|Publication number||USRE39735 E1|
|Application number||US 10/282,449|
|Publication date||Jul 17, 2007|
|Filing date||Oct 29, 2002|
|Priority date||Jun 9, 1997|
|Also published as||EP0884918A2, EP0884918A3, EP0884918B1, EP1511343A2, EP1511343A3, EP1511344A2, EP1511344A3, US6141555, US20040229617, USRE41891|
|Publication number||10282449, 282449, US RE39735 E1, US RE39735E1, US-E1-RE39735, USRE39735 E1, USRE39735E1|
|Original Assignee||Nec Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (56), Non-Patent Citations (1), Referenced by (1), Classifications (11), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
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. 10/282,449 (the present application) and 10/868,886, both of which are divisional 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:
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 ).
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 pre-determined 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 best 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 to 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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|U.S. Classification||455/437, 370/332, 455/442, 370/331, 455/438|
|International Classification||H04W36/18, H04W48/12|
|Cooperative Classification||H04W36/18, H04W48/12|
|European Classification||H04W36/18, H04W48/12|