as shown in fig4 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 fig4 will be described below with reference to fig5 . 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 fig6 . 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 fig6 the mobile stations are divided into two groups . while the mobile stations are shown as being grouped based on their positions in fig6 the mobile stations are actually grouped based on the propagation loses . in fig6 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 fig1 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 fig3 c . 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 fig8 . 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 .