GB2338153A

GB2338153A - Mobile data communication with frequency diversity and re-transmission

Info

Publication number: GB2338153A
Application number: GB9920436A
Authority: GB
Inventors: Masahiko Tanaka
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1997-07-29
Filing date: 1998-07-29
Publication date: 1999-12-08
Also published as: GB9920436D0

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. When a block of data of low quality is received the mobile station transmits a radio retransmission request for the block of low transmission quality data to the base station.

Description

2338153 MOBILE DATA COM4UNICATION SYSTEM AND METHOD 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 and is di fkm acatkn nD. 9B16515.2 Recently, a mobIle telephone system has been used f or not only telephony but also da ta 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 integrat-ing 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 roof top of a building- However, it is clearly f ound 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 so.me extent. For instance, 2 transmiss-lon power of 1 Watt Is needed. The increased transmisslon 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 near the edge of a radio zone formed by the base station. the radio communication channel is disconnected. Further, a 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 3 communication channel to be disconnected. In other words, the substantially operable radio zone of each base station becomes smaller. Therefore, the operable service area of the mobile communication system is substantially reduced.

An object of at least the preferred embodiments of the present invention is to provide a mobile communication system and data transmission method which can substantially enlarge the service area thereof.

Another such object 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.

In one aspect the inventor provides a method for transmitting data from a base station to a mobile station in a mobile communication system, the base station being adapted to transmit data at a plurality of predetermined frequencies, the method comprising the steps of:

a) monitoring a received signal strength at at least one of the base station and the mobile station; and b) 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 radio signal is transmitted at a different one of the predetermined frequencies and conveys the same data.

According to another aspect of the present invention, there is provided a method of transmitting data from a base station to a mobile station in a mobile communication system, the base station being adapted to transmit data using a plurality of predetermined frequency channels, the method comprising monitoring received signal strength at at least one of the base a 4 station and the mobile station; the monitoring comprising the steps of:

at the base station, a) concurrently transmitting a plurality of radio signals to the mobile station each through a respective frequency channel, when the received signal strength reduces to less than a predetermined level, wherein each of the radio signals conveys the same data in data blocks; at the mobile station, b) receiving the radio signals from the base station to produce streams of received data in data blocks, the streams of received data corresponding to respective said radio signals; c) determining the transmission quality of data block for each of the streams of the received data; d) determining whether a block of low transmission quality data of quality lower than a predetermined quality level exists in the streams of the received data; e) transmitting a retransmission request for the block of low transmission quality data to the base station when the block of low transmission quality data exists; and at the base station, f) transmitting a radio retransmission signal conveying data corresponding to the block of low transmission quality data to the mobile station.

In another aspect the present invention provides a mobile communication system comprising at least one base station and at least one mobile station, the at least one base station comprising at least one receiving means; a plurality of transmission means each for transmitting at a different one of a plurality of predetermined frequencies; and means for controlling the transmission means respectively to transmit concurrently a plurality of radio signals to the at least one mobile station communicating therewith when a received signal strength at at least one of the base station and the mobile station reduces to less than a predetermined level; the at least one mobile station comprising at least one transmission means; a plurality of receiving means for receiving the radio signals from the base station; and processing means for producing streams of received data respectively corresponding to the receiving means, wherein the processing means is adapted to determine whether a block of low transmission quality of data of quality lower than a predetermined transmission quality level exists in the streams of the received data, and, when this is the case, controls the transmission means to transmit a retransmission request for the block of low transmission quality data to the base station. In another aspect the present invention provides a mobile station adapted to be used in the above communication system, comprising: at least one transmission means a plurality of receiving means corresponding to the predetermined frequencies, wherein a predetermined one of the receiving means is used for a normal receiving mode; a display; a power switch; power controlling means for controlling power supply from a battery; and processing means for reproducing original data from a plurality of streams of received data corresponding to the radio signals concurrently received by the respective receiving means, wherein the processing means is adapted to determine whether a block of low transmission quality of data of quality lower than a predetermined transmission quality level exists in the streams of the received data, and, when this is the case, 6 controls the transmission means to transmit a retransmission request for the block of low transmission quality data to the base station.

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, the likelihood of an unintentional disconnection of the communication channel is reduced, resulting in a substantially enlarged service area of the mobile communication system.

Further, since erroneously received data is retransmitted, reliable data transmission can be achieved.

Preferred features of the present invention will now be described, purely by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic block diagram showing features of a system configuration of a mobile data communication system according to the present invention; 7 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 f irst embodiment of the 5 present invention; FIG. 3 is a sequence diagram showing a receive-only mode operation init. lated 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 retransmisslon 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.

