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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/8275


    Title: 以CRC為基礎之無線網路資料錯誤更正器;A CRC Based Error Corrector (CEC)for Wireless Networks
    Authors: 李惠玉;Huei-yu Lee
    Contributors: 通訊工程研究所
    Keywords: 自動重送請求;循環冗餘校驗;混合式自動重送請求;CRC;HARQ;ARQ
    Date: 2008-06-24
    Issue Date: 2009-09-22 11:22:19 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: CRC是一種錯誤偵測機制,在有線及無線網路的媒體存取控制層中被廣泛應用。由於CRC field只傳送有限的資訊,接收端偵測出有錯誤發生之後,幾乎沒有辦法更正錯誤的封包,所以接收端自動將有錯誤的封包捨棄。為了在雜訊較多的通訊環境中提供可靠的傳輸,重傳機制常被用來和CRC機制一起運作;也就是說,當偵測出錯誤時,重傳機制會立刻要求傳送端再一次重新傳送序列號碼相同的封包。 從理論的觀點來說,假如重傳時又偵測出錯誤,那麼這些被偵測出有錯誤的封包,它們可能隱藏著一些有用的資訊,接收端可能可以利用這些資訊得到正確的封包。為了達到這個目標,我們在MAC層裡提出了一個新的錯誤更正機制,由於它是以舊有的CRC為基礎,所以我們將它命名為CEC (CRC based Error Corrector)。CEC主要的想法就是比對所有接收到的錯誤封包,然後在這些封包中找出有不同位元值的位置。根據這些位置,將所有可能的值放入到這些位置,並且將每一個封包都視為可能正確的封包,最後再利用CRC去確認是否有正確的封包。 若比較器可以檢查出所有可能有錯誤的位元位址,原始的封包就可以被CEC還原,反之若錯誤發生在多個錯誤封包的同一個位址,那麼比較器就無法偵測出這一個位元是有錯誤的。在這一篇論文的第四個部分,我們必須去分析在多個封包中,錯誤發生同一個位址的可能性,進而去探討CEC系統的錯誤還原機率。為了提升效能,總共提出兩種CEC架構 ,分別命名為CEC-I及CEC-II,這兩種架構的差別主要在於硬體的成本與所需要執行時間的不同。最後經由第五部份的模擬來評估,我們所提出的CEC可以在重傳次數很低的情況之下,有效率的還原出正確封包。 Cyclic redundancy check (CRC) scheme, which was particularly designed for medium access control (MAC) layer to detect erroneous frames, has been widely used in wired and wireless networks. Due to the limited information carried in the CRC field, receiver is not able to correct any erroneous frame and receiver drops all the erroneous frames automatically. To provide reliable communications on an error-prone communication link, retransmission scheme is often adopted to cooperate with the CRC scheme. From theoretical viewpoint, erroneous frame still possesses useful information which could be helpful for receiver to figure out the original data frame in case that retransmission is failed again. As a solution, we propose a new error recovery scheme at the MAC layer, namely CRC based error corrector (CEC), which simply amends legacy CRC error checker to become a CRC-based error corrector. The key concept of CEC is to compare all received erroneous frames and thereby derive the positions of differentiated bits among them. According to the positions of unmatched bits, all possible data frames are formed and rechecked by a fabric with a number of CRC encoders. As all error bits are detectable by the comparator, the original data frame is found. The error recoverable probability (ERP) of CEC is analyzed in this thesis. For evaluations, we also develop two types of hardware based CEC, namely CEC-I scheme and CEC-II scheme, as they equip different numbers of CRC encoders. As confirmed by performance evaluations, the proposed CEC with limited hardware cost can easily achieve excellent performance in term of the mean number of retransmissions per frame.
    Appears in Collections:[通訊工程研究所] 博碩士論文

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