When a mobile station becomes a soft handover state, a reception quality monitor 104 measures reception quality of a pilot signal from each base station. base station specifying means 105 generates a base-station select signal (BSse1) to specify base stations that have transmitted pilot channels of inferior reception quality. The base-station select signal is multiplexed with transmission data (r-txd), and the multiplexed signal is spread at a spreading circuit. The spread signal is transmitted to plural base stations through a transmitter 111 and a duplexer 102. Then the base stations specified by this base-station specify signal stops transmission of downward signals.
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0. 34. A communication method in a mobile station, comprising:
monitoring, in a soft handover state, reception qualities of pilot signals from a plurality of base stations;
comparing the reception qualities monitored;
generating a base station select signal specifying a base station on the basis of the reception qualities compared; and
transmitting the base station select signal to all of the plurality of base stations including a base station that is to stop transmission of a downward communication channel to a mobile station if the base station is not specified by the base station select signal.
0. 28. A base station among a plurality of base stations in a communication system, comprising:
a receiver which receives, from a mobile station, a base station select signal that is transmitted to all base stations, among the plurality of base stations, including a base station that is to stop transmission of a downward communication channel to said mobile station if the base station is not specified by the base station select signal; and
a transmitter which transmits the downward communication channel to the mobile station when said base station comprising said transmitter is specified by the base station select signal received.
0. 31. A base station among a plurality of base stations in a communication system, comprising:
a receiving means for receiving, from a mobile station, a base station select signal being transmitted to all base stations, among the plurality of base stations, including a base station that is to stop transmission of a downward communication channel to said mobile station if the base station is not specified by the base station select signal; and
a transmitting means for transmitting the downward communication channel to the mobile station when said base station comprising said transmitting means is specified by the base station select signal received.
0. 36. A communication method in communication system which comprises a plurality of base stations, comprising:
at a base station, receiving, from a mobile station, a base station select signal that is transmitted to all base stations, among the plurality of base stations, including a base station that is to stop transmission of a downward communication channel to said mobile station if the base station is not specified by the base station select signal; and
transmitting the downward communication channel to the mobile station when said base station receiving the base station select signal is specified by the base station select station receiving the signal received.
0. 22. A mobile station in a communication system, comprising:
a monitor which monitors, in a soft handover state, reception qualities of pilot signals from a plurality of base stations;
a base station selector which compares the reception qualities monitored by said monitor and generates a base station select signal specifying a base station on the basis of the reception qualities compared; and
a transmitter which transmits the base station select signal to all of the plurality of base stations including a base station that is to stop transmission of a downward communication channel to said mobile station if the base station is not specified by the base station select signal.
0. 24. A mobile station in a communication system, comprising:
a monitoring means for monitoring, in a soft handover state, reception qualities of pilot signals from a plurality of base stations;
a base station selecting means for comparing the reception qualities monitored by said monitor and generating a base station select signal specifying a base station on the basis of the reception qualities compared; and
a transmitting means for transmitting the base station select signal to all of the plurality of base stations including a base station that is to stop transmission of a downward communication channel to said mobile station if the base station is not specified by the base station select signal.
0. 33. A communication method in a mobile station, comprising:
monitoring, in a soft handover state, reception qualities of pilot signals from a plurality of base stations;
comparing the reception qualities monitored;
generating a base station select signal specifying a base station on the basis of the reception qualities compared; and
transmitting the base station select signal to a first base station that transmits a downward communication channel to said mobile station if the first base station is specified by the base station select signal and to a second base station, among the plurality of base stations, that stops a downward communication channel to said mobile station if the second base station is not specified by the base station select signal.
0. 19. A mobile station in a communication system, comprising:
a monitor which monitors, in a soft handover state, reception qualities of pilot signals from a plurality of base stations;
a base station selector which compares the reception qualities monitored by said monitor and generates a base station select signal specifying a base station on the basis of the reception qualities compared; and
a transmitter which transmits the base station select signal to a first base station that transmits a downward communication channel to said mobile station if the first base station is specified by the base station select signal and to a second base station, among the plurality of base stations, that stops a downward communication channel to said mobile station if the second base station is not specified by the base station select signal.
0. 23. A mobile station in a communication system, comprising:
a monitoring means for monitoring, in a soft handover state, reception qualities of pilot signals from a plurality of base stations;
a base station selecting means for comparing the reception qualities monitored by said monitor and generating a base station select signal specifying a base station on the basis of the reception qualities compared; and
a transmitting means for transmitting the base station select signal to a first base station that transmits a downward communication channel to said mobile station if the first base station is specified by the base station select signal and to a second base station, among the plurality of base stations, that stops a downward communication channel to said mobile station if the second base station is not specified by the base station select signal.
