Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-03-30
2002-08-20
Kincaid, Lester G. (Department: 2685)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S437000, C455S443000, C455S448000, C455S067700, C370S333000
Reexamination Certificate
active
06438378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to handoff devices used for mobile communication systems employing CDMA system.
This application is based on Patent Application No. Hei 10-84718 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Generally, the term “handoff” is defined as the transfer of a cellular telephone call between cells when transmission quality drops below a specified connection threshold.
For example, the paper of Japanese Patent Application, Publication No. Hei 9-284827 discloses a handoff control system for the CDMA mobile communication system, which uses pilot signal sending devices to send pilot signals to the neighboring cells.
The paper of Japanese Patent Application, Publication No. Hei 8-149551 discloses a support system for a CDMA-to-CDMA different frequencies handoff in the cellular phone system of the CDMA system, which provides determination of the handoff to be effected between the base stations of different frequencies by using the primary frequency modulation.
In addition, the paper of Japanese Patent Application, Publication No. Hei 8-130766 discloses a mobile communication system, which selects the communication systems such as TDMA, CDMA and FDMA in response to the priority of the user.
Further, the paper of Japanese Patent Application, Publication No. Hei 10-42369 discloses an incoming call destination registration method for the mobile communication network, which automatically establishes an incoming call connection by managing a location of a radio mobile station which travels from one area to another.
By the way, according to the hard handoff method of the mobile communication system employing the CDMA system (where “CDMA” is an abbreviation for “Code Division Multiple Access”), a handoff from the service area of the CDMA system to the service area of another system is performed as follows:
A transmitter employing the CDMA system transmits pilot signals as identification signals. A “mobile” terminal device (or MS, i.e., mobile station) measures intensities of the identification signals. If measured values become greater than a prescribed value which is set in advance, a handoff request is issued with respect to a base station radio transceiver (or BTS, i.e., base transceiver station) employing the CDMA system.
However, the aforementioned handoff method of the mobile communication system suffers from a problem with regard to interference, as follows:
In
FIG. 7
, a radio communication area E used by the base transceiver station of the CDMA system partially overlaps with a radio communication area F used by the transmitter of the CDMA system. Herein, interference occurs in an overlapping area between the areas E and F. To avoid such interference, it is necessary to narrow down the radio communication area E that enables the communication service based on the CDMA system.
One reason why the aforementioned problem occurs is that in the CDMA system, both of the base transceiver station and transmitter use the same frequency. So, transmission waves of the transmitter act as interference waves as well. In the case where the transmitter amplifies input signals thereof up to a high output level when transmitting them, it is necessary to provide linearity in amplification up to such a high output level (or high electric power level). In consideration of the efficiency in such amplification, the transmitter consumes a large amount of electric power which is approximately twenty times as much as an amount of electric power generally consumed. For this reason, the transmitter inevitably demands a large amount of electric power. This raises a problem that the mobile communication system as a whole requires high cost and large scale of system configuration.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a handoff device for the mobile communication system that is capable of performing hard handoff from the service area of the CDMA system to the service area of another system without reducing the radio communication area of the CDMA system which is placed adjacent to the radio communication area of another system.
According to this invention, a mobile communication system actualizing a handoff is constructed by a mobile station, a first base transceiver station having a first radio communication area based on a CDMA system, a second base transceiver station having a second radio communication area based on a non-CDMA system, receiver stations and a base station controller. Herein, the mobile station travels from the first radio communication area to the second radio communication area while communicating with the first base transceiver station. When the mobile station detects an event that an Ec/Io value of a pilot signal transmitted thereto from the first base transceiver station exceeds a first threshold value, the base station controller determines the receiver stations which are located adjacent to the second base transceiver station. Then, the base station controller requests the receiver stations to receive uplink traffic signals respectively transmitted from the mobile station. The base station controller selects a receiver station receiving the uplink traffic signal having a best Eb/No value within the receiver stations. Such a best Eb/No value of the uplink traffic signal of the selected receiver is compared with a second threshold value. When it exceeds the second threshold value, the base station controller requests the mobile station to perform hard handoff from the first base transceiver station to the second base transceiver station while simultaneously requesting the second base transceiver station to enable handoff.
Incidentally, the first threshold value is determined in such a way that the first radio communication area is narrower than a limit radio communication area in which the mobile station is capable of communicating with the first base transceiver station. In addition, an Eb/No value of the uplink traffic signal received by the first base transceiver station is set to the second threshold value.
Thus, it is possible to broaden the radio communication area of the CDMA system in a direction toward the service area of the non-CDMA system at a hard handoff mode.
REFERENCES:
patent: 5313489 (1994-05-01), Menich et al.
patent: 5594718 (1997-01-01), Weaver, Jr. et al.
patent: 5697055 (1997-12-01), Gilhousen et al.
patent: 5754542 (1998-05-01), Ault et al.
patent: 5978679 (1999-11-01), Agre
patent: 6021328 (2000-02-01), Curtis et al.
patent: 6026301 (2000-02-01), Satarasinghe
patent: 6240292 (2001-05-01), Haberman et al.
patent: 6263205 (2001-07-01), Yamaura et al.
patent: WO 97/14261 (1997-04-01), None
patent: WO 98/27776 (1998-06-01), None
patent: 8-130766 (1996-05-01), None
patent: 8-149551 (1996-06-01), None
patent: 9-284827 (1997-10-01), None
patent: 10-42369 (1998-02-01), None
European Search Report dated Feb. 14, 2000.
Holeman et al., “CDMA Intersystem Operations” 1994, pp.590-594.
Satarasinghe, P. “A Novel Method for CDMA Hard Handoff” 1996, pp.1766-1768.
Kincaid Lester G.
McGinn & Gibb PLLC
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