1. Institute of Electrical Engineering, Yanshan UniversityQinhuangdao066004People's Republic of China
2. Department of Electrical EngineeringUniversity of Texas at ArlingtonArlingtonTX76019USA
3. Department of Electrical EngineeringUniversity of North TexasDentonTX76207USA
4. Department of Electrical and Computer EngineeringUniversity of Puerto Rico at MayagüezPR 00681‐9000Puerto Rico