0. 29. A base station among a plurality of base stations in a communication system comprising:
a receiver which receives, from a mobile station, a base station select signal specifying a base station that is to perform transmission of a downward communication channel to said mobile station, wherein said receiver receives the base station select signal that specifies its own base station or, alternatively, the base station select signal that does not specify its own base station, in a soft handover state with respect to the mobile station; and
a transmitter which transmits a downward communication channel to the mobile station when said base station comprising said transmitter is specified by the base station select signal received, and stops the transmission of the downward communication channel to the mobile station when said base station comprising said transmitter is not specified by the base station select signal received.
0. 35. A communication method, comprising:
at a base station, receiving, from a mobile station, a base station select signal specifying a base station that is to perform transmission of a downward communication channel to said mobile station, comprising
receiving the base station select signal that specifies said base station receiving the base station select signal or, alternatively, the base station select signal that does not specify said base station receiving the base station select signal, in a soft handover state with respect to the mobile station; and
transmitting, from a transmitter, a downward communication channel to the mobile station when said base station comprising the transmitter is specified by the base station select signal received, and stopping the transmission of the downward communication channel to the mobile station when the base station comprising the transmitter is not specified by the base station select signal received.
0. 32. A base station among a plurality of base stations in a communication system comprising:
a receiving means for receiving, from a mobile station, a base station select signal specifying a base station that is to perform transmission of a downward communication channel to said mobile station; and
a transmitting means for transmitting a downward communication channel to the mobile station when said base station comprising said transmitting means is specified by the base station select signal received, and stopping the transmission of the downward communication channel to the mobile station when said base station comprising said transmitting means is not specified by the base station select signal received, wherein
said receiving means receives the base station select signal that specifies its own base station or, alternatively, the base station select signal that does not specify its own base station, in a soft handover state with respect to the mobile station.
0. 25. A base station among a plurality of base stations in a communication system comprising:
a receiver which receives, from a mobile station, a base station select signal specifying a base station that is to perform transmission of a downward communication channel to said mobile station, wherein said receiver receives the base station select signal that specifies said base station comprising said receiver or, alternatively, the base station select signal that does not specify said base station comprising said receiver, in a soft handover state with respect to the mobile station; and
a transmitter which transmits a downward communication channel to the mobile station when said base station comprising said transmitter is specified by the base station select signal received, and stops the transmission of the downward communication channel to the mobile station when said base station comprising said transmitter is not specified by the base station select signal received.
0. 30. A base station among a plurality of base stations in a communication system comprising:
a receiving means for receiving, from a mobile station, a base station select signal specifying a base station that is to perform transmission of a downward communication channel to said mobile station; and
a transmitting means for transmitting a downward communication channel to the mobile station when said base station comprising said transmitting means is specified by the base station select signal received, and stopping the transmission of the downward communication channel to the mobile station when said base station comprising said transmitting means is not specified by the base station select signal received, wherein
said receiving means receives the base station select signal that specifies said base station comprising said receiving means or, alternatively, the base station select signal that does not specify said base station comprising said receiving means in a soft handover state with respect to the mobile station.
0. 37. A communication system comprising a mobile station and a plurality of base stations, wherein
said mobile station comprises:
a monitor which monitors, in a soft handover state, reception qualities of pilot signals from the plurality of base stations;
a base station selector which compares the reception qualities monitored by said monitor and generates a base station select signal on the basis of the reception qualities compared; and
a transmitter which transmits the base station select signal to a first base station that transmits a downward communication channel to said mobile station if the first base station is specified by the base station select signal and to a second base station, among the plurality of base stations, that stops a downward communication channel to said mobile station if the second base station is not specified by the base station select signal, and
each of the plurality of base stations comprises
a receiver which receives, from said mobile station, the base station select signal that specifics said base station comprising said receiver or, alternatively, the base station select signal that does not specify said base station comprising said receiver in a soft handover state with respect to the mobile station.
0. 1. A cellular system using a direct spread code division multiple access system, in which a mobile station performs soft handover with plural base stations, said mobile station comprising:
downward reception quality monitoring means for monitoring reception quality of downward signals from the plural base stations that are in the soft handover state;
base-station specifying means for comparing reception quality of downward signals from respective base stations monitored at said downward reception quality monitoring means, and for generating a base-station specify signal which specifies a base-station having the best reception quality monitoring result to maintain a signal transmission;
multiplexing means for multiplexing the base-station specify signal into an upward signal to be sent to said plural base stations; and
reception means for performing combined reception of receive signals from said plural base stations that are in the soft handover state, and
said base stations each comprising:
demodulation means for demodulating the base-station specify signal multiplexed in the upward signal; and
transmission control means for receiving the base-station specifying signal demodulated by said demodulation means, for keeping a downward signal transmission when its own base station is specified in the base-station specify signal, and for stopping a downward signal transmission when its own base station is not specified in the base-station specify signal.
