Abstract:
A CDMA wireless system which exchanges a signal between a plurality of mobile stations and a wireless base station by a CDMA (Code Division Multiple Access) method includes first and second hierarchical cells, and a base station control device. The first and second hierarchical cells constitute hierarchical cover areas of the wireless base station and are different in only transmission/reception frequency. The base station control device preferentially hands over a mobile station having a low spread rate among the mobile stations to either one of the first and second hierarchical cells. An interference wave suppressing method in this system is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a CDMA (Code Division Multiple Access) wireless system and an interference wave suppressing method of avoiding degradation in the quality of a mobile station near a base station in the system.  
           [0002]    As a CDMA feature, a large transmission power supplied from a base station to a mobile station located at a distant place greatly interferes with reception at a mobile station relatively close to the base station, degrading the quality at the near mobile station. As a technique which avoids this problem, there is proposed transmission power control.  
           [0003]    The transmission power technique increases/decreases a downlink transmission power from the base station so as to satisfy the SIR (Signal Interference Ratio) at the mobile station. If, however, the traffic increases at a given cell to increase the interference within the cell, all the mobile stations within the cell cannot satisfy the SIR, and transmission powers to all the mobile stations increases.  
           [0004]    In the conventional transmission power control described above, an increase in transmission power simultaneously increases signal and interference components at a given mobile station, failing to improve the SIR. This further increases downlink transmission powers to respective mobile stations, and the power balance is lost. In this case, the quality of each mobile station excessively degrades, and communication stops in the worst case.  
           [0005]    This phenomenon is considered to readily occur due to the coexistence of a mobile station with a large power and a mobile station with a small power within the same cell. To satisfy a specific reception quality when the spread rate is low (communication rate is high) at a mobile station, a larger transmission power is required in comparison with the case in which the spread rate is high (communication rate is low). In addition, a longer distance requires a larger transmission power.  
           [0006]    More specifically, when a mobile station in communication at a position relatively close to a CDMA wireless base station device and a mobile station which is downloading an image at a high rate by, e.g., IP (Internet Protocol) packets exist near the boundary of cells in the CDMA wireless base station device, the relative difference becomes large between a downlink transmission power to the mobile station close to the CDMA wireless base station device and a downlink transmission power to the IP packet mobile station apart from the CDMA wireless base station device. The CN (Carrier to Noise) of a signal received by the mobile station close to the CDMA wireless base station device decreases, degrading the quality of a downlink radio signal. The mobile station in this state requests the CDMA wireless base station device to increase the downlink transmission power in order to compensate for the degradation of the quality.  
           [0007]    A plurality of mobile stations close to the CDMA wireless base station device simultaneously request the device to increase downlink transmission powers under the influence of a high-rate IP packet mobile station apart from the CDMA wireless base station device. The high-rate IP packet mobile station also requests the CDMA wireless base station device to increase a downlink transmission power in order to maintain the reception quality. In the worst case, the power within the cell is unbalanced, and some speech communication processes or data transmission processes are interrupted, extremely degrading the communication quality.  
           [0008]    Hence, demands have arisen for suppressing the degradation of the reception quality of a mobile station within a given cell and the power unbalance within the cell caused by the interference of a mobile station which requires a large power due to a long distance, a high transmission rate (low spread rate), or the like.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide a CDMA wireless system capable of stabilizing the power balance within a cell and an interference wave suppressing method in the system.  
           [0010]    To achieve the above object, according to the present invention, there is provided a CDMA wireless system which exchanges a signal between a plurality of mobile stations and a wireless base station by a CDMA (Code Division Multiple Access) method, comprising first and second hierarchical cells which constitute hierarchical cover areas of the wireless base station and are different in only transmission/reception frequency, and base station control means for preferentially handing over a mobile station having a low spread rate among the mobile stations to either one of the first and second hierarchical cells. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a block diagram showing a CDMA wireless system according to the first embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a graph showing a temporal change in the transmission power of each mobile station in a CDMA wireless base station device shown in FIG. 1;  
         [0013]    [0013]FIG. 3 is a flow chart showing the operation of the CDMA wireless system according to the first embodiment;  
         [0014]    [0014]FIG. 4 is a flow chart showing the operation of a CDMA wireless system according to the second embodiment of the present invention;  
         [0015]    [0015]FIG. 5 is a flow chart showing the operation of a CDMA wireless system according to the third embodiment of the present invention; and  
         [0016]    [0016]FIG. 6 is a block diagram showing the CDMA wireless base station device shown in FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    The present invention will be described in detail below with reference to the accompanying drawings.  
