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In IEEE 802.16 networks, initial ranging is a primary and important procedure of connection setup between subscriber stations and base station. The mandatory method defined in the standard of contention resolution is based on a truncated binary exponential backoff, with a fixed initial contention window size. However, the original algorithm neglects the possibility that the number of actively contending stations may change dynamically over time, leading to dynamically changing contention intensity. The major contribution of this paper is twofold: 1) we propose an accurate analytical model to analyze the performance of initial ranging requests in IEEE 802.16 networks. Two metrics, connection probability and average connection delay, are investigated to evaluate the network performance; 2) based on the above analysis, we propose an efficient performance improvement method by using dynamic window adjustment for initial ranging. Unlike the standard algorithm, this algorithm automatically adjusts the initial contention window to an optimal trade-off point between connection probability and connection delay. The performance revels that improving the service capacity and buffer size of base station can optimize the connection probability and the average connection delay. The numerical results also show that the optimal contention window adjustment outperforms the algorithm in the standard.