Abstract:
In data transmission from a base station to a mobile station, a received signal strength is monitored at the base station and the mobile station and, when the received signal strength reduces to less than a predetermined level, the base station concurrently transmits a plurality of radio signals each having a different frequencies and conveying the same data. At the mobile station, the original data is reproduced from the radio signals received from the base station.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mobile data communication system and, particularly, to a system and method for transmitting data from a base station to a mobile terminal. 
     2. Description of the Prior Art 
     Recently, a mobile telephone system has been used for not only telephony but data communication. For example, there have been commercially available a mobile data communication device which is detachably connected to the mobile telephone terminal and a mobile terminal integrating a telephone terminal with a mobile data communication terminal. 
     In general, such a mobile terminal is battery-powered and it is desirable that its power consumption is as small as possible. For this purpose, the transmission power of a mobile terminal is smaller than that of a base station. To achieve balanced received signal strength at either side of the mobile terminal and the base station, it is necessary to provide the base station with a high-gain antenna placed at a high position such as the rooftop of a building. However, it is clearly found that there are the limitations of the antenna of a base station in gain and installation height. 
     Therefore, it is necessary for a mobile terminal to increase its transmission power to same extent. For instance, a transmission power of 1 Watt is needed. The increased transmission power causes the size of a power IC and its heat sink and further the capacity of a battery to be large, resulting in difficulty in miniaturization. In other words, there has been a tradeoff between miniaturization and transmission power. 
     In such a condition, movement of a mobile terminal causes a received signal strength to fluctuate at either side of the base station and the mobile station. When the received signal strength is reduced, especially in the case of the mobile terminal located at an end of a radio zone formed by the base station, the radio communication channel is disconnected. Further multipath fading also causes a significant fluctuation of received signal strength which would result in channel disconnection. 
     To avoid such undesired channel disconnection, there has been proposed a polling communication method in Japanese Patent Unexamined Publication No. 60-148238. According to the conventional method, a stationary station optimally selects an antenna and a use frequency for each mobile terminal to improve the quality of channel transmission. 
     However, even when the antenna and the use frequency is optimally selected, the influence of fading cannot be avoided. Therefore, in the case where the mobile terminal is located away from the base station and the received signal strength is weak, the occurrence of fading easily causes the communication channel to be disconnected. In other words, the operable radio zone of each base station becomes substantially smaller. Therefore, the operable service area of the mobile communication system is substantially reduced. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a mobile communication system and data transmission method which can substantially enlarge the service area thereof. 
     Another object of the invention is to provide a mobile communication system and data transmission method which can achieve reliable data transmission from a base station to a mobile terminal. 
     According to an aspect of the present invention, in data transmission from a base station to a mobile station in a mobile communication system, the method is comprised of the steps of 
     a) preparing a plurality of predetermined frequencies; 
     b) monitoring a received signal strength in at least one side of the base station and the mobile station; and 
     c) concurrently transmitting a plurality of radio signals from the base station to the mobile station when the received signal strength reduces to less than a predetermined level, wherein each of the radio signals has a different one of the predetermined frequencies and conveys the same data. 
     According to another aspect of the present invention, the method is further comprised of 
     d) monitoring quality of the data conveyed by the radio signals at the mobile station; and 
     e) transmitting a radio retransmission signal from the base station to the mobile station when the radio retransmission signal conveys a part of the data of a quality lower than a predetermined quality level. 
     As described above, a plurality of radio signals each conveying the same data are concurrently transmitted from the base station to the mobile station when the received signal strength is weak as in the case where the mobile station is located away from the base station. Therefore, even when multipath fading occurs, it is hard to disconnect the communication channel, resulting in substantially enlarged service area of the mobile communication system. 