Referring to FIG. 1, a mobile data communication system is composed of a plurality of base stations and mobile 8 terminals - The respective base stations form radio zones in cllular 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 communicate$ 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 fD1 to the mobile terminal 20 and receives a mobile-to-base radio signal of frequency fup from the mobile terminal 20. The transmitter 103 transmits a base-to-mobile radio signal of frequency f,,, to the mobile terminal 20. Further, the transceiver 101 detects a received signal strength RS,, from the received radio signal of frequency f,,, 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., and f.2. 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- 9 The processor 105 is connected to a data memory 106 f or 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 fl depending on the data to produce the base-to-mobile radio signal.

The transmitter 103 Is activated when necessary. when p activated, the transmitter 103 receives the same data as the transceiver 101 from the processor 105 and modulates a carrier of the frequency f,, 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 103The processor 105 runs a control program sto red in a read-only memory (not shown) to perform the receive-only mode communication with the mobile terminal 2 0 and then to perform retransmission of erroneously transmitted data, as will be described later. The mobile terminal 20 is composed of a transceiver 25 201 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., to the base station 10 and receives the base-to-mobile radio signal of frequency f,,, from the base station 10. The receiver 203 receives the base- to -mobile radio signal of frequency f., from the base station 10. Further, the transceiver 201 detects a received signal strength RS,, 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-tomobile radio signal of the frequency f,,, to output received data S,, and Its signal strength RS, to the processor 205. Further, the transceiver 201 modulates a carri-er depending on transmission dataST received from the processor 205 to produce the mobile-to-base radio signal of the frequency fup. 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., to output received dataS,2 to the processor 205.

As described before. the base-to-moblle radio signal of the frequencyfD2 conveys the same data as the base-to--mobile radio signal of the frequency f,,, 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,, and S1,2 in frames onto the received data memory 206 and then performs the error correcting to produce error rates ER1 and ER,. of the received data SR, and S.2 in frames. The error rates ER, and ER2 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, f or 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 12 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 fD1 and f,., 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) Baseinitiated sequence and 2) mobile- initiated sequence. In the base-inItiated sequence, at the base station 10, when the received signal strength RS, falls to less than a predetermined threshold TH,,,, a receive-only mode switch instruction is transmitted from the bass station 10 to the mobile terminal 20. In the mobile- initiated sequence, at the 13 mobile terminal 20, when the received signal strength RS, falls to less than a predetermined threshold TH..., 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.

Referring to Fig- 2, f irst 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 S01)- 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 S02). Thereafter, the communication between the base station 10 and the mobile terminal 20 is perf ormed using the frequencies f,, and f,p (step S03).

During the location registration or thereafter communication, the processor 105 of the base station 10 monitors the received signal strength RS,, at all times - When it is determined that the received signal strength RSB falls to less than the predetermined threshold THRSBF which may be 14 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 S04).

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-6nly mode in which the transceiver 201 receives the base-to-mobile signal of the frequency f.1 and at the same time the receiver 203 receives the base-to-moblle signal of the frequency f.2 from the base station 10. After having switched to the recelve-only mode, a receive-only mode switch completion signal is transmitted to the base station 10 (step S05).

The processor 105 of the base station 10, when receiving the receive-only mode switch completion signal from the mobile terminal 20, starts receiveonly 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 baseto-mobile radio signals to the mobile terminal 20 through the two frequency channels of f,, and f,,,, respectively (step S06).

The mobile terminal 20 receives the two base-to-mobile radio signals of the frequencies f.1 and fD2 from the base station 10 in the receive-only mode. It is preferably that an indicator of the receiveonly 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 s 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 S07).

M9 b ll-e Referring to Fig. 3, the descriptions of steps S11 -S13 are omitted because they are the same as the steps S01-S03 of Fig. 2.

During the location registration or thereafter communIcation, the processor 205 of the mobile terminal 20 monitors the received signal strength RS, at all times - When it is determined that the received signal strength RS, falls to less than the predetermined threshold TH,,,., 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 S14).

In response to the receive-only mode switch request, the processor 105 of the base station 10 transmits a rece:Lve-only mode switch permissiLon signal back to the mobile terminal (step S15).

16 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 receiveonly mode, a receive-only mode switch completion signal is transmitted to the base station 10 (step S16).

The processor 105 of the base station 10, when receiving the receive-only mode switch conpletlon signal from the mobile terminal 20, starts receiveonly mode transmission (step S17).

The mobile terminal 20 receives the two base-to-mobile radio signals of the frequencies f.1 and f,,, 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 S18).

RECEIVE-ONLY MODE The receiving control of the receive-only mode will be described hereinafter.

