近年來，正交分頻多工存取(OFDMA)的技術因為其更高的傳輸速率，而受到廣泛關注，並且在IEEE 802.16的標準中也採用了此項存取技術。頻譜資源分配問題是在正交分頻多工存取技術應用的一個瓶頸問題，由於基地台必須挑選在一個時幀(Frame)中要傳送的資料叢(Burst)，並且決定這些資料叢所使用的資源、調變和編碼，因此若是分配做的不好，就會產生許多不必要的浪費。我們在本篇論文中證明了OFDMA系統的下行(Downlink)頻譜資源分配問題是個NP-hard的問題，因此我們提出了一個名叫HORA的啟發式方法來增進頻譜資源的使用效率。HORA會逐個頻道分配資源，並在資源分配規劃方面採用了動態規劃(Dynamic programming)來減低各方面浪費產生的可能性。通過模擬實驗，我們證實了HORA與使用暴力法並運用分枝界定法(branch and bound)加快執行速度所計算出來的最佳配置相比，在頻譜使用率上有不錯的結果，並在計算時間上比起暴力法有著大幅的優勢，而且在平均可以達到95%以上的頻譜使用率。

Recently, Orthogonal Frequency Division Multiple Access (OFDMA) transmission technique is gaining popularity because of its higher transmission capacity. The IEEE 802.16 standard has also adopted the OFDMA as its access technique. The problem of resources allocation in time and frequency is the bottleneck of OFDMA system. Due to the base station has to choose a group of bursts to transfer and decide the using of radio resource, modulation and coding rates by the bursts, it will generate a lot of unnecessary waste when the allocation of radio resource is bad. We show that the resources allocation problem is NP-hard. Therefore, we proposed an approximation downlink bandwidth resource allocation scheme which is called AOBA to improve the downlink bandwidth utilization in this thesis. AOBA allocates every sub-channel one by one to prevent generating wastage. In the resources allocation, we adopted dynamic programming strategy to reduce the wastage which may occur in the allocation process. The simulation results show that compared AOBA with the optimal allocation which is calculated by brute force with branch and bound algorithm, our method is not only having good bandwidth utilization and stability, but also has a lot of superiority in execution time.

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