     Further, since erroneously received data is retransmitted, reliable data transmission can be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing a system configuration of a mobile data communication system according to the present invention; 
     FIG. 2 is a sequence diagram showing a receive-only mode operation initiated by the base station in a data communication method according to a first embodiment of the present invention; 
     FIG. 3 is a sequence diagram showing a receive-only mode operation initiated by the mobile terminal in the data communication method according to the first embodiment; 
     FIG. 4 is a flow chart showing a receive-only mode operation of the mobile terminal according to the first embodiment; 
     FIG. 5 is a sequence diagram showing a retransmission operation of a data communication method according to a second embodiment of the present invention; and 
     FIG. 6 is a flow chart showing the retransmission operation of the mobile terminal according to the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     System 
     Referring to FIG. 1, a mobile data communication system is composed of a plurality of base stations and mobile terminals. The respective base stations form radio zones in cellular configuration to form a service area of the system. Each mobile terminal located within the radio zone of a base station can communicate with that base station by radio. Hereinafter, the case where a base station  10  forms a radio zone in which a mobile terminal  20  is located and communicated with the base station  10  will be described for simplicity. 
     Schematically, the base station  10  is composed of a transceiver  101  connected to an antenna  102  and a transmitter  103  connected to an antenna  104 . The transceiver  101  transmits a base-to-mobile radio signal of frequency f D1  to the mobile terminal  20  and receives a mobile-to-base radio signal of frequency f UP  from the mobile terminal  20 . The transmitter  103  transmits a base-to-mobile radio signal of frequency f D2  to the mobile terminal  20 . Further, the transceiver  101  detects a received signal strength RS B  from the received radio signal of frequency f UP  to output it to a processor  105  which controls the operations of the base station  10 . 
     There is provided a frequency difference between the base-to-mobile frequency channels: f D1  and f D2 , so that the occurrence of multipath fading has little effect on either of the frequency channels to keep its receive sensitivity acceptable. These frequencies are selected from predetermined frequency bands assigned to the system 
     The processor  105  is connected to a data memory  106  for storing data to be transmitted. The data stored in the data memory  106  is processed by the processor  105  and then is transferred to the transceiver  101 . The transceiver  101  modulates a carrier of the frequency f D1  depending on the data to produce the base-to-mobile radio signal. 
     The transmitter  103  is activated when necessary. When activated, the transmitter  103  receives the same data as the transceiver  101  from the processor  105  and modulates a carrier of the frequency f D2  depending on the data to produce another base-to-mobile radio signal. 
     The internal circuits including the transceiver  101  and the transmitter  103  are supplied with power as necessary by a power supply controller  107  under the control of the processor  105 . More specifically, power is normally supplied to the transceiver  101  and, when receive-only mode communication of the mobile terminal  20  is started, power is supplied to both the transceiver  101  and the transmitter  103 . The processor  105  runs a control program stored in a read-only memory (not shown) to perform the receive-only mode communication with the mobile terminal  20  and then to perform retransmission of erroneously transmitted data, as will be described later. 
     The mobile terminal  20  is composed of a transceiver  201  connected to an antenna  202  and a receiver  203  connected to an antenna  204 . The transceiver  201  transmits the mobile-to-base radio signal of frequency f UP  to the base station  10  and receives the base-to-mobile radio signal of frequency f D1  from the base station  10 . The receiver  203  receives the base-to-mobile radio signal of frequency f D2  from the base station  10 . Further, the transceiver  201  detects a received signal strength RS M  from the received radio signal of frequency f D1  to output it to a processor  205  which controls the operations of the mobile terminal  20 . 
     More specifically, the transceiver  201  demodulates a base-to-mobile radio signal of the frequency f D1  to output received data S R1  and its signal strength RS M  to the processor  205 . Further, the transceiver  201  modulates a carrier depending on transmission data S T  received from the processor  205  to produce the mobile-to-base radio signal of the frequency f UP . The transmit/receive timing is controlled by the processor  205 . For example, the well-known TDMA/TDD scheme may be used in the system. On the other hand, the receiver  203  demodulates a base-to-mobile radio signal of the frequency f D2  to output received data S R2  to the processor  205 . As described before, the base-to-mobile radio signal of the frequency f D2  conveys the same data as the base-to-mobile radio signal of the frequency f D1  received by the transceiver  201 . The receive timing of the receiver  203  is also controlled by the processor  205 . 
