To whom correspondence should be addressed. E-mail: chenehanghai@gmail. com Chen Changhai, born in 1980. He is a Ph. D. candidate in Key Laboratory of Universal Wireless Communications, Ministry of Education, Beijing University of Posts and Telecommunications. He also received his B.S. and M.S. degrees from Beijing Uni- versity of Posts and Telecommunications in 2003 and 2006 respectively. His research interests include channel estimation, MIMO, OFDM and transmission capacity of ad hoc networks.