1. School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China
2. National Key Laboratory of Application Specific Integrated Circuit, Hebei Semiconductor Research Institute, Shijiazhuang 050051, China
3. College of Communication Engineering, Chongqing University, Chongqing 400044, China
4. Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA