Wireless base station and control method thereof

1. A wireless base station comprising:

a plurality of antennas; and

a signal processor to take in reception signals from a mobile station to perform a signal transmission and reception with the mobile station by a first antenna providing a higher reception signal level, in which the signal processor switches over the signal transmission to the mobile station by the first antenna to that by a second antenna while keeping the signal reception from the mobile station by the first antenna, when a frequency variation characteristic measured for an allocated frequency of the reception signal from the first antenna indicates that the mobile station is moving away from the first antenna.

2. The wireless base station as claimed in claim 1 , wherein the signal processor determines that the mobile station is moving away from the first antenna when the frequency variation characteristic presents a negative characteristic and a variation range of the frequency variation characteristic exceeds a specified value.

3. The wireless base station as claimed in claim 1 , wherein after having switched over the signal transmission to the mobile station by the first antenna to that by the second antenna, when a variation range of the frequency variation characteristic measured with the second antenna falls below a specified value, the signal processor determines that the mobile station has moved within a fixed distance area of the second antenna, thereby switching over the signal reception from the mobile station by the first antenna to that by the second antenna as well.

4. The wireless base station as claimed in claim 1 , wherein the antennas and the signal processor are provided for each sector managed by the wireless base station, in which the adjacent antennas on adjacent sectors are set close to each other to present equivalent frequency variation characteristics.

5. A wireless base station system comprising:

a plurality of slave base stations; and

a master base station to take in reception signals of the slave base stations from a mobile station to select a first slave base station providing a higher reception signal level to perform a signal transmission and reception with the mobile station, in which the master base station selects a second slave base station to perform a signal transmission to the mobile station while keeping the signal reception from the mobile station by the first slave base station, when a frequency variation characteristic measured for an allocated frequency of the reception signal for the first slave base station indicates that the mobile station is moving away from the first slave base station.

6. A wireless base station system comprising:

a plurality of wireless base stations, each of which including:

a plurality of antennas; and

a signal processor to take in reception signals from a mobile station to perform a signal transmission and reception with the mobile station by a first antenna providing a higher reception signal level, in which the signal processor switches over the signal transmission to the mobile station by the first antenna to that by a second antenna when a frequency variation characteristic measured for an allocated frequency of the reception signal from the first antenna indicates that the mobile station is moving away from the first antenna; and after the signal transmission to the mobile station by the first antenna has been switched over to that by the second antenna, when a variation range of the frequency variation characteristic measured with the second antenna falls below a specified value, the signal processor determines that the mobile station has moved within a fixed distance area of the second antenna, thereby switching over the signal reception from the mobile station by the first antenna to that by the second antenna as well while keeping the signal reception from the mobile station by the first antenna;

the adjacent antennas on adjacent wireless base stations being set close to each other to present equivalent frequency variation characteristics.

7. A control method of a wireless base station including a plurality of antennas, the control method comprising:

taking in reception signals from a mobile station to perform a signal transmission and reception with the mobile station by a first antenna providing a higher reception signal level;

measuring, during the signal transmission and reception, a frequency variation characteristic for an allocated frequency of the reception signal from the first antenna; and

switching over the signal transmission to the mobile station by the first antenna to that by a second antenna while keeping the signal reception from the mobile station by the first antenna, when the frequency variation characteristic indicates that the mobile station is moving away from the first antenna.