17 Referring to Fig- 4, when receiving the receive-only mode switch ins truction/permis s ion signal from the base station 10 (YES in step S301), the processor 205 of the mobile terminal 20 switches the communication mode from the normal mode to the recelve-only mode (step S302). As descrIbed Def ore. it Is pref erably 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 S303). When receiving the two base-to-mobile radio signals of the frequencies f,, and f D2 from the base station 10 (YES 15 in step S304). the transceiver 201 outputs the received data SR1 and the receiver 203 outputs the received data SR2 to the processor 205. Since the base station 10 transmits the same data, the received data S R1 and S, , are Ideally identical to each other.. The received data S R1 and S.. are stored onto the received data memory 206 (step S305).

Subsequently, the error correcting section 207 performs the error correcting of the received data SR1 and S.2 In frames and produces error rates ER1 and ER2 respectively (step S306). The processor 205 selects one of the received data S.1 and S.. depending on which error rate ls smaller and the selected data is output to the external device through 18 the user interface 208 (step S307). Since the frequencies fDl and fD2 are selected so that occurrence of multipath fading has little ef f ect 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.1 and SR2 is acceptable.

When detecting the end of the data transmission (YES in step S308), the processor 205 switches the communication mode from the receive-only mode to the normal mode (step S309).

DATA RETRANSMISSION According to the second embodiment of the p-resent invention, after the end of he 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, is smaller than the predetermined threshold TH,,, If the received s-lgnal strength RS, Is not smaller than the predetermined threshold THRS1. the location registration is performed again (steps S21 and S22).

Subsequently, if there is frame data which falls to be received correctly, the mobile terminal 20 transmits a request f or retransmission of erroneously received f rame data to the base station 10 (step S23).

19 When receiving the retransmission recluest signal from the mobile. terminal 20, the processor 105 of the base station 10 searches the data Aemory 106 for the requested frame data and transmits the requested frame data to the mobile terminal 20 (step S24). Such a retransmission steps s23 and S24 is repeatedly perf ormed until no erroneously received f rame data remain in the mobile terminal 20 (step S25).

Ref erring to Fig. 6, after the communication mode of the mobile terminal 20 switches from the receive-only mode to the normal mode, the proces-sor 205 determines whether the received signal strength RS, is smaller than the predetermined threshold TH,,, (step S401). If the received signal strength RSx Is not smaller than the predetermined threshold TH,., (NO in step S401), the location registration is performed again (step S402).

Subsequently, the processor 205 determines whether there is any frame data satisfying that the error rates ER, and ER, are both higher than a predetermined threshold TH., (stepS403). It is determined that frame data satisfying such a condition has never been recelved Therefore, if such frame data is found (YES in step S403), the retransmission request for such erroneously received data is transmitted to the base station 10 (step S404). And when receiving the requested frame data from the base station 10 (YES in step S405), the processor 205 stores the received data onto the received data memory 206 (step S406). In this manner, all the frame data can be received.

It should be noted that the communication mode can switch to the receiveonly 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/permisdion. 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 mobile terminal 20 need three or more communication devices, more reliable data transmission can be achieved.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Statements in this specification of the,objects of the invention,' relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

The description of the invention with reference to the drawings is by way of example only.

The text of the abstract filed herewith is repeated here as part of the specification.

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 21predetermined level, the base station concurrently transmits a plurality of radio signals each having a different frequency and conveying the same data. At the mobile station, the original data is reproduced from the radio signals received from the base station.

When a block of data of low quality is received the mobile station transmits a radio retransmission request for the block of low transmission quality data to the base station.

22 -

Claims

CLAIMS:

1 - A method of transmitting data from a base station to a mobile station in a mobile communication system, the base station being adapted to transmit data using a plurality of predetermined frequency channels, the method comprising monitoring a received signal strength at at least one of the base station and the mobile station; the monitoring comprising the steps of:

at the base station, a) concurrently transmitting a plurality of radio signals to the mobile station each through a respective frequency channel, when the received signal strength reduces to less than a predetermined level, wherein each of the radio signals conveys the same data in data blocks; at the mobile station, b) receiving the radio signals from the base station to produce streams of received data in data blocks, the streams of received data corresponding to respective said radio signals; c) determining the transmission quality of data block for each of the streams of the received data; d) deter-mining whether a block of low transmission quality data of quality lower than a predetermined quality level exists in the streams of the received data; e) transmitting a retransmission request for the block of low transmission quality data to the base station when the block of low transmission quality data exists; and at the base station, f) transmitting a radio retransmission signal conveying data corresponding to the block of low transmission quality data to the mobile station.

2. The method according to claim 1, wherein, at the mobile station, the retransmission request is transmitted to the base station when the received signal strength is not smaller than the predetermined level.

23 3. The method according to claim 1, wherein the retransmission request is transmitted to the base station through a normally-used frequency channel.

