One disclosure of the present invention suggests a method for synchronizing a downlink. The method for synchronizing downlinks comprises the steps of: receiving from at least one cell, which belongs to a random group, a synchronization signal; synchronizing the downlink in accordance with the synchronization signal; and using the downlink synchronization, which is performed on the at least one cell, to synchronize the downlink with a target cell, when the target cell that belongs to the random group is converted from an off state to an on state, wherein the random group comprises geographically adjacent cells, and wherein the cells that are comprised in the random group can have mutually different cell IDs, but a portion of the synchronization signals can be identical.