近年來，多媒體在無線環境上面的應用越來越多。藉由近幾年開始發展的標準IEEE 802.16又被稱為WiMAX的技術，高容量、高速率的資料傳輸、QoS的需求可以被滿足。IEEE 802.16的MAC層支援兩種網路拓樸，第一種是點對多點的架構，第二種是mesh的架構。在點對多點的架構下，會有一個基地台和許多被服務的用戶在一個服務的區間裡。雖然在IEEE 802.16的使用下，許多用戶可以被同時服務，但是不可避免的是封包在無線環境下傳輸的遺失。當封包遺失率漸漸提高的時候，這些多媒體的影像品質將會產生嚴重的衰減。為了在單向傳播和群播的應用下，可以減少封包遺失造成的品質衰減，錯誤更正碼(FEC)和自動重傳機制(ARQ)被廣泛地當作錯誤控制的方法。
　　在這篇論文當中，一個混合型自動重傳機制的方法被提出並且應用在WiMAX的無線廣播環境中。這篇論文採用了封包式的Reed-Solomon erasure correction code (RS code)，而並不是採用位元式的RS code保護。使用一個傳統的封包式的RS code基地台在一次的傳輸群組裡會傳送固定的數目的資料封包和固定數量的保護封包。但是提出的方法中，基地台適應性地去調整保護封包的數量，藉此可以達到節省傳輸固定數量的保護封包的頻寬。並且基地台在封包遺失率比較高的時候會分配好頻寬給保護封包和需要被重傳的封包。另外當封包遺書率繼續升高的同時，基地台會調整資料封包和保護封包的比率去降低錯誤的發生。最後，傳輸的多媒體資料可以受到更好的保護以及達到節省頻寬的效果。

In recent years, the demand for transmitting multimedia over wireless environment is greatly needed. With IEEE 802.16, also called world interoperability for microwave access (WiMAX), technology coming to maturity in the next few years, the demand for high data rate, high capacity and QoS to each user can be satisfied. The IEEE 802.16 MAC layer supports two kinds of topology. One is point-to-multipoint architecture and the other is mesh topology. In PMP architecture, there is a central base station (BS) and several subscriber stations (SSs) in one sector. Although the IEEE 802.16 can satisfy the demand for each user, packet loss can’t be avoided in the wireless environment. The quality of multimedia will be greatly degraded with the increasing packet loss rate in the wireless radio environment. Forward error correction (FEC) techniques and automatically repeat request (ARQ) are the preferred error control schemes for unicast or multicast.
In this thesis, a novel Hybrid ARQ (HARQ) algorithm is proposed for video multicasting over WiMAX network. The proposed algorithm adopts Reed-Solomon erasure correction code (RS code) at packet level instead of at bit level. With traditional RS code at packet level, BS needs to send original data packets and constant number of redundant packets in one transmission group. The proposed algorithm adaptively adjusts the number of redundant packets to conserve the bandwidth in lower packet loss rate. If the packet loss rate increases continuously, the proposed algorithm adjusts the number of data packets and redundant packets adaptively. In the end, the number of retransmitted packets is reduced and the multimedia can be protected more robustly.

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