0. 2. The cellular system according to
0. 3. The cellular system according to
0. 4. The cellular system according to
0. 5. In a cellular system using a direct spread code division multiple access system, a mobile station used in the cellular system for performing soft handover with plural base stations, comprising:
downward reception quality monitoring means for monitoring reception quality of downward signals from the plural base stations that are in the soft handover state;
base-station specifying means for comparing reception quality of downward signals from respective base stations monitored at said downward reception quality monitoring means, and for generating a base-station specify signal which specifies a base-station having the best reception quality monitoring result to maintain a signal transmission;
multiplexing means for multiplexing the base-station specify signal into an upward signal to be sent to said plural base stations; and
reception means for performing combined reception of receive signals from said plural base stations that are in the soft handover state.
0. 6. The mobile station according to
0. 7. The mobile station according to
0. 8. A base station provided in a cellular system in which a plurality of mobile stations according to
demodulation means for demodulating the base-station specify signal multiplexed in the upward signal; and
transmission control means for receiving the base-station specify signal demodulated by said demodulation means, for keeping a downward signal transmission when its own base station is specified in the base-station specify signal, and for stopping a downward signal transmission when its own base station is not specified in the base-station specify signal.
0. 9. The base station according to
0. 10. A cellular system using a direct spread code division multiple access system, in which a mobile station performs soft handover with plural base stations,
said mobile station comprising:
reception quality monitoring means for monitoring reception quality of downward signals from the plural base stations that are in the soft handover state and outputting a sub-signal composed of the monitoring results and signals indicative of base stations corresponding to the respective results;
multiplexing means for multiplexing the sub-signal into an upward signal to be sent to said plural base stations; and
reception means for performing combined reception of receive signals from said plural base stations that are in the soft handover state, and
said base stations each comprising:
demodulation means for demodulating the sub-signal multiplexed in the upward signal;
transmission halt signal generating means for receiving the sub-signal demodulated by said demodulation means, comparing reception quality of downward signals from respective base stations monitored at the mobile station indicated in the sub-signal, and for generating a transmission halt signal which indicates a signal transmission instruction whether transmission should be allowed or halted at its own base station in accordance with the reception quality monitoring result indicated in the sub-signal; and
transmission control means for receiving the transmission halt signal from said transmission halt signal generating means, and for setting a signal transmission state of a downward signal from its own base station, whether a signal transmitting or a signal stopping, in accordance with the instruction indicated in the transmission halt signal.
0. 11. The cellular system according to
0. 12. The cellular system according to
0. 13. The cellular system according to
0. 14. The cellular system according to
0. 15. A base station provided in a cellular system in which a plurality mobile stations, each equipped with means for monitoring reception quality of downward signals from plural base stations with soft handover state and transmitting a sub-signal composed of the monitoring results for each base station, are utilized, said base station comprising:
demodulation means for demodulating the sub-signal;
transmission halt signal generating means for receiving the sub-signal demodulated by said demodulation means, comparing reception quality of downward signals from respective base stations monitored at the mobile station indicated in the sub-signal, and for generating a transmission halt signal which indicates a signal transmission instruction whether allowed or halted at its own base station in accordance with the reception quality monitoring result indicated in the sub-signal; and
transmission control means for receiving the transmission halt signal from said transmission halt signal generating means, and for setting a signal transmission state of a downward signal from its own base station, whether a signal transmitting or a signal stopping, in accordance with the instruction indicated in the transmission halt signal.
0. 16. The base station according to
0. 17. The base station according to
0. 18. The base station according to
0. 20. A mobile station according to
a multiplexer which multiplexes the base station select signal and upward data into an upward signal, wherein said transmitter transmits the upward signal.
0. 21. A mobile station according to
the base station select signal specifies a base station that is transmitting a pilot signal of the best reception quality.
0. 26. A base station according to
the base station select signal and upward data are multiplexed into an upward signal, and said receiver receives the upward signal.
0. 27. A base station according to
the base station select signal specifies the base station that is transmitting a pilot signal of the best receiving quality at the mobile station.
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Notice: More than one reissue application has been filed for the reissue of U.S. Pat. No. 6,141,555. The reissue applications are application Ser. Nos. 10/868,886(the present application) and 10/282,449, now U.S. Pat. No RE39,735, of which the present application is a continuation, and both of which are reissues of U.S. Pat No. 6,141,555.
