在資料廣播環境的架構下，要求式廣播是一個有效率的無線資料傳輸方式。在真實的廣播環境中，使用者所發出的請求通常是包含多個資料項的，因此近來有很多相關的研究開始在探討多個資料項請求的廣播排程。然而，傳統的要求式資料廣播排程假設每一個廣播時間單位（one time slot）只能存放一個資料項，因此一旦使用者錯過所需要的資料項，則必須要等待下一個廣播週期才能收到所需要的資料項。在這裡本研究採用了一個網路編碼的技術，將多個資料項編碼成一個編碼資料項，配置到一個廣播時間單位上並且資料項可以重複出現在不同的排程位置，因此一個廣播時間單位就有可以服務不同資料項的請求，要是使用者錯過所需要的資料項時，就可以在不同排程位置上，利用本身已經Cache的資料項來解碼，就可以獲得所需要的資料項。在本研究中，我們以網路編碼為基礎並且考量了請求的時效性，提出了一個新的衡量標準來決定哪一個請求要優先被服務，其計算出來的權重值越高的越優先服務。根據我們的方法與[3]比較的實驗結果顯示，我們的方法在平均存取時間上只多了5%的存取時間，但是在請求成功的比率上，我們的方法提高了20％的請求成功比率。 On-demand broadcast is an effective wireless data dissemination technique in data broadcast environment. In real world, users' queries usually include multiple data items. Recently, there are many related papers began to explore multi-item queries of data scheduling. However, traditional on-demand data broadcast scheduling assume that each time slot includes only one data item. Therefore, the above constraint demands a mobile user to wait until the next broadcast cycle to retrieve the data item if he misses the item in the cycle. Using network coding technique, server can encode and broadcast multiple data items in a time slot. The data item was repeated allocated different positions in the broadcast cycle. A user can retrieve data items which they needed on the places in a broadcast cycle by using the user's cached data items to decode it. In this paper, we based on network coding model and we propose a new weight metrics that decided which query should be service with the highest priority. The metrics considers the factor of requests deadline to reduce deadline miss rate. Experiment results show that our algorithm can increase 20% success rate (which only increase 5% user's access time) compared with [3].