         [0018]    [0018]FIG. 1 shows a CDMA wireless system according to the first embodiment of the present invention. In FIG. 1, a CDMA wireless base station system  100  comprises a CDMA wireless base station device  101 , a CDMA wireless base station control device (base station control means)  102  connected to the CDMA wireless base station device  101 , a cell (Cell_ 1 )  103  and cell (Cell_ 2 )  104  which receive services from the CDMA wireless base station device  101 , and CDMA mobile stations  105  to  108  which move within the cells  103  and  104 .  
         [0019]    As shown in FIG. 6, the CDMA wireless base station device  101  comprises a wireless unit  11  which exchanges radio signals via an antenna, a modulation/demodulation unit  12  which modulates/demodulates a spread code, an encoding/decoding unit  13  which performs error correction encoding/decoding processing, a baseband processing unit  14  which performs baseband processing of transmission power control, an interface processing unit  15  which performs interface processing with a line, and a control unit  16  which determines a mobile station to be handed over and controls the entire wireless base station. The cells  103  and  104  constitute hierarchical cells having antenna beams which are emitted by the same antenna of the CDMA wireless base station device  101  and cover areas in the same direction, and having different transmission/reception frequencies.  
         [0020]    One CDMA wireless base station control device  102  is installed for a plurality of CDMA wireless base station devices  101 , and executes wireless resource management including handover control for the CDMA wireless base station devices  101  managed by the control device  102 . The mobile stations  105  to  107  are located within the cells  103  and  104 , and perform communication relatively close to the CDMA wireless base station device  101 . The mobile station  108  is moving toward the boundary from the center of the cell  103 , and downloads a downlink image or the like at a high rate by IP packets.  
         [0021]    [0021]FIG. 2 shows an example of a temporal change in transmission power from the CDMA wireless base station device  101  to the mobile stations  105 ,  106 , and  108  in a situation as shown in FIG. 1. In FIG. 2, the total transmission power threshold to all the mobile stations in this cell is  200 .  
         [0022]    Processing when the transmission power exceeds the total transmission power threshold of  200  will be explained with reference to FIG. 3.  
         [0023]    In the CDMA wireless base station device  101 , a monitoring unit  14   a  of the baseband processing unit  14  calculates and monitors a total transmission power in a given cell (step S 1 ). A determination unit  16   a  of the control unit  16  determines whether the total transmission power output from the monitoring unit  14   a  continuously exceeds the maximum total transmission power threshold of  200  for a predetermined time or more (steps S 2  and S 3 ). If “YES” in step S 2  or S 3 , the control unit  16  sequentially extracts, in a descending order for mobile stations (a 1 ), resources which transmit data to the mobile stations with the largest power within the cell (step S 4 ).  
         [0024]    For the extracted mobile stations (a 1 ), the control unit  16  estimates a transmission power (whether each mobile station moves close or apart) on the basis of the communication rate in use and past data (step S 5 ). The control unit  16  sorts mobile stations sequentially from a mobile station which has a high communication rate and moves apart, and selects mobile stations (a 2 ) to be actually handed over between frequencies from the sorted mobile stations (step S 6 ).  
         [0025]    The control unit  16  determines whether handover is possible from information such as the total transmission power of a lower hierarchical cell serving as a handover destination and an uplink interference amount (step S 7 ). If the status of the lower hierarchical cell permits handover (“YES” in step S 8 ), the control unit  16  transfers the resource of a mobile station to be handed over to the CDMA wireless base station control device  102  (step S 9 ). The CDMA wireless base station control device  102  receives the resource information of the mobile station to be handed over from the CDMA wireless base station device  101 , and executes handover of the mobile station to the lower hierarchical cell by using a protocol with the mobile station (step S 10 ). If “NO” in step S 2 , S 3 , or S 8 , the flow returns to step S 1 .  