The present invention relates to a mobile communication system, and particularly to a handover technique for mobile or portable telephone systems (cellular systems) using a direct spread code division multiple access (DS-CDMA) system. More particularly, it relates to a transmit power control of down links for transmission from plural base stations to a mobile station during soft handover where the mobile station is simultaneously connected to the plural base stations.
A conventional technique well known in the art is a cellular system using a code division multiple access (CDMA) system conforming a North American TIA/EIA IS-95 standard. This IS-95 standard uses soft handover. In a system using such a soft handover technique, when a mobile station approaches a boundary between cells (or sectors), the mobile station simultaneusly communicates with plural base stations each having one of the cells as its service area.
When the mobile station detects a base station or stations at reception levels larger than the base station currently in communication therewith, the mobile station regards itself as approaching a cell boundary and starts communication with plural base stations including the currently connected base station and other base stations detected at the larger reception levels. The mobile station then receives the same downward information from the plural base stations, where the downward information from the plural base stations is received at the mobile station by a maximal ratio combining diversity.
Upward information transmitted from the mobile station is received at the plural base stations. These received signals are maximal ratio combined or one of these signal is by a base station controller (BSC) which supervises the plural base stations.
In general, any mobile station located near a cell boundary is likely far at a distance from the base station in the cell, and the reception level of downward signals from the base station would be low. Further, the channel quality is likely deteriorated near the cell boundary due to radio interference from plural cells. Such deterioration of the channel quality can be prevented by connecting the mobile station to plural base stations (by a soft handover).
In a conventional technique such as one conforming the IS-95 standard, the downward signals are simultaneously transmitted from plural base stations during soft handover. Such a system extremely increases the number of currently used down links compared with cellular systems not using the soft handover. An increase in the number of mobile stations during soft handover limits the number of links that can be used simultaneously because of a limited capacity of down links to cause a latent problem that efficiency in channel utilization can not be improved.
Although the IS-95 standard seems to use such an ineffective method that it transmits downward signals from plural base stations, the capacity of down links has not caused any problem because the transmission system of down links has been more effective than that of up links.
However, the capacity of up links is now improving as much as that of down links, and the deterioration in the capacity of down links due to the soft handover needs to be solved.
It is therefore the main object of the present invention to provide a soft handover scheme capable of improving efficiency of down links in a CDMA mobile communication system by improving a downstream transmission system in a soft handover with a terminal located near a cell or sector boundary.
According to one aspect of the invention, there is provided a cellular system using a direct spread code division multiple access system, in which a mobile station performs soft handover with plural base stations,
said mobile station comprising:
The mobile station monitors quality of downward signals from plural base stations in a handover state and multiplexes into an upward signal a signal to specify the best-quality base station. This permits only the base station in good channel conditions to perform downstream transmission even at the time of handover with inhibiting transmission from the other base stations, thus improving the frequency utilization efficiency of down links.
Referring next to the accompanying drawings, the present invention will be described.
As shown in
The RAKE receiver 108 includes a correlator 106 and a maximal ratio combiner 107 for outputting downward communication channel data (f-txd).
The downward reception quality monitoring means 104 of the mobile station measures the level of reception quality using a pilot channel transmitted from each base station to all the mobile stations.
The base-station specifying means 105 selects base stations out of plural ones as having good downward reception quality, and outputs a base-station select signal indicative of the selected base stations. For this selection, any of the following criteria can be used:
Referring to
The common section comprises an antenna 201 for receiving radio signals from mobile stations; a duplexer (DUP) 202; a radio receiving part (Rx) 203 for converting a radio signal to a receive base-band signal; a spreading circuit 218 for spreading a pilot channel (PLCH); and an adder 209 for adding and combining the pilot channel and transmit signals of plural channels from the plural channel sections 208. The pilot channel and the plural communication channels use different spread codes from each other.
The channel section 208 necessary for each communication channel comprises an RAKE receiver 204 for reversely spreading signals received on a multipath channel and combining them at the maximum ratio; a demultiplexer (DMUX) 205 for separating the output of the RAKE receiver 204 into upward communication channel data (r-rxd) and the associated control channel including the base-station select signal (Bsse1); a spreading circuit 206 for spreading downward communication channel data (f-txd); and transmission control means 207 for controlling transmission ON/OFF according to the base-station select signal (Bsse1) included in the associated control channel data from the mobile station.
