I. Introduction
Source localization remains one of the key requirements utilized in many audio applications, such as surveillance [1], video-conference, speech enhancement [2], and source separation [3]. However, there remain common limitations with regard to resolving closely-spaced sources in strong reverberant environments due to distortions caused by acoustic reflections and inner-interference among sources. Note that the resolution of localization improves largely by increasing the number of microphones under adverse conditions. With the proliferation of portable devices, acoustic research using small size microphone arrays has become increasingly promising [4], [5], and it will apparently turn into a great challenge to localize closely-spaced sources with a small number of microphones.