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The objective of the present paper is to build a model which permits to capture and analyze the principal impacts of fading and multiuser diversity gain on the dynamic performance of an OFDMA cellular network. To this end, assuming Markovian arrivals and departures of customers that transmit some given data-volumes, as well as some temporal channel variability (fading), we study the mean throughput (and delay) that the network offers to users in the long-term evolution of the system. Explicit formulas are obtained in the case of allocation policies, which may or may-not take advantage of the fading, called respectively opportunistic and non-opportunistic. The main practical results of the present work are the following. Firstly we evaluate for the non-opportunistic allocation the degradation due to fading compared to AWGN (that is, a decrease of at least 13% of the throughput). Secondly, we evaluate the gain induced by the opportunistic allocation. In particular, when the traffic demand per cell exceeds some value (about 2 Mbps in our example), the gain induced by opportunism compensates the degradation induced by fading compared to AWGN.