1.1 Motivation and Objectives
The Orthogonal Frequency Division Multiple Access (OFDMA) technique is one of the key technologies for future broadband radio systems. Due to its benefits concerning data transmissions in frequencyselective fading channels, it is employed in the broadband radio system standard MobileWiMAX (IEEE 802.16e) and the 3GPP Long Term Evolution (LTE). It is known that the exploitation of multiuser and resource diversity in an OFDMA system can significantly increase the system capacity compared to the use of conventional single carrier transmissionschemes [Hanzo et al., 2003]. For this reason, theOFDMAor multiuserOFDMtransmission technique has been in the focus of extensive research activities in the last years (e.g. [Wong et al., 1999, Rhee and Coiffi, 2000]). However, the emphasis of most activities was the physical layer (PHY) which means that the impact of different traffic load characteristics have been neglected in performance evaluation. The complete optimization of dynamic resource allocation in an OFDMA system has to take into account the interaction of medium access control (MAC) and PHY. This approach is known as crosslayer optimization [Song and Li, 2005] and represents one of the most important issues when designing guidelines for the development of future broadband OFDMA systems.