Abstract:
A process of assigning frequencies uses propagation loss in a CDMA communication system. A base station measures propagation loss over a channel between the base station and a mobile station. If the measured propagation loss is greater than a certain fixed value, the base station determines that since the mobile station is present in a position distant from the base station, the mobile station may possibly interfere with a base station of another CDMA communication system. The base station assigns a frequency channel which is not adjacent to the frequency band assigned to the other CDMA communication system to the mobile station.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of assigning a frequency from a base station to mobile stations in a CDMA (Code Division Multiple Access) communication system. 
     2. Description of the Related Art 
     The CDMA communication system is expected to be the next-generation mobile communication system. The CDMA communication system is advantageous in that a plurality of adjacent base stations can share one frequency band. 
     In a CDMA communication system, different codes are assigned to respective channels, and signals are demodulated on the basis of the fact that there is a difference between autocorrelated values, which are correlated values between the same codes, and cross-correlated values, which are correlated values between different codes. It is therefore necessary that the cross-correlated values between the codes assigned to the respective channels are sufficiently lower than the autocorrelated values. It is most preferable that all the codes which are used be orthogonal to each other (the cross-correlated values be 0). However, since it is difficult for all the codes which are used by base stations that are adjacent to each other to be orthogonal to each other, the cross-correlated values are generally not 0. 
     If the cross-correlated values are not 0, then there is a possibility that when a mobile station A 1  is present near a base station and a mobile station A 2  is present far from the base station, depending on the ratio of transmission powers of the mobile stations A 1 , A 2  at the time signals from the mobile stations A 1 , A 2  arrive at the antenna of the base station, the correlated value (cross-corrected value) between the signal from the mobile station A 1  and the code assigned to the mobile station A 2  may be greater than the correlated value (autocorrelated value) between the signal from the mobile station A 2  and the code assigned to the mobile station A 2 . 
     In such a case, because the cross-corrected value is not lower than the autocorrelated value, the signal from the mobile station A 2  cannot properly be demodulated. This could lead to a fatally dangerous situation where if the signal power sent from one mobile station and received by the antenna of a base station is much greater than the signal power sent from another mobile station and received by the antenna of the base station, then the base station is unable to decode signals other than the signal from the mobile station whose reception power is larger at the antenna of the base station. This problem is widely known as a near-far problem. In order to solve the problem, it is necessary to perform a transmission power control process on transmitters for equalizing signal powers from all the transmitters in input signals received by a receiver. 
     If a mobile communication system is operated by a sole network operator and a handoff is properly carried out as a mobile station moves, then communications are effected via a path whose transmission loss is smallest between the base station and the mobile station. It is thus possible to perform a transmission power control process in order to minimize the transmission power of the mobile station for thereby solving the near-far problem. 
     Another problem occurs when a plurality of mobile communications are operated in one area by a plurality of network operators. In this situation, if the base stations are located at one site or mutual roaming is carried out between the different network operators, then no difficulty arises because a path whose transmission loss is smallest between base and mobile stations is practically selected. Since, however, such a solution is generally difficult to achieve, it has been customary to solve the above problem by assigning different frequency bands-to the respective network operators, as shown in FIG. 1 of the accompanying drawings. FIG. 1 shows the assignment of four frequency channels to each of network operators A, B, C. 
     If the frequency bands assigned to the respective network operators are successive, then in frequency channels located at a boundary between different frequency bands, the leaking power in an adjacent channel of a transmission signal from a mobile station causes a near-far problem to a frequency channel of another network operator. For example, in frequency channels fa 1 , fa 4  located at a boundary of the frequency band assigned to a network operator A, the leaking power in an adjacent channel of a transmission signal from a mobile station causes a near-far problem to a frequency channel fc 4  of a network operator C and a frequency channel fb 1  of a network operator B. 
