1. A method for decoding a bitstream for a video signal, the method comprising: constructing a motion vector candidate group of a current block, wherein the current block refers to a first reference picture in a first reference picture list and a neighboring block of the current block refers to a second reference picture in a second reference picture list; assigning a motion vector candidate specified by index information from among the motion vector candidate group to a motion vector predictor of the current block, the index information being obtained from the bitstream; and decoding the current block based on the motion vector predictor, wherein the motion vector candidate group comprises a motion vector of the neighboring block when picture order count (POC) of the first reference picture is same as POC of the second reference picture and the first reference picture list is different from the second reference picture list.