     The processor  205  is connected to a received data memory  206  and an error correcting section  207  which may be implemented with error correcting program running on the processor  203 . As will be described later, in the case where the mobile terminal  20  is set to the receive-only mode, the processor  205  stores the received data S R1  and S R2  in frames onto the received data memory  206  and then performs the error correcting to produce error rates ER 1  and ER 2  of the received data S R1  and S R2  in frames. The error rates ER 1  and ER 2  are used to select better received data. 
     The mobile terminal  20  is further provided with a user interface  208  which controls a display  209  such as LCD, a keypad for data and instruction input, and communication with external equipment such as a personal computer which is detachably connected by an external connector. The keypad includes a power switch  210  which is used by a user to power the mobile terminal  20  on and off. 
     The mobile terminal  20  is powered by a battery  211 . A power supply controller  212  supplies the battery power to the transceiver  201 , the receiver  203  and other necessary circuits under control of the processor  205 . In normal receive mode, the processor  205  controls the power supply controller  212  so that the power is not supplied to the receiver  203  but to the transceiver  201  to perform normal communication with the base station  10 . When the radio channel is impaired, at a request of the mobile terminal  20  or on the instruction of the base station  10 , the mobile terminal  20  changes from the normal receive mode to the receive-only mode. 
     In the receive-only mode, the processor  205  controls the power supply controller  212  so that the power is supplied to both the receiver  203  and the transceiver  201  to receive the same data from the base station  10  through two different frequency channels. In other words, the respective base-to-mobile signals of frequencies f D1  and f D2  are concurrently received by the transceiver  201  and the receiver  203 . Further, in the receive-only mode, the processor  205  controls the user interface  208  so that an indicator of the receive-only mode is displayed on the display  209  and the power switch  210  is inoperative. 
     Communication Control 
     In the case where the mobile terminal  20  operates in the receive-only mode, the transceiver  201  and the receiver  203  are both activated to concurrently receive the same data from the base station  10  through the two different frequency channels. The mobile terminal  20  switches to the receive-only mode in one of the following ways: 1) Base-initiated sequence and 2) mobile-initiated sequence. In the base-initiated sequence, at the base station  10 , when the received signal strength RS B  falls to less than a predetermined threshold TH RSB , a receive-only mode switch instruction is transmitted from the base station  10  to the mobile terminal  20 . In the mobile-initiated sequence, at the mobile terminal  20 , when the received signal strength RS M  falls to less than a predetermined threshold TH RSM , a receive-only mode switch request is transmitted from the mobile terminal  20  to the base station  10  and then the receive-only mode switch permission is transmitted back to the mobile terminal  20 . The details will be described hereinafter. 
     Base-Initiated Sequence 
     Referring to FIG. 2, first of all, when the power switch  210  is turned on by the user, the processor  205  starts location registration operation. That is, a location registration signal is transmitted to the base station  10  through the transceiver  201  (step S 01 ). 
     When receiving the location registration signal from the mobile terminal  20 , the base station  10  performs the location registration of the mobile terminal  20  and then transmits a location registration completion signal to the mobile terminal  20  (step S 02 ). Thereafter, the communication between the base station  10  and the mobile terminal  20  is performed using the frequencies f D1  and f UP  (step S 03 ). 
     During the location registration or thereafter communication, the processor  105  of the base station  10  monitors the received signal strength RS B  at all times. When it is determined that the received signal strength RS B  falls to less than the predetermined threshold TH RSB , which may be caused by multipath fading or the mobile terminal  20  moving to the end of the radio zone, the processor  105  produces a receive-only mode switch instruction and transmits it to the mobile terminal  20  through the transceiver  101  (step S 04 ). 
