data (voice data) generated in the portable telephone set into one having a given data format by adding to it a header it receives from the controller 35 before sending it out to the digital MODEM section 31 in predetermined time slots. The speech CODEC section 33 expands digital voice data it receives from the TDMA processor 32 that are compressed by a technique such as ADPCM and sends them out to the voice conversion circuit 34. It also compresses digital voice data coming from the voice conversion circuit 34 typically by ADPCM and sends them out to the TDMA processor 32. The voice conversion circuit 34 is equipped with an A/D converter and a D/A converter, and converts the digital speech data derived from the speech CODEC section 33 into an analog speech signal which will then be outputted to a speaker 24. The voice conversion circuits 34 also converts a speech signal inputted from a microphone 25 into a digital speech signal which will then be outputted to the speech CODEC section 33.

The controller 35 comprises a CPU (Central Processing Unit), a timer and other components, and executes a communication control process program of the portable telephone set stored in the ROM 36 for executing a communication control operating, controlling the operation of each and every component of the portable telephone set. It also controls the operation of displaying the telephone number input by the key input section 23 on the LCD 22 and the operation of the speaker 26 for the ringer of the portable telephone set and executes a determination process program stored in the ROM 36 for determining if the portable telephone set is located within the service area of a radio base station so that the result of the program execution may be displayed on the LCD 22.

As will be described hereinafter, the controller 35 also control the operation of transmitting the telephone number and a remote control signal depending on the results of the above execution of the program for determining if the portable telephone set is located within the service area of a radio base station.

The ROM (Read Only Memory) 36 stores the ID number of the portable telephone set, that of the base station telephone set, the communication control process program executed by the controller 35, and the determination process program for determining if the portable telephone set is located within the service area of a radio base station. The RAM (Random Access Memory) 37 is used as data areas for temporarily storing various data when the controller 35 executes the communication control process and the determination process used for determining if the portable telephone set is located within the service area of a radio base station. The RAM 37 also has memory areas for temporarily storing the telephone number of the base station telephone set, a # signal, the ID number for canceling the recording mode of the base station telephone set, a time to be set in the timer, information to be notified, and data necessary for remote control operation, and setting a timer flag, a flag of a private reception area, a flag of a public service area and a flag of out-of public service area. The LCD 22 displays various data provided by the controller 35 as well as the result of the determination process of determining if the portable telephone set is in or out of the service area of the base station telephone set or the public radio base station. The ringer 26 is driven by a ringer signal which is output by means of the communication process executes by the controller 35 to generate a ringer sound whenever the portable telephone set is called.

Operations of the preferred embodiment having the above described configuration will be described below.

First, slots (channels) to be used by the portable telephone set to communicate with the private radio base station (base station telephone set 3a) and the public radio base station will be described with reference to FIG. 6.

Referring to FIG. 6, two different types of slots are used for radio communication by the portable telephone set. One is the control physical slot (control channel) and the other is communication physical slot (communication channel). The control channel is divided into a private (home) control channel and a public control channel. Note that a portable telephone set can establish communication channels with a public radio base station and with the private radio base station, and respective base radio stations have predetermined respective service areas for establishing communication channels therein. That is, if the portable telephone set is out of the service area of the public radio base station (if it can no longer receive control signals from the public radio base station through the established public control channel), a message "OUT-OF PUBLIC SERVICE AREA" is displayed on the LCD 22 to inform the user that he or she cannot call or be called via the public radio base station. On the other hand, if it enters the service area of the public radio base station, a message "PUBLIC SERVICE AREA" is displayed on the LCD 22 to inform the user that he or she now can call or be called via the public radio base station.

If the portable telephone set enters the service area of the private radio base station (if it can now receive control signals from the private radio base station through the established private control channel), message "PRIVATE RECEPTION AREA" is displayed on the LCD 22 to inform the user that he or she can exchange information through the private radio base station.

Now the process of determining if the portable telephone set is located within the service area of the base station telephone set (in the home), that of the public radio base station (except for that of the base station telephone set) and the out-of service area of the public radio base station will be described by referring to the flow chart of FIG. 7.

