A method in a present frequency sharing system of assigning a frequency in an environment where an incumbent system exists, the present frequency sharing system using a radio access technology, RAT, for communications, the method comprising: determining whether a sub-frequency resource solely assignable to the present frequency sharing system discriminated from the incumbent system exists, the sub-frequency resource being generated by dividing a target frequency resource assigned to the incumbent system and indicating a portion of an entire frequency band, which is used by the incumbent system; assigning to the present frequency sharing system the sub-frequency resource solely assignable to the present frequency sharing system, when it is determined that the sub-frequency solely assignable to the present frequency sharing system exists; determining whether a sub-frequency resource assigned to a frequency sharing system using the same RAT as the present frequency sharing system exists, when it is determined that the sub-frequency resource solely assignable to the frequency sharing system does not exist; assigning to the present frequency sharing system the sub-frequency resource assigned to the frequency sharing system using the same RAT, when it is determined that the sub-frequency resource assigned to the frequency sharing system using the same RAT exists, wherein the present frequency sharing system in the sub-frequency resource and the frequency sharing system using the same RAT share the sub-frequency resource according to a self-coexistence mechanism; and otherwise assigning to the present frequency sharing system a sub-frequency resource assigned to a frequency sharing system using a different RAT than the present frequency sharing system, when it is determined that the sub-frequency resource assigned to the frequency sharing system using the same RAT does not exist, wherein the present frequency sharing system in the sub-frequency resource and the frequency sharing system using the different RAT share the sub-frequency resource according to a inter-system coexistence mechanism. determining whether a sub-frequency resource solely assignable to the present frequency sharing system discriminated from the incumbent system exists, the sub-frequency resource being generated by dividing a target frequency resource assigned to the incumbent system and indicating a portion of an entire frequency band, which is used by the incumbent system; assigning to the present frequency sharing system the sub-frequency resource solely assignable to the present frequency sharing system, when it is determined that the sub-frequency solely assignable to the present frequency sharing system exists; determining whether a sub-frequency resource assigned to a frequency sharing system using the same RAT as the present frequency sharing system exists, when it is determined that the sub-frequency resource solely assignable to the frequency sharing system does not exist; assigning to the present frequency sharing system the sub-frequency resource assigned to the frequency sharing system using the same RAT, when it is determined that the sub-frequency resource assigned to the frequency sharing system using the same RAT exists, wherein the present frequency sharing system in the sub-frequency resource and the frequency sharing system using the same RAT share the sub-frequency resource according to a self-coexistence mechanism; and otherwise assigning to the present frequency sharing system a sub-frequency resource assigned to a frequency sharing system using a different RAT than the present frequency sharing system, when it is determined that the sub-frequency resource assigned to the frequency sharing system using the same RAT does not exist, wherein the present frequency sharing system in the sub-frequency resource and the frequency sharing system using the different RAT share the sub-frequency resource according to a inter-system coexistence mechanism.