     FIG. 2 of the accompanying drawings shows CDMA communication systems provided by a plurality of network operators and located in one area. As shown in FIG. 2, two network operators A, B provide respective CDMA communication systems in one area. The CDMA communication system provided by the network operator A has base stations (not shown) which communicate with mobile stations present in cells A 1 -A 7 . It is assumed that the cells A 1 -A 7  overlap a cell B 1  of a base station of the CDMA communication system provided by the network operator B, and that the frequency channels shown in FIG. 1 are assigned to each of the network operators A, B. 
     In FIG. 2, α represents the position of the base station of the cell A 1 , β represents the position of the base station of the cell B 1 , and γ represents the position of the base station of the cell A 4 . 
     In order to solve a near-far problem in one CDMA communication system, each of the base stations control the transmission powers of mobile stations linked thereto, and the reception powers at the antenna of each of the base stations are controlled at a constant level at all times irrespective of the positions of the mobile stations. Specifically, if the distance between a mobile station and a base station linked thereto is large, then the transmission power of the mobile station is increased, and if the distance between a mobile station and a base station linked thereto is small, then the transmission power of the mobile station is reduced. 
     In FIG. 2, when a mobile station belonging to the CDMA communication system provided by the network operator A moves from the position α to the position β to the position γ, the transmission power of the mobile station is first progressively increased from a low level to a maximum level at the position β. Near the position β, a handoff takes place from the base station of the cell A 1  to the base station of the cell A 4 . As the mobile station approaches the position γ, the transmission power of the mobile station is gradually lowered. 
     The transmission power of a mobile station in the cell A 1  which uses the frequency channel fa 4  changes as shown in FIG. 3 a  of the accompanying drawings. The reception power at the base station of the cell A 1  in the position a and the reception power at the base station of the cell A 4  in the position γ change as shown in FIG. 3 b  of the accompanying drawings. 
     As shown in FIG. 3 b , when the mobile station is located between the position α and the position β, the reception power at the base station of the cell A 1  is constant, and when the mobile station is located between the position β and the position γ, the reception power at the base station of the cell A 1  is gradually reduced. The reception power at the base station of the cell A 4  is gradually increased when the mobile station is located between the position α and the position β, and is constant when the mobile station is located between the position β and the position γ. When the reception powers are constant, the reception powers have the same level at the base stations of the cells A 1 , A 4 . At this time, the reception power in the frequency channel fb 1  of the base station in the position β of the cell B 1  of the network operator B is indicated by the solid-line curve in FIG. 3 c  of the accompanying drawings because of the leaking power in an adjacent channel from a mobile station. The conventional CDMA communication system has strict standards for the leaking power in an adjacent channel with respect to a transmitted signal from a mobile station, so that any interference which occurs with the base station of the cell when the mobile station is in the position β can be reduced to a level low enough not to cause practical problems to the CDMA communication system. 
     The reception power in the frequency channel fb 1  in the base station of the cell B 1  at the time the mobile station has a transmission frequency channel fa 3  is indicated by the dotted-line curve in FIG. 3 c . The reception power in the frequency channel fb 1  is low because the leaking power is exponentially reduced as the reception frequency channel is away from the transmission frequency channel. 
     As described above, the conventional CDMA communication system has strict standards for the leaking power in an adjacent channel with respect to a transmitted signal from a mobile station so as to solve the near far problem. Therefore, the transmitters of the mobile stations, particularly the final-stage power amplifiers thereof, are required to have a a high level of linearity, and hence have a large consumption power requirement and an increase in size. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of assigning a frequency from a base station to mobile stations in a CDMA communication system for solving a nearfar problem without strict. standards for the leaking power in an adjacent channel with respect to the transmitter of a mobile station and for reducing a consumption power requirement and size of base stations. 
     To achieve the above object, in a method of assigning a frequency in a CDMA communication system according to the present invention, a base station measures a propagation loss over a channel between the base station and a mobile station. If the measured propagation loss is greater than a certain fixed value, then the base station determines that since the mobile station is present in a position distant from the base station, the mobile station may possibly interfere with a base station of another CDMA communication system. The base station assigns a frequency channel which is not adjacent to the frequency band assigned to the other CDMA communication system, to the mobile station which may possibly interfere with a base station of another CDMA communication system. Therefore, it is possible to reduce interference with a base station belonging to another network operator, without strict standards for the leaking power in an adjacent channel of a transmitted signal of a mobile station. Therefore, the consumption power requirement and size of the transmitter of the mobile station can be reduced. 
