1. Science & Technology Laboratory on Reliability & Environment Engineering, School of Reliability and Systems Engineering, Beihang University, Beijing 100191, P. R. China
2. Department of Engineering & Physics, Oral Roberts University, 7777 South Lewis Avenue, Tulsa, OK 74171, USA
3. Department of Electrical and Computer Engineering, Duke University, 132 Hudson Hall, Box 90291, Durham, NC 27708, USA