     Upon receipt of the receive-only mode switch instruction, the processor  205  of the mobile terminal  20  switches the communication mode from the normal mode to the receive-only mode in which the transceiver  201  receives the base-to-mobile signal of the frequency f D1  and at the same time the receiver  203  receives the base-to-mobile signal of the frequency f D2  from the base station  10 . After having switched to the receive-only mode, a receive-only mode switch completion signal is transmitted to the base station  10  (step S 05 ). 
     The processor  105  of the base station  10 , when receiving the receive-only mode switch completion signal from the mobile terminal  20 , starts receive-only mode transmission. More specifically, the same data to be transmitted is output to both the transceiver  101  and the receiver  103  which concurrently transmit the two base-to-mobile radio signals to the mobile terminal  20  through the two frequency channels of f D1  and f D2 , respectively (step S 06 ). 
     The mobile terminal  20  receives the two base-to-mobile radio signals of the frequencies f D1  and f D2  from the base station  10  in the receive-only mode. It is preferably that an indicator of the receive-only mode is displayed on the display  209  to inform the user. Further, during the receive-only mode, the power switch  210  is preferably inoperative to prevent accidental power-off. 
     When the data transmission in the receive-only mode has been terminated, the base station  10  transmits a data transmission completion signal to the mobile terminal  20 . When detecting the end of the data transmission, the processor  205  switches the communication mode from the receive-only mode to the normal mode (step S 07 ). 
     Mobile-Initiated Sequence 
     Referring to FIG. 3, the descriptions of steps S 11 -S 13  are omitted because they are the same as the steps S 01 -S 03  of FIG.  2 . 
     During the location registration or thereafter communication, the processor  205  of the mobile terminal  20  monitors the received signal strength RS M  at all times. When it is determined that the received signal strength RS M  falls to less than the predetermined threshold TH RSM , which may be caused by multipath fading or the mobile terminal  20  moving to the end of the radio zone, the processor  205  produces a receive-only mode switch request and transmits it to the base station  10  through the transceiver  201  (step S 14 ). 
     In response to the receive-only mode switch request, the processor  105  of the base station  10  transmits a receive-only mode switch permission signal back to the mobile terminal (step S 15 ). 
     When receiving the receive-only mode switch permission signal, the processor  205  of the mobile terminal  20  switches the communication mode from the normal mode to the receive-only mode. After having switched to the receive-only mode, a receive-only mode switch completion signal is transmitted to the base station  10  (step S 16 ). 
     The processor  105  of the base station  10 , when receiving the receive-only mode switch completion signal from the mobile terminal  20 , starts receive-only mode transmission (step S 17 ). 
     The mobile terminal  20  receives the two base-to-mobile radio signals of the frequencies f D1  and f D2  from the base station  10  in the receive-only mode. It is preferably that an indicator of the receive-only mode is displayed on the display  209  to inform the user. Further, during the receive-only mode, the power switch  210  is preferably inoperative to prevent accidental power-off. 
     When the data transmission in the receive-only mode has been terminated, the base station  10  transmits a data transmission completion signal to the mobile terminal  20 . When detecting the end of the data transmission, the processor  205  switches the communication mode from the receive-only mode to the normal mode (step S 18 ). 
     Receive-Only Mode 
     The receiving control of the receive-only mode will be described hereinafter. 
     Referring to FIG. 4, when receiving the receive-only mode switch instruction/permission signal from the base station  10  (YES in step S 301 ), the processor  205  of the mobile terminal  20  switches the communication mode from the normal mode to the receive-only mode (step S 302 ). As described before, it is preferably that an indicator of the receive-only mode is displayed on the display  209  to inform the user. Further, during the recelve-only mode, the power switch  210  is preferably inoperative to prevent accidental power-off. After having switched to the receive-only mode, the receive-only mode switch completion signal is transmitted to the base station  10  (step S 303 ). 