     In another method of assigning a frequency in a CDMA communication system according to the present invention, a frequency channel which is not adjacent to the frequency band assigned to another CDMA communication system is assigned to a mobile station whose transmission power level is greater than a certain fixed value. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the assignment of four frequency channels to each of network operators; 
     FIG. 2 is a diagram showing cells of conventional CDMA communication systems provided by network operators A, B shown in FIG. 1; 
     FIG. 3 a  is a graph showing the manner in which the transmission power of a mobile station changes in the conventional CDMA communication systems changes; 
     FIG. 3 b  is a graph showing the manner in which the reception powers in the base stations of cells A 1 , A 4  in the conventional CDMA communication systems change; 
     FIG. 3 c  is a graph showing the manner in which the reception power in the frequency channel fb 1  of the base station of a cell B 1  in the conventional CDMA communication systems changes; 
     FIG. 4 is a diagram showing a CDMA communication system according to a first embodiment of the present invention; 
     FIG. 5 is a flowchart of a process of assigning a frequency in the CDMA communication system shown in FIG. 4; 
     FIG. 6 is a diagram showing cells of the CDMA communication system shown in FIG. 4; 
     FIG. 7 is a graph showing the manner in which the reception power in the frequency channel fb 1  of the base station of a cell B 1  in the CDMA communication system shown in FIG. 4 changes; and 
     FIG. 8 is a flowchart of a process of assigning a frequency in a CDMA communication system according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1st Embodiment 
     As shown in FIG. 4, a CDMA communication system according to a first embodiment of the present invention has a mobile station  10  and a pair of base stations  20 ,  21 . The base station  20  communicates with mobile stations in a cell A 4 , and the base station  21  communicates with mobile stations in a cell A 1 . For example, the mobile station  10  moves in the cell A 4 , and hence communicates with the base station  20 . 
     A process of assigning a frequency in the CDMA communication system shown in FIG. 4 will be described below with reference to FIG.  5 . 
     At first, in step  101 , value 1 and value 2 are set to V 0 . In step  102 , a base station measures a propagation loss in a channel established between the base station and each of the mobile stations. The base station can obtain a propagation loss by calculating the difference between a transmission power level of each of the mobile stations, which transmission power level is reported therefrom, and a reception power level of a signal received from each of the mobile stations by the base station. The base station can also obtain a propagation loss by calculating the difference between a reception power level of a signal received from the base station by each of the mobile stations, which reception power level is reported from the mobile station, and a transmission power level of the base station for each of the mobile stations. 
     In step  103 , the base station decides whether the propagation loss with respect to a mobile station is greater than value 1 or not. If the propagation loss is greater than value 1 in step  103 , then the base station assigns a frequency channel which is not adjacent to a frequency channel assigned to another CDMA communication system, to the mobile station in step  105 . If the propagation loss is equal to or smaller than value 1 in step  103 , the base station decides whether the propagation loss with respect to a mobile station is equal to or smaller than value 2 or not in step  104 . If the propagation loss is greater than value 2 in step  104 , the process is forwarded to step  103 . 
     If the propagation loss is equal to or smaller than value 2 in step  104 , then the base station preferentially assigns a frequency channel which is adjacent to a frequency channel assigned to another CDMA communication system, to the mobile station in step  106 . Finally, in step  107 , value 1 is set to V 1  and value 2 is set to V 2 , then process is forwarded to step  102 . 
     The process of assigning a frequency in the CDMA communication system will be described in specific detail below with reference to FIG.  6 . 
     As described above, the base station measures a propagation loss in a channel established between the base station and each of the mobile stations. Based on the measured propagation loss, the base station divides the mobile stations present in its own area into a plurality of groups. In FIG. 6, the mobile stations are divided into two groups. While the mobile stations are shown as being grouped based on their positions in FIG. 6, the mobile stations are actually grouped based on the propagation loses. 