Referring to FIG. 7, a first private control channel is made ready for signal reception (step S1) and then it is determined if the ID number of the base station telephone set being transmitted by radio from the base station set can be detected through the control channel (step S2). If the ID number of the base station set is not detected through the currently activated control channel and if all the private control channels have not been completely scanned (step S3), the reception frequency of another private control channel is set in order to make the another private control channel ready for signal reception (step S4). The process returns to step S2 to once again find out if the ID number of the base station set is detected through the currently activated control channel.

If, on the other hand, the ID number of the base station set is detected in step S2, a flag of private reception area is set up (step S5) and a private reception area message is displayed on the LCD 22 (step S6) so that the process moves to a stand-by status and monitors any out-of synchronism (step S7). If an out-of synchronism is found, the process goes back to step S1.

Meanwhile, if the ID number of the base station set has not been detected even when the operation of scanning all the private control channels is over in step S3, the controller 35 sets up a first public control channel (step S8) and determines if a synchronism is established or not (step S9). If yes, a flag of public service area is set up (step S10) and a public service area message is displayed on the LCD 22 (step S11). The process moves to a stand-by status and monitors any out-of synchronism (step S7). If an out-of synchronism is found, the process 35 goes back to step S1.

If, on the other hand, no synchronism is established in step S9, it is determined if all the reception frequencies have been scanned for the public control channel (step S12). If not, reception frequencies are set up at another control channel and the process returns to the process of determining if a synchronism is established or not (step S13). If all the reception frequencies have been scanned for the public control channel, an out-of public service area message is displayed on the LCD 22 (step S14) and the timer is started (step S15) before the process returns to a stand-by status (step S7).

Now, the operation of the controller 35 in a stand-by status in step S7 will be described by referring to the flowchart of FIG. 8.

After completing the display of an out-of public service area message in step S11 and a private reception area message in step S6 and the start of the timer in step S15, the process moves to a stand-by status. First it is determined if the key input section 23 is operated by the user (step Q1) and, if it is not operated, it is determined if a timer flag which is set up by the timer start operation in step S15 is set or not (step Q2). If the timer flag is set, it is determined if a time up exists or not (step Q3). If the time up exists, the process returns to the above described process of setting up the reception at the first private control channel in step S1. If the time up does not exist, the processes in steps Q1 to Q3 are repeatedly executed until the time is up or the key operation is executed.

If, on the other hand, it is determined in step Q2 that no timer flag is set up for a public service area or private reception area, it is determined if it is the timing signal for intermittent reception (step Q4). If it is determined that it is not the timing signal for intermittent reception, the process returns to step Q1. Steps Q1, Q2 and Q4 are repeatedly executed until the key input section 23 is operated by the user or the timing signal comes for intermittent reception.

If the key input section 23 is operated by the user in step Q1, it is determined if the operated key is the calling key or not (step Q5). If yes, a calling process is carried out and then the process returns to step Q1 (step Q6). If the operated key is not the calling key but some other key, a process designates by the depressed key is carried out and the process returns to step Q1 (step Q7).

If, on the other hand, it is determined in step Q4 that it is now the reception timing, the reception process is carries out (step Q8) and then it is determined if an out-of synchronism exists or not (step Q9). If it is determined that an out-of synchronism exists, the process returns to step S1 for setting up a reception at a first private control channel. If, on the other hand, it is determined that no out-of synchronism exists, it is determined if notice information (those concerning the reception timing etc.) are received or not (step Q10). If notice information are received, they are stored (step Q11) and the process returns to step Q1. If no notice information are received, it is determined if own calling information is received or not (step Q12). If own calling information is received, a call incoming process is carried out (step Q13) and the process goes back to step Q1. If no own calling information is received, the process directly goes back to step Q1.

