I. Introduction
Orthogonal frequency division multiplexing access(OFDMA) networks have no doubt of becoming one of the dominant strength in the future wideband wireless networks and mobile communication systems. It sufficiently takes advantage of the OFDM technology which is immune to frequency selective fading and quite suitable to be used for wideband communication. By using the frequency band in orthogonal subcarriers and time slots, OFDMA systems give a great freedom for their resource rescheduling to get large multi-user and multi-carrier gain. Many standardization communities have taken or would like to take OFDMA as a part of their transmission technologies for their wireless communication standards, such as wireless metropolitan area networks (IEEE 802.16), 3GPP's long time evolution (LTE), and wireless RAN (IEEE 802.22).