1. A method of decoding a video signal, comprising: obtaining residual samples of a current block; selecting a collocated picture having a different temporal order from a current picture, the current picture comprising the current block, the collocated picture comprising a temporal neighboring block which is used to derive a temporal merging candidate of the current block; deriving a spatial merging candidate from a spatial neighboring block; deriving the temporal merging candidate from the temporal neighboring block; generating a merging candidate list including the spatial merging candidate and the temporal merging candidate; determining a motion vector and a reference picture index of the current block by using the merging candidate list, the reference picture index of the current block specifying a reference picture of the current block, the reference picture of the current block having a different temporal order from the collocated picture; obtaining prediction samples of the current block by using reconstruction samples within the reference picture of the current block, the reconstruction samples being indicated by the motion vector of the current block, wherein while a reference picture index of the spatial merging candidate is determined based on a reference picture of the spatial neighboring block, a reference picture index of the temporal merging candidate is set to fixed value of zero, the reference picture index of the temporal merging candidate being used to determine the reference picture index of the current block but not used to determine the collocated picture of the current block; and reconstructing the current block by using the residual samples and the prediction samples.