1. School of Computer Science, Zhongyuan University of Technology, Zhengzhou 450007 China (e-mail: changlin@zut.edu.cn).
2. Electrical Engineering Department, Columbia University, New York, NY 10027 USA (e-mail: wangx@ee.columbia.edu).
3. Communications and Statistical Sciences Research Department, Nokia Bell Laboratories, Murray Hill, NJ 07974-0636 USA (e-mail: aa2354@columbia.edu).