Referring next to
The base station (BS2) informs the base station controller (BSC) 304 the reception of the downward communication channel data from the moble station. The base station controller maximal-ratio-combines the received downward communication channel data at the base stations 301 and 302. the combined data is transmitted to a mobile station exchnger (MSC) 305. Then, the mobile station exchnger transfers the downward communication channel data not only to the base station (BS1) but also to the base station (BS2). These operations are the same as the prior art, such as IS-95 standards.
The operations of the base stations (BS1, BS) and the mobile station on the downward communication channel data are now described. In this example of
The transmission control means 207 of each of the base stations 301 and 302 stops transmission when its own base station is not specified in the base-station select signal. The transmission control means 207 performs transmission to the mobile station when its own base station is specified in the base-station select signal or a transmission error in the base-station select signal is detected; otherwise, it will stop transmission.
In a conventional scheme (e.g., IS-95) as shown at a) and b) in
On the other hand, shown at c) to e) in
The procedure for soft handover in the case of
By operating the base stations and the mobile station 303 in the above procedure, only some base stations of good propagation quality perform downstream transmission and the other base stations of inferior propagation quality do not need to perform downstream transmission. It is therefore possible to improve the frequency utilization efficiency of down links.
The mobile station 303 measures the quality of down links using the pilot channel, which is always transmitted at each base station and commonly used by all the mobile station to perform base station search. For this reason, the mobile station can also monitor the quality of down links of other base stations that have stopped transmission to the mobile station, and hence instruct these base stations to restart downstream transmission thereto.
When judging that the down links of plural base stations show almost equal quality and the received signals from the plural base stations can be combined at the maximum ratio without any loss, the mobile station 303 instructs the corresponding plural base stations to perform downstream transmission so that a diversity gain can be obtained.
When judging that the quality of down links of all the base stations is deteriorated and required quality can not be obtained by the transmission from one base station, the mobile station instructs plural base stations of relatively good reception quality to perform downstream transmission so that the required quality can be obtained.
The danger of an error in receiving the base-station select signal transmitted from the mobile station 303 must be considered here. Particularly, in the case the soft handover is performed across plural base stations, respective base stations may cause a different demodulation result and no base station may judge to be instructed for transmission. To avoid such a case, each base station performs transmission without receiving the instruction when an error in the demodulation result has been detected, thus extremely reducing the probability of no base station performing transmission [see the above procedure [5]).
However, if it is assured that plural base stations cause the same demodulation result of the base-station select signal, they can perform transmission ON/OFF according to the base-station select signal irrespective of the presence or absence of the error detection. For example, when the soft handover is performed for plural sectors within one base station, the demodulation result of the base-station select signal can be easily used among the sectors in common. In this case, the transmission sectors may be selected according to the base-station select signal.
Referring next to
As apparent from a comparison between
Also as apparent from a comparison between
As apparent from the above, the first embodiment and the second embodiment differ in the following point: In the first embodiment the mobile station decides which of base stations is to stop downstream transmission during soft handover, while in the second embodiment the base station decides which of base stations is to stop downstream transmission during the soft handover. Since the second embodiment does not need the base station specifying means 105 essential for the mobile station (
The transmission halt signal generating means 105a of the second embodiment can use any criterion such as one of the following three criteria for outputting the transmission halt signal.
(1a) The transmission halt signal generating means outputs the transmission halt signal when the downward signal from its own station to the mobile station shows a large difference in reception quality from that of the best-quality base station.
(2a) The transmission halt signal generating means outputs the transmission halt signal when the reception quality of the downward signal from its own station to the mobile station is ranked lower than a predetermined one graded according to their reception quality.
(3a) The transmission halt signal generating means outputs the transmission halt signal when the reception quality of the downward signal from its own station to the mobile station is smaller than a predetermined value. When all the base stations show reception quality smaller than the predetermined value and the reception quality of its own base station is ranked higher than the predetermined one graded according to their reception quality, the transmission halt signal generating means does not output the transmission halt signal even if the reception quality of the downward signal is smaller than the predetermined value. This optional condition is provided to prevent all the base station in the soft handover state from stopping transmission to the mobile station when the reception quality of the downward signals is low at the mobile station.
As described above, according to the present invention, since only a small number of base stations of the bast quality perform downstream transmission even at the time of soft handover, the frequency utilization efficiency of down links is not reduced. It is therefore possible to avoid limitations on the channel capacity.
The present invention is also to prevent transmission from the base station in an inferior propagation state even at the time of soft handover, so that mean transmission power of the base station can be reduced, thereby simplifying the transmission power amplifier at the base station.
Modifications of the invention herein disclosed will occur to a person skilled in the art and all such modifications are deemed to be within the scope of the invention as defined by the appended claims.
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