Now, the calling process to be carried out in step Q6 will be described below in detail. When the calling key of the key input section 23 of the portable telephone set is operated, a communication channel assignment request signal is transmitted to the related radio base station by way of the control channel. When a channel is assigned to it by the radio base station, a call request signal is transmitted through the assigned communication channel. Thereafter, as the push button keys are operated, the data representing the operated keys are sent to the radio base station. Then, a communication line is established by the above calling process between the calling portable telephone set and the called telephone set so that the calling party may communicate with the called party through the microphone 25 and the speaker 24.

The call incoming process to be carried out in step Q13 for receiving a call will be described below in detail. If the portable telephone set is called by means of own calling information transmitted as the radio signal from the related radio base station, that is, if the received ID number agrees with the ID number stored in advance in the ROM 36, the speaker 26 for ringer emits a ringing sound to inform the called party of the incoming call. If the called party then operates the communication key in response to the incoming call, a communication channel assignment request signal is transmitted to the radio base station. As a communication channel is assigned by the radio base station, an acknowbedge signal is transmitted to the radio base station via the assigned communication channel. When the above described call incoming process is over, a communication line is secured between the calling telephone set and the called portable telephone set so that the called party may speak with the calling party through the microphone 25 and the speaker 24.

Now, the another process of determining if the portable telephone set is within the private reception area of the base station telephone set or within or out-of the service area of the public radio base station will be described by referring to the flow chart of FIG. 9.

Referring to FIG. 9, first the reception frequency at the first private control channel is set up (step S21). It is determined if the ID number of the base station telephone set transmitted from the latter by radio is detected in the private control channel or not (step S22). Unless the ID number of the base station telephone set is detected in the control channel which is currently used and unless the operation of scanning all the private control channels is completed (step S23), the reception frequency is set up at another private control channel (step S24). The process returns to the process in step S22 to determine if the ID number of the base station telephone set is detected in the private control channel or not.

If, on the other hand, the ID number of the base station telephone set is detected in step S22, the name of the control channel is stored in the RAM 37 (step S25). After a flag of private reception area is set (step S26) or after the operation of scanning all the private control channels is over in step S23, the reception frequency is set up at the first public control channel (step S27). Subsequently, it is determined if a synchronism is established for the first public control channel or not (step S28). If yes, the name of the control channel where synchronism is established is stored in the RAM 37 (step S29) and a flag of a public service area is set up (step S30).

If, on the other hand, no synchronism is established in step S28, or a flag of public service area is set up in step S30, it is determined if the operation of scanning all the public control channels is completed or not (step S31). If no, the reception frequency is set up at another public control channel and the process then returns to the determination process in step S28 to determine if a synchronism is established or not (step S32). If the operation of scanning all the public control channels is completed, it is checked if a flag of private reception area is set up or not (step S33). If yes, the reception frequency is set up at the private control channel which is stored in step S25 and a private reception area message is displayed on the LCD 22 (step S34).

Next, the process proceeds to stand-by status (step S35) and, when out-of synchronism is detected, the process returns to step S21. On the other hand, unless the flag of private reception area is set in step S33, it is checked whether the flag of the public service area is set up or not (step S36). If so, the reception frequency is set up at the public control channel which is stored in step S29 and a public service area message is displayed on the LCD 22 (step S39). If the flag of the public service area is not set, the LCD 22 displays an out-of public service area (step S37) and the timer is started (step S38). After the process moves into a stand-by status and waits for the time period set in the timer (step S35), the process returns to step S21.

Referring to the flowchart of FIG. 10, a stand-by process status in step S35 will be described. The steps of FIG. 10 that are same as those of the corresponding steps of FIG. 8 are respectively indicated by the same symbols and will not be described here any further.

If an out-of synchronism is detected in step Q9, all the channels are searched again to reestablish a synchronism (step Q14) since the names of those channels available for signal reception are stored in the RAM 37 before the out of synchronism starts. Then, it is determined if a synchronism is established in any of the channels which are stored in the RAM 37 (step Q15) and, if yes, the process returns to step Q1.

If the resynchronized channel is shifted from the one in the private reception area to the one in the public service area or vice versa, the flag and the message on the display will be appropriately changed correspondingly.

If, on the other hand, a synchronism is not reestablished, the process goes back to step S21 in FIG. 9 to set the reception at the first private control channel.

