隨著無線網路裝置的數量將會很快地超過有線網路，無線網路上的需求也隨著漸漸增加。因此，IEEE 802.16標準被制定為一個新的無線寬頻技術，並且有非常高的資料傳送速率和廣大的覆蓋範圍。同時隨著網路上串流應用受到歡迎，視訊群播服務成為一個在WiMAX上深具潛力的應用。這篇論文的主要目標是要提出一個低複雜度的視訊層調變決策機制以確保頻寬使用的效率。
群播的主要問題是在於如何應付接收端的異質性。接收端的異質性會導致接收端有不同的調變接收限制，因此傳送端應該要選擇能夠滿足大多數接收端需求的調變方式。所以這篇論文期望能夠根據每個視訊層的大小、接收者分佈情況和一個視訊任務中已知可得的正交分頻多工存取(OFDMA)符號數量，在每個圖像群組(GOP)時間調整每個視訊層的調變。根據上述提到的影響因素，本篇論文分別使用加權平均(weighted average)和最大最小化(minimax)兩種方法來公式化調變的最佳化問題。同時，一個以基因演算法為基礎的演算法被提出用來降低決定最佳調變時所產生的複雜度。最後，實驗結果證明可調性調變的優點並且比較三種方法的結果。除此之外，本篇論文還討論當多個視訊任務時，正交分頻多工存取(OFDMA)符號數量的分配問題。

With the number of wireless network devices soon surpassing the number of wired network devices, the demand over wireless networks is increasing. Therefore, the IEEE 802.16 standard (commonly known as WiMAX), which has been proposed as a new wireless broadband standard, is capable of delivering very high data rate and covering wide area. Furthermore, the video multicast service would become one potential application over WiMAX with the popularity of streaming applications in the Internet. The main purpose of this thesis is to propose a low-complexity decision mechanism for the modulation of video coding layers to ensure the efficiency of bandwidth usage.
The main problem of multicasting is the diversity of receivers. The diversity may cause different receiving constrains for modulations. So, the transmitter should choose proper modulations to achieve the satisfactions of most receivers. Therefore, this thesis intends to adjust the modulation scheme of each coding video layer in each group of picture (GOP) time according to the size of video layer, receiver distribution, and available symbols of this video session. From above-mentioned factors, an optimization problem can be formulated and then solved with weighted average and minimax methods. Furthermore, a GA-based algorithm is proposed to reduce the complexity of modulation decision. Simulation results are shown to compare the above-mentioned three optimization methods.
In addition, this thesis also makes a study on allocation of given total downlink symbols to OFDMA subcarriers in several multicast video sessions.

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