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It is well known that channel-dependent OFDMA resource assignment algorithms provide a significant performance improvement compared to static (i.e. channel-unaware) approaches. Such dynamic algorithms constantly adapt resource assignments to current channel states according to some objective function. Due to these dynamics, it is difficult to predict the resulting performance for such schemes given a certain scenario (characterized by the number of terminals in the cell and their average channel gains). Hence, previous work on admission control for OFDMA systems neglects the performance improvement from channel-dependent resource assignments and bases analysis on the average channel gains instead. In this paper we provide for the first time an analytical framework for admission control in OFDMA systems applying channel-dependent resource assignments. The framework is based on fundamental transformations of the channel gains caused by the channel-dependent assignment algorithms. We provide closed-form expressions for these transformations and derive from them probability functions for the rate achieved per terminal and frame. These functions can then be used for admission control as demonstrated in this paper for Voice-over-IP streams in IEEE 802.16e systems.