Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1998-06-29
2002-05-07
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000
Reexamination Certificate
active
06385186
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a down-traffic-signal transmission system for a mobile communication system in which communication between base stations and mobile stations is performed in accordance with the CDMA (Code Division Multiple Access) scheme.
2. Description of the Related Art
In a mobile communication system in which communication between base stations and mobile stations is performed in accordance with the CDMA scheme, when routing is switched from one base station to another base station in accordance with movement of a mobile station, such switching must be performed instantaneously such that a user does not notice it. Therefore, during switchover of base stations, a mobile station communicates with two or more base stations simultaneously (this technique will be called “soft handoff” hereinafter). At this time, the mobile station performs maximum ratio composition in which energies of signals received from the respective base stations are added together.
In the CDMA scheme, in order to allow a mobile station to communicate with a plurality of base stations simultaneously, each of the base stations modulates a signal through use of a different spread code, and the mobile station demodulates the signal through use of the same spread code as that used in the corresponding base station. Thus, the mobile station can extract the signal transmitted from each base station.
For example, International Patent Publication No. WO 95/08901 discloses a technique in which signals are modulated at respective base stations through use of different spread codes; the signals modulated through use of different spread codes are respectively received by means of independent reception units provided in a mobile unit; and demodulated signals output from the reception units are combined.
With reference to
FIG. 6
, a description will be given of such a conventional mobile communication system, particularly of a system for transmitting a down traffic signal from a base station to a mobile station.
FIGS.
6
(
a
) and
6
(
b
) show the structure of a conventional down-traffic-signal transmission system, wherein FIG.
6
(
a
) is a block diagram of the down-traffic-signal transmission system, and FIG.
6
(
b
) is a block diagram showing the structure of a spread-coding apparatus provided in a base station.
In FIG.
6
(
a
), numeral
51
denotes a base station controller
51
which establishes connection between a mobile communication exchange and base stations and controls the base stations. First and second base stations
52
and
53
are connected to the base station controller
51
via respective lines. A mobile station
54
communicates with the first and second base stations
52
and
53
via respective radio means. although FIG.
6
(
a
) shows an example in which the mobile station
54
communicates with two base stations (the first and second base stations
52
and
53
), three or more base stations may be installed.
As shown in FIG.
6
(
b
), each of the first and second base stations
52
and
53
has a spread-coding apparatus
60
which performs modulation through use of a spread code. The spread-coding apparatus
60
includes a modulator
64
. The spread-coding apparatus
60
of the first base station
52
performs modulation through use of a spread code C
1
, and the spread-coding apparatus
60
of the second base station
53
performs modulation through use of a spread code C
2
.
In the first and second base stations
52
and
53
there are composed a voice signal and a control signal (e.g., a signal for identifying a base station under communication)—which are separately transmitted from the base station controller
51
—and a thus-obtained composite signal is input to the spread-coding apparatus
60
as a traffic signal (input signal). An output signal from the spread-coding apparatus
60
is transmitted to the mobile station
54
as a down traffic signal.
The down traffic signal is not necessarily composed of both a voice signal and a control signal and in some cases may be composed of only a voice signal or a control signal. Further, the mobile station
54
includes reception units that can demodulate signals modulated through use of the spread codes C
1
and C
2
.
However, in the above-described conventional down-traffic-signal transmission system, when a fault occurs in communication between the base station controller and the base stations, the base stations may send different down traffic signals to the mobile station.
For example, if the first base station correctly receives a voice signal and a control signal and the second base station correctly receives only the voice signal at a timing where both the voice signal and the control signal have to be received by the first and second base stations, the mobile station receives different signals from the first and second base stations. In such a case, the mobile station cannot compose them, and therefore cannot reproduce a signal.
In order to solve such a problem, the down-traffic-signal transmission system may be modified such that a base station that has correctly received only a voice signal or a control signal does not transmit a down traffic signal to the mobile station. However, in this case, the voice signal or the control signal that is correctly received is not used in the mobile station. Further, if the first base station correctly receives only the control signal and the second base station correctly receives only the voice signal, no down traffic signal is transmitted to the mobile station.
SUMMARY OF THE INVENTION
The present invention has been accomplished to solve the above-described problems involved in the prior art techniques, and an object of the present invention is to provide a down-traffic-signal transmission system that allows a mobile station to effectively use a correctly received signal even when a base station correctly receives only one of a voice signal and a control signal transmitted from a base station controller.
Another object of the present invention is to provide a down-traffic-signal transmission system that can prevent a signal that is not correctly received by the base station from being received by a mobile station as noise that has a strong correlation with a correctly received signal and therefore makes reproduction of the signal impossible.
To achieve the foregoing objects, the present invention provides an improved down-traffic-signal transmission system for a mobile communication system comprising:
a plurality of base stations;
a mobile station capable of being connected with at least one base station via radio means; and
a base station controller provided for connecting a mobile communication exchange and the base stations and for controlling the base stations;
wherein communication between the base stations and the mobile station is performed in accordance with the CDMA scheme.
The down-traffic-signal transmission system is designed such that each of the base stations has a spread-coding apparatus for modulating, through use of a different spread code, a traffic signal obtained through composition of a voice signal and a control signal separately received from the base station controller; and the thus-modulated traffic signal is transmitted from the base station to the mobile station.
The spread-coding apparatus comprises:
a valid signal determination unit for determining, as valid information, a control signal and/or a voice signal contained in a traffic signal from the base station controller when correctly received, and determining, as BE invalid information, the control signal and/or the voice signal when incorrectly received;
a selection instruction unit for outputting a timing signal based on results of the determination of the valid signal determination unit, the timing signal indicating that the control signal is valid or invalid information and that the voice signal is valid or invalid information;
a selection unit which, in accordance with the timing signal, assigns to the control signal and/or the voice signal de
Abelson Ronald
NEC Corporation
Nguyen Chau
Whitham Curtis & Christofferson, P.C.
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