1. A method utilized in a wireless communication system comprising a mobile device and a plurality of base stations (i=1−n), the method comprising: obtaining a geometric matrix as H = [ e 11 e 12 e 13 1 e 21 e 22 e 23 1 ⋮ ⋮ ⋮ ⋮ e n 1 e n 2 e n 3 1 ] according to a plurality of relative distances between the mobile device and the plurality of base stations, wherein e i 1 = x ^ - X i r ^ i , e i 2 = y ^ - Y i r ^ i , e i 3 = z ^ - Z i r ^ i , (X i ,Y i ,Z i ), represents a position coordinate of the i th base station, ({circumflex over (x)},ŷ,{circumflex over (z)}) is utilized as a simplified position coordinate, and e j1 2 +e j2 2 +e j3 2 =1 is satisfied; obtaining a weighted matrix as W = [ 1 / σ 1 2 0 0 0 0 0 1 / σ 2 2 0 0 0 0 0 1 / σ 3 2 0 0 0 0 0 ⋱ 0 0 0 0 0 1 / σ n 2 ] = [ k 1 0 0 0 0 0 k 2 0 0 0 0 0 k 3 0 0 0 0 0 ⋱ 0 0 0 0 0 k n ] according to the mobile device and the plurality of base stations, wherein o represents a covariance related to a corresponding one of the plurality of base stations, and k i represents a reciprocal value of each corresponding covariance; obtaining a weighted geometric dilution of precision (WGDOP) as WGDOP=√{square root over (tr(H T WH) −1 )} according to the weighted matrix and the geometric matrix, so as to obtain a WGDOP closed form; and choosing a plurality of selected base stations from the plurality of base stations according to the WGDOP closed form, so as to position the mobile device.