1. School of Electrical and Computer Engineering, Georgia Institute of Technology Atlanta, GA 30332, USA
2. Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Hong Kong SAR, P. R. China
3. School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30313, USA
4. School of Automation, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China