     When receiving the two base-to-mobile radio signals of the frequencies f D1  and f D2  from the base station  10  (YES in step S 304 ), the transceiver  201  outputs the received data S R1  and the receiver  203  outputs the received data S R2  to the processor  205 . Since the base station  10  transmits the same data, the received data S R1  and S R2  are ideally identical to each other. The received data S R1  and S R2  are stored onto the received data memory  206  (step S 305 ). 
     Subsequently, the error correcting section  207  performs the error correcting of the received data S R1  and S R2  in frames and produces error rates ER 1  and ER 2 , respectively (step S 306 ). The processor  205  selects one of the received data S R1  and S R2  depending on which error rate is smaller and the selected data is output to the external device through the user interface  208  (step S 307 ). Since the frequencies f D1  and f D2  are selected so that occurrence of multipath fading has little effect on either of the frequency channels to keep its receive sensitivity acceptable, there is a high possibility that at least one of the received data S R1  and S R2  is acceptable. 
     When detecting the end of the data transmission (YES in step S 308 ), the processor  205  switches the communication mode from the receive-only mode to the normal mode (step S 309 ). 
     Data Retransmission 
     According to the second embodiment of the present invention, after the end of the data transmission in the receive-only mode, erroneously received data can be retransmitted in frames. 
     As shown in FIG. 5, after the communication mode of the mobile terminal  20  switches from the receive-only mode to the normal mode, the processor  205  determines whether the received signal strength RS M  is smaller than the predetermined threshold TH RSM . If the received signal strength RS M  is not smaller than the predetermined threshold TH RSM , the location registration is performed again (steps S 21  and S 22 ). Subsequently, if there is frame data which fails to be received correctly, the mobile terminal  20  transmits a request for retransmission of erroneously received frame data to the base station  10  (step S 23 ). 
     When receiving the retransmission request signal from the mobile terminal  20 , the processor  105  of the base station  10  searches the data memory  106  for the requested frame data and transmits the requested frame data to the mobile terminal  20  (step S 24 ). Such a retransmission steps S 23  and S 24  is repeatedly performed until no erroneously received frame data remain in the mobile terminal  20  (step S 25 ). 
     Referring to FIG. 6, after the communication mode of the mobile terminal  20  switches from the receive-only mode to the normal mode, the processor  205  determines whether the received signal strength RS M  is smaller than the predetermined threshold TH RSM  (step S 401 ). If the received signal strength RS M  is not smaller than the predetermined threshold TH RSM  (NO in step S 401 ), the location registration Is performed again (step S 402 ). 
     Subsequently, the processor  205  determines whether there is any frame data satisfying that the error rates ER 1  and ER 2  are both higher than a predetermined threshold TH ER  (step S 403 ). It is determined that frame data satisfying such a condition has never been received. Therefore, if such frame data is found (YES In step S 403 ), the retransmission request for such erroneously received data is transmitted to the base station  10  (step S 404 ). And when receiving the requested frame data from the base station  10  (YES in step S 405 ), the processor  205  stores the received data onto the received data memory  206  (step S 406 ). In this manner, all the frame data can be received. 
     It should be noted that the communication mode can switch to the receive-only mode when the received signal strength is reduced during data retransmission as shown in FIG.  6 . 
     The sequence as shown in FIGS. 2 and 3 may be further simplified. For example, the base station  10  can transmit the data to the mobile terminal  20  after a predetermined time interval during which the mobile terminal completely switches to the receive-only mode has elapsed since the receive-only mode switch instruction/permission. In this case, the receive-only mode switch completion signal is not needed. 
     Further, the present invention is not limited to the above embodiments. Three or more frequencies may be used in the receive-only mode. In this case, although both of the base station  10  and the mobile terminal  20  need three or more communication devices, more reliable data transmission can be achieved.