Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-08-06
2004-03-16
Tran, Cong Van (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S438000, C455S439000, C455S442000, C370S331000, C370S332000, C370S335000
Reexamination Certificate
active
06708030
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) radio data communication system, particularly to a selection method of a handoff system between a soft handoff system and a hard handoff system.
2. Description of the Related Art
As is well known, as multiple access (MA) systems, there are known a time division multiple access (TDMA) system, a frequency division multiple access (FDMA) system and a code division multiple access (CDMA) system. Among the MA systems, the CDMA system is adopted in a mobile communication system in view of the fact that it is strong at noise and excellent in concealing performance. Further, in the mobile communication system of the CDMA system, when a mobile station is moved from one cell (a radio zone covered by a radio base station, sometimes called as a service area) to other cell, communication can be carried out with the both radio base stations covering the cells. Because the same frequency for radio communications in any cell is utilized in the CDMA system, and even when the mobile station moves between the cells, a reception frequency or a transmission frequency needs not to change. This is referred to as soft handoff or soft hand over in this technical field.
In contract thereto, according to the TDMA system or the FDMA system, only so-to-speak hard handoff (hard hand over) can be carried out. An interruption of communication occurs in the hard handoff when a communication channel is switched from one radio base station to another radio base station. Incidentally, caution is required to the fact that the CDMA system is not only able to carry out only soft handoff but also able to carry out both of soft handoff and hard handoff.
As described above, according to the mobile communication system of the CDMA system, even when the mobile station moves between the cells, the mobile station can always communicate with at least one radio base station with no momentary interruption. Therefrom, the mobile communication system of the CDMA system is provided with an advantage in which not only voice communication but also data communication can be carried out. In this specification, a mobile radio system of a CDMA system capable of dealing with also data communication is referred to as “CDMA radio data communication system”.
Conventionally, the CDMA radio data communication system of this kind includes a radio base station control equipment (BSC) connected to a mobile switching center (MSC), a plurality of radio base stations (BTS) connected to the radio base station control equipment and a mobile station (MS) for carrying out data communication with the radio base stations while moving cells covered by the radio base stations. Further, the radio base station control equipment is also referred to as a base station control equipment or a radio control base station.
An explanation will be given of operation of a CDMA system in a hard handoff control in reference to FIG.
10
. Here, an explanation will be given of the operation when a mobile station is switched from a state of carrying out data communication with an radio base station (hereinafter, referred to as “old radio base station”) to data communication with other radio base station (hereinafter, referred to as “new radio base station”).
First, the mobile station carries out data communication with the old radio base station and the old radio base station carries out data communication with the base station control equipment. Assume that under the state, by movement of the mobile station to a vicinity of a boundary of a cell covered by the old radio base station, a reception electric field intensity of a pilot signal transmitted from the old radio base station at the mobile station is relatively weakened and by approach of the mobile station to a boundary of a cell covered by the new radio base station, a reception electric field intensity of a pilot signal transmitted from the new radio base station at the mobile station is relatively strengthened. In this case, the mobile station transmits a hard handoff request signal to the old radio base station. The old radio base station transmits the received hard handoff request signal to the base station control equipment.
In response to the hard handoff request signal, the base station control equipment carries out a channel assignment to the new radio base station. When the channel assignment can be confirmed, the new radio base station transmits a channel assignment confirmation signal to the base station control equipment. In response to the channel assignment confirmation signal, the base station control equipment transmits a handoff message to the mobile station via the old radio base station.
In response to the handoff message, the mobile station returns a handoff message response signal to the base station control equipment via the old radio base station. Thereafter, the mobile station transmits a preamble signal to the new radio base station. In response to the preamble signal, the new radio base station transmits establishment of synchronization to the base station control equipment. In response to the establishment of synchronization, the base station control equipment transmits a base station confirmation request signal to the mobile station via the new radio base station. In response to the base station confirmation request signal, the mobile station returns a base station confirmation response signal to the new radio base station.
Thereafter, the base station control equipment notifies channel release to the old radio base station. Further, the mobile station carries out data communication with the new radio base station and the new radio base station carries out data communication with the base station control equipment.
Next, an explanation will be given of operation of a CDMA system in a soft handoff control in reference to FIG.
11
. Also in this case, an explanation will given of the operation when the mobile station is switched from a state of carrying out data communication with the old radio base station to data communication with the new radio base station.
First, the mobile station carries out data communication with the old radio base station and the old radio base station carries out data communication with the base station control equipment. Assume that under the state, by movement of the mobile station to a vicinity of a boundary of a cell covered by the old radio base station, a reception electric field intensity of a pilot signal transmitted from the old radio base station at the mobile station is relatively weakened and by approach of the mobile station to a boundary of a cell covered by the new radio base station, the reception electric field intensity of a pilot signal transmitted from the new radio base station at the mobile station is relatively strengthened. In this case, the mobile station transmits a first soft handoff request signal to the old radio base station. The old radio base station transmits the received first soft handoff request signal to the base station control equipment. The first soft handoff request signal is a request signal for assigning other channel since radio wave of a channel which has been used in the communication becomes weak.
In response to the first soft handoff request signal, the base station control equipment carries out channel assignment to the new radio base station. When the channel assignment can be confirmed, the new radio base station transmits a channel assignment confirmation signal to the base station control equipment. In response to the channel assignment confirmation signal, the base station control equipment transmits a handoff message to the mobile station via the old radio base station.
In response to the handoff message, the mobile station returns a handoff message response signal to the base station control equipment via the old radio base station. Thereby, “soft handoff state”, surrounded by a quadrangle of dotted lines in
FIG. 11
, is brought about. The mobile station carries out data communica
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