A soft decision method for determining a soft decision coordinate associated with a constellation is provided. The soft decision coordinate includes a first soft decision sub-coordinate and a second soft decision sub-coordinate. The method includes receiving an input signal including a coordinate value; defining a first coordinate range on a coordinate axis in the constellation, the first coordinate range having a first limit and a second limit; obtaining the first soft decision sub-coordinate according to the first coordinate range; defining a second coordinate range on the coordinate axis in the constellation, the second coordinate range having a third limit and a fourth limit; and obtaining the second soft decision sub-coordinate according to the second coordinate range; wherein the first and the third limit do not simultaneously equal to the second and the fourth limit.