To whom correspondence should be addressed. E-mail: mxu725@gmail.com Mao Xu, born in 1982. He received the B. S. degree in electronic information science and technology from Liaocheng University, Shandong, China, in 2005 and the M.S. degree in communication and informa- tion system from Chongqing University of Posts and Telecommunications, in 2008. He has been pursuing his Ph. D degree in Beijing University of Posts and Tel- ecommunications since 2008. His research interests include the heterogeneous wireless networks, cognitive radio systems and radio resource management.