It should be noted that the controller 35, the ROM 36 and the RAM 37 need to be modified by, for example, adding a storage area for storing channel information to the RAM 37 in order to make the configuration of FIG. 5 adapted to the above operation of going back to step 21.

Referring to the flowchart of FIG. 11, the process executed by the portable telephone set when any of the remote control buttons 23b in FIG. 4 is depressed will be described.

Referring to FIG. 11, if any of the remote control buttons 23b is depressed by the user (step P1), it is determined where the portable telephone set is located as the result of the above described process of determining if the portable telephone set is within the private reception area or within or out-of the public service area (step P2, P5). If a private reception area message is displayed on the LCD 22, a radio line is connected between the portable telephone set and the base station telephone set (step P3) and a remote control signal corresponding to the depressed remote control button is transmitted to the base station telephone set to terminate the process (step P4).

If, on the other hand, a public service area message is displayed on the LCD 22 as a result of the determination process, the public radio line is connected to the portable telephone set (step P6) and the telephone number of the base station telephone set stored in the RAM 37 is transmitted (step P7). Once the portable telephone set is connected to the public radio base station by way of a radio line (step P8), # signal stored in the RAM 37 is transmitted to suspend the transmission of an acknowledge message by the base station telephone set. Then, the ID number for canceling the recording mode of the base station telephone set 3a stored in the RAM 37 is transmitted (step P9) and a signal corresponding to the depressed remote control button is transmitted to the base station telephone set to terminate the process (step P4).

If it is determined in step P5 that the portable telephone set is located out of the public service area, the process in FIG. 11 is immediately terminated.

FIG. 12A shows the message to be transmitted from the portable telephone set of the preferred embodiment when any of the remote control buttons 23b of the key input section 23 is depressed by the user and a public service area message is displayed on the LCD 22, whereas FIG. 12B shows the message to be transmitted from the portable telephone set of the preferred embodiment while a private reception area message is displayed on the LCD 22.

When any of the remote control buttons 23b of the portable telephone set of the preferred embodiment is depressed, the telephone number, the # signal, the ID number and the control information (remote control signal) stored in the RAM 37, as shown in FIG. 12A, are sequentially transmitted in the public service area, whereas only the control information shown in FIG. 12B is transmitted in the private reception area.

Therefore, when the user of the portable telephone set controls remotely the base station telephone set in the public service area, the user can automatically transmit the telephone number, the # signal, and the ID number, which are required to control remotely the base station set in the public service area, by conducting the same operation of the remote control buttons 23b as those conducted in the private reception area. As a result, the same procedures of the remote control operation can be used in both the private reception area and the public service area and thus the procedures of the remote control operation of the portable telephone set can be simplified and the remote operation can be improved.

While the process of determining if the portable telephone set 3b is within the private reception service area or within or out-of the public service area is carried out before any of the remote control buttons is depressed in the above preferred embodiment, the process may alternatively be carried out after any of the remote control buttons is depressed.

While separate remote control buttons 23b are provided in the key input section 23 of the portable telephone set of FIG. 4, some of the numbered push buttons may alternatively be used for remotely controlling the base station telephone set. If such is the case, the extension mode selection button may be depressed to select the extension mode and thereafter the # button may be used as a switch for selecting any of the remote control buttons and, a specific one of the remote control buttons may be depressed after the # button to remotely control the operation of the base station telephone set.

In short, with the portable telephone set of the above described preferred embodiment, the base station telephone set can be remotely controlled only by transmitting from the portable telephone set a signal for selecting a specific operation of the base station telephone set that varies depending on whether the portable telephone set is in a private reception area or in a public service area.

The above operation of remotely controlling the base station telephone set may alternatively be so modified that, if the portable telephone set is within the public service area, the network mode selection button may be depressed before the extension mode selection button in order to use different procedures for remotely controlling the base station telephone set in a public service area and in a private reception area. If such is the case, the only difference between the two procedures lies in which of the extension mode selection button and the network mode selection button is depressed first and the operation of remotely controlling the base station telephone set remains as simple as ever.