         [0026]    The first embodiment forcibly moves a large-transmission-power mobile station in a given cell to a lower hierarchical cell in response to, as a trigger, an excess of the total transmission power of the cell over a preset threshold. Consequently, upper cells in the hierarchy of hierarchical cells are used as small- to intermediate-power cells, and lower cells are used as large-power cells. The relative power difference between mobile stations within one cell decreases to stabilize the power balance within the cell. The influence of a mobile station requiring a large transmission power on other mobile stations within the cell can be reduced, thus stabilizing the communication quality within the cell.  
         [0027]    The operation of a CDMA wireless system according to the second embodiment of the present invention will be described with reference to FIG. 4. The arrangement of the CDMA wireless system according to the second embodiment is the same as that of the CDMA wireless system according to the first embodiment shown in FIG. 1.  
         [0028]    The above-described first embodiment moves a large-transmission-power resource to a lower hierarchical cell when the traffic of a given cell rises and exceeds the threshold. To the contrary, the second embodiment executes the following operation instead of performing processing after the traffic rises.  
         [0029]    In a CDMA wireless base station device  101 , a monitoring unit  14   a  monitors a transmission power to each mobile station in a normal state (step S 11 ). A determination unit  16   a  determines whether the transmission power to each mobile station exceeds the threshold. If the transmission power does not exceed the threshold, a control unit  16  hands over the mobile station to an upper hierarchical cell (step S 13 ). If the transmission power exceeds the threshold of the transmission power value, the control unit  16  hands over the mobile station to a lower hierarchical cell (step S 14 ). The CDMA wireless base station control device  102  executes resource management in accordance with the handover result.  
         [0030]    The operation of a CDMA wireless system according to the third embodiment of the present invention will be described with reference to FIG. 5. The arrangement of the CDMA wireless system according to the third embodiment is the same as that of the CDMA wireless system according to the first embodiment shown in FIG. 1.  
         [0031]    Whether a radio channel is set between a mobile station and a CDMA wireless base station device  101  for the first time is checked (step S 21 ). If “YES” in step S 21 , a CDMA wireless base station control device  102  sets the initial downlink transmission power value of the CDMA wireless base station device  101  (step S 22 ). That is, when the mobile station transmits the first channel setting request message, the CDMA wireless base station control device  102  determines an initial transmission power value by calculation based on the quality of a pilot channel always transmitted by the CDMA wireless base station device  101 .  
         [0032]    Each hierarchical cell has a limited transmission power range. By determining which of the transmission power ranges of hierarchical cells corresponds to the initial transmission power value determined by calculation, the mobile station is handed over to the hierarchical cell at the beginning of the channel setting (step S 23 ).  
         [0033]    While the radio channel is set, a monitoring unit  14   a  always monitors the downlink transmission power of each mobile station in a predetermined cycle (steps S 24  and S 25 ). Using an output from the monitoring unit  14   a , a determination unit  16   a  determines whether a downlink transmission power to a given mobile station exceeds the threshold for a predetermined period regardless of the total transmission power of the cell (step S 26 ). If “YES” in step S 26 , the determination unit  16   a  hands over the mobile station to a lower or upper hierarchical cell (step S 27 ).  
         [0034]    If “NO” in step S 24  or S 26 , the flow returns to step S 21 . If “NO” in step S 21 , the flow skips steps S 22  and S 23  and jumps to step S 24 .  
         [0035]    As has been described above, according to the present invention, a mobile station requiring a large transmission power which unbalances the power within a cell is detected, and the downlink transmission power of each resource within the cell is monitored in a predetermined cycle. If the downlink transmission power exceeds the threshold, the mobile station is handed over from the current cell to a lower hierarchical cell. Accordingly, power unbalance within a cell can be prevented, and the degradation of the quality of a downlink radio signal in a mobile station can be suppressed.  
         [0036]    Since a mobile station with a low spread rate (high communication rate) is preferentially handed over to a lower hierarchical cell, the power balance within a cell can be stabilized. The influence of a mobile station requiring a large transmission power on other mobile stations within the cell can be reduced, thus stabilizing the communication quality within the cell.