School of computer and communication, Hunan University, Changsha, Hunan, China. dragonbw@163.com|School of computer and communication, Hunan University, Changsha, Hunan, China. dragonbw@163.com|School of computer and communication, Hunan University, Changsha, Hunan, China. dragonbw@163.com|School of computer and communication, Hunan University, Changsha, Hunan, China. dragonbw@163.com