1. A method of identifying directions of arrival from a plurality of radiation sources to an antenna array having N antennas (N being an integer) by a processor using an estimator of circular cumulants of a predetermined even-order (2q, q being an integer equal to or greater than 2), the method comprising: receiving signals by the N antennas and arranging the received signals into a plurality of sampled vectors x(t), “t” being the index denoting different sampled vector; calculating an estimated cumulant matrix Ĉ 2q,x (l) for a cumulant matrix C 2q,x (l) according to the sampled vectors x(t), where “1” referring to a predetermined cumulant ranking, and each component of C 2q,x (l) being Cum[x i 1 (t),. .. , x i q (t), x i q+1 (t)*,. .. , x i 2q (t)*] (1≦i j ≦N)(1≦j≦2q); decomposing the estimated matrix Ĉ 2q,x (l) into eigenelements, and calculating an estimate unit matrix Û 2q,n (l) for an unit matrix U 2q,n (l) of the eigenvectors of C 2q,x (l); calculating an estimated pseudo-spectrum {circumflex over (P)} Music-2q(l) (θ,φ) for a direction space (θ,φ), {circumflex over (P)} Music-2q(l) (θ,φ) being defined as: P ^ Music ⁢ - ⁢ 2 ⁢ q ⁡ ( l ) ⁡ ( θ , φ ) ⁢ = Δ ⁢ [ a ⁡ ( θ , φ ) ⊗ l ⊗ a ⁡ ( θ , φ ) * ⊗ ( q - l ) ] † ⁢ U ^ 2 ⁢ q , n ⁡ ( l ) ⁢ U ^ 2 ⁢ q , n ⁡ ( l ) † [ a ⁡ ( θ , φ ) ⊗ l ⊗ a ⁡ ( θ , φ ) * ⊗ ( q - l ) ] [ a ⁡ ( θ , φ ) ⊗ l ⊗ a ⁡ ( θ , φ ) ⊗ ( q - l ) ] † ⁡ [ a ⁡ ( θ , φ ) ⊗ l ⊗ a ⁡ ( θ , φ ) * ⊗ ( q - l ) ] a(θ,φ) being a matrix comprising directional vectors of the radiation sources; and identifying the directions of arrival of the radiation sources according to local minimums of the pseudo-spectrum {circumflex over (P)} Music-2q(l) (θ,φ).