I. Introduction
Orthogonal Frequency Division Multiplexing Access (OFDMA) is an ideal multi-access method for next generation wireless networks with its high spectrum efficiency, flexible resource allocation features. In OFDMA system, multiuserss can transmit data simultaneously in different subcarrier during an OFDM symbol. Since the channel fading in different subcarriers among different users are quite different, intricate subcarriers allocation and packet scheduling can effectively improve the system throughput and guarantee QoS(Qualify of Service) requirement.