Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-07-29
2001-03-27
Le, Thanh Cong (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S062000, C455S067700, C455S513000
Reexamination Certificate
active
06208855
ABSTRACT:
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.
REFERENCES:
patent: 4355411 (1982-10-01), Reudink et al.
patent: 5345600 (1994-09-01), Davidson
patent: 5663990 (1997-09-01), Bolgiano et al.
patent: 5915210 (1999-06-01), Cameron et al.
patent: 5930248 (1999-07-01), Langlet et al.
patent: 5993421 (1999-08-01), Alamouti et al.
patent: 60-148238 (1985-08-01), None
patent: WO88/04496 A1 (1988-06-01), None
Cong Le Thanh
NEC Corporation
Nguyen Thuan T.
Scully Scott Murphy & Presser
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