4. The method according to claim 1, wherein the radio signals are concurrently transmitted when at the base station the received signal strength reduces to less than the predetermined level.

5. The method according to claim 4,wherein the radio signals are concurrently transmitted at a request of the mobile station when at the mobile station the received signal strength reduces to less than the predetermined level.

is 6. The method according to any preceding claim, comprising, at the mobile station, receiving the radio signals from the base station to produce streams of received data corresponding to the radio signals, respectively; and selecting a data block of better quality from the streams of received data to reproduce original data.

7. The method according to claim 6, wherein a communication mode of the mobile station switches to a receiveonly mode before the radio signals are concurrently received from the base station.

8. The method according to claim 7, wherein the communication mode of the mobile station switches to the receive-only mode on an instruction of the base station.

9. The method according to claim 8, comprising, at the mobile station, transmitting a request for receive-only mode switch to the base station; receiving receive-only mode switch permission from the base station; and switching to the receive-only mode.

10. The method according to claim 9, wherein, when the mobile station has switched to the receive-only mode, an indicator of the receive-only mode is displayed on a display device provided in the mobile station.

11. The method according to claim 9 or 10, wherein when the mobile station has switched to the receive-only mode, a power switch provided in the mobile station is inoperative until the receive-only mode is released.

12. A mobile communication system comprising at least one base station and at least one mobile station, the at least one base station comprising at least one receiving means; a plurality of transmission means each for transmitting at a different one of a plurality of predetermined frequencies; and means for controlling the transmission means respectively to transmit concurrently a plurality of radio signals to the at least one mobile station communicating therewith when a received signal strength at at least one of the base station and the mobile station reduces to less than a predetermined level; the at least one mobile station comprising at least one transmission means; a plurality of receiving means for receiving the radio signals from the base station; and processing means for producing streams of received data respectively corresponding to the receiving means, wherein the processing means is adapted to determine whether a block of low transmission quality of data of quality lower than a predetermined transmission quality level exists in the streams of the received data, and, when this is the case, controls the transmission means to transmit a retransmission request for the block of low transmission quality data to the base station.

13. The mobile communication system according to claim 12, wherein the base station further comprises a monitor for monitoring whether a received signal strength of a radio signal received from the mobile station, reduces to less than the predetermined level.

14. The mobile communication system according to claim 12, wherein the mobile station comprises a monitor for monitoring a received signal strength of a radio signal received from the base station, wherein the base station concurrently transmits the radio signals at a request of the mobile station when at the mobile station the received signal strength reduces to less than the predetermined level.

15. The mobile communication system according to any of claims 12 to 14, wherein the processing means switches a communication mode to a receive-only mode before the radio signals are concurrently received from the base station.

16. The mobile communication system according to any of claims 12 to 15, wherein the processing means selects a data block of better transmission quality from the streams of received data to reproduce original data.

17. A mobile station adapted to be used in the mobile communication system according to claim 12, comprising:

at least one transmission means a plurality of receiving means corresponding to the predetermined frequencies, wherein a predetermined one of the receiving means is used for a normal receiving mode; display; power switch; power controlling means for controlling power 26 supply from a battery; and processing means for reproducing original data from a plurality of streams of received data corresponding to the radio signals concurrently received by the respective receiving means, wherein the processing means is adapted to determine whether a block of low transmission quality of data of quality lower than a predetermined transmission quality level exists in the streams of the received data, and, when this is the case, controls the transmission means to transmit a retransmission request for the block of low transmission quality data to the base station.

18. The mobile station according to claim 17, wherein the processing means selects a data block of better transmission quality from the streams of received data to reproduce the original data.

19. The mobile station according to claim 17 or 18, wherein the processing means switches a communication mode of the mobile station from a normal mode to a receive-only mode before the radio signals are concurrently received from the base station, wherein the transmission means and the predetermined receiving means are used to communicate with the base station in the normal mode.

20. The mobile station according to claim 19, wherein, when the mobile station has switched to the receive-only mode, the processing means instructs the display to display an indicator of the receive-only mode.

21. The mobile station according to claim 19 or 20, wherein, when the mobile station has switched to the receive-only mode, the processing means makes the power switch inoperative until the receive-only mode is released.

22. The mobile station according to claim 19, wherein when in the normal mode, the processing means controls the power controlling means so that power is supplied to the transmission means and the predetermined receiving means and when in the receive-only mode, the processing means controls the power controlling means so that power is supplied to all the receiving means.

23. A method of transmitting data from a base station to a mobile station including the step of transmitting a radio retransmission signal, a mobile communication system or a mobile station wherein the mobile station includes means for transmitting a retransmission request, substantially as described hereinabove with reference to the accompanying drawings.