1. A method of decoding a video signal, comprising: selecting a collocated picture having a different temporal order from a current picture, the current picture comprising a current prediction block, the collocated picture comprising a temporal neighboring block that is used to derive a temporal merging candidate of the current prediction 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 comprising the spatial merging candidate and the temporal merging candidate; determining a motion vector and a reference picture index of the current prediction block by using the merging candidate list, the reference picture index of the current prediction block specifying a reference picture of the current prediction block, the reference picture of the current prediction block having a different temporal order from the collocated picture; obtaining prediction samples of the current prediction 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 prediction block; obtaining residual samples of a current transform block; and generating a reconstruct block by adding the residual samples and the prediction samples, 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 a fixed value of zero wherein the spatial neighboring block and the temporal neighboring block are determined based on a position of a coding block, a size of a block on which a parallel merge processing is applicable, and a size of the coding block, wherein the coding block comprises the current prediction block.