為了有效率地傳輸耗費頻寬的應用，群播/廣播服務是其中一項被 Mobile WiMAX 所支援的重要功能。群播/廣播服務，例如，影音串流，通常容易受傳輸延遲所影響，所以群播/廣播服務區的概念被用來減少換手的延遲，因為在同一個群播/廣播服務區中，所有的基地台都同時傳送相同的群播/廣播服務內容，行動裝置換手後，不需要重新註冊與重新建立群播連線。然而，當服務基地台與目標基地台屬於不同的群播/廣播服務區，行動裝置就需要執行群播/廣播服務區間的換手，跨群播/廣播服務區的換手有較長的延遲時間，與相鄰群播/廣播服務區間的訊框位移，都將造成大量的封包遺失，使得即時應用的服務品質難以達成。群播/廣播服務區間的換手造成大量封包遺失的問題，目前為止還鮮少被研究。在這篇論文中，我們改進了群播/廣播服務區間換手的流程，大幅地減少換手的延遲，除此之外，我們也提出一個資料重獲機制，藉由使用額外的群播頻道做為群播/廣播服務資料重獲的頻道，減少封包的遺失。模擬的結果顯示，只需要使用相當少的資料重獲頻道，我們所提出的機制就能讓封包遺失率幾乎減少為零。

To transmit bandwidth-intensive applications efficiently, multicast broadcast service (MBS) is one of the important features supported by Mobile WiMAX. As MBS services such as audio and video streaming are usually delay-sensitive, the concept of an MBS zone is also introduced to reduced the handover delay for MBS services. In an MBS zone, the participant BSs transmit the