Liu Qianlin, born in 1966, received his B. S. degree in radio communications engineering, and M. S. degree in Communications and Information Systems from PLA University of Science and Technology ( www. plaust, edu. en) , Nanjing, P.R. China, in 1989 and 1995 respectively. Currently, he is studying for his Ph. D. degree in Communications and Information Systems in Yunnan University (www. ynu. edu. cn) , Kunming, P. R. China. His main research interests inelude polling multiple-access communications system, satellite communications and wireless communications. To whom correspondence should be addressed. E-mail: Zhaodf123@ 263. net