1. Assistant Professor, Northwestern Polytechnical Univ., Xi’an 710072, China (corresponding author).
2. Assistant Professor, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332.
3. Assistant Professor, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332.
4. Professor, Northwestern Polytechnical Univ., Xi’an 710072, China.