     In FIG. 6, the mobile stations in the cell A 1  are divided into a group A 1   1  and a group A 1   2 , and the mobile stations in the cell A 4  are divided into a group A 4   1  and a group A 4   2 . The mobile stations whose propagation losses are equal to or smaller than a value are classified into the group A 1   1  or A 4   1 , and the mobile stations whose propagation losses are greater than the value are classified into the group A 1   2  or A 4   2 . 
     The mobile stations belonging to the groups A 1   2 , A 4   2  are assigned a frequency channel fa 2  or fa 3  in FIG. 1 which is not adjacent to a frequency channel assigned to another network operator from among the frequency channels assigned to the CDMA communication system. The mobile stations belonging to the groups A 1   1 , A 4   1  are preferentially assigned a frequency channel fa 1  or fa 4  which is adjacent to a frequency channel assigned to another network operator. 
     In the CDMA communication system according to the present embodiment, when a mobile station moves from a position α, to a position γ, the following frequency channel assignment is performed. 
     When the mobile station is in the position α, the base station of the cell A 1  handles the mobile station as belonging to the group A 1   2 , and preferentially assigns the frequency channel fa 1  or fa 4  to the mobile station. When the mobile station moves and its propagation loss becomes greater than the value, the base station of the cell A 1  classifies the mobile station into the group A 1   2 , and changes the frequency channel being used to fa 2  or fa 3 . When the mobile station further moves to the position β, a handoff occurs from the base station of the cell A 1  to the base station of the cell A 4 . At this time, the frequency channel being used is not changed and base station of the cell A 4  handles the mobile station as belonging to group A 4   2 . When the mobile station further moves and its propagation loss as measured by the base station of the cell A 4  becomes lower than the value, the base station of the cell A 4  handles the mobile station as belonging to the group A 4   1 , and preferentially changes the frequency channel being used to fa 1  or fa 4 . 
     The above frequency channel assignment, as shown in FIG.7, makes it possible to reduce an upper limit for the reception power in the frequency channel fb 1  in the base station of a cell B 1  of a network operator B at the position β,to a level lower than the interference power indicated by the solid-line curve in FIG. 3 c.    
     2nd Embodiment 
     In the first embodiment, a group to which a mobile station belongs is determined on the basis of a propagation loss over a channel with the mobile station, as measured by the base station. According to the second embodiment, a group to which a mobile station belongs is determined on the basis of the transmission power level of the mobile station, rather than a propagation loss. According to a transmission power control process carried out on the mobile station by the base station, the transmission power of the mobile station is controlled so as to be greater if the propagation loss is greater, and the transmission power of the mobile station is controlled so as to be smaller if the propagation loss is smaller. Therefore, the second embodiment can offer the same advantages as the first embodiment even though it employs the transmission power level of the mobile station. 
     A process of assigning a frequency in the CDMA communication system according to the second embodiment will be described below with reference to FIG.  8 . 
     At first, in step  601 , value 1 and value 2 are set to V 0 . In step  602 , a base station receives a transmission power level from a mobile station. The base station decides whether the transmission power level is greater than value 1 or not in step  603 . If the transmission power level is greater than value 1 in step  603 , then the base station assigns a frequency channel which is not adjacent to a frequency channel assigned to another CDMA communication system, to the mobile station in step  605 . If the transmission power level is equal to or smaller than value 1 in step  603 , the base station decides whether the transmission power level is equal to or smaller than value 2 or not in step  604 . If the transmission power level is greater than value 2 in step  604 , then the process is forwarded to step  603 . 
     If the transmission power level is equal to or smaller than value 2 in step  604 , then the base station preferentially assigns a frequency channel which is adjacent to a frequency channel assigned to another CDMA communication system, to the mobile station in step  606 . Finally, in step  607 , value 1 is set to V 1  and value 2 is set to V 2 , then process is forwarded to step  602 . 
     In the first and second embodiments described above, the mobile stations in one cell are divided into two groups. However, the principles of the present invention are also applicable to CDMAM communication systems where the mobile stations in one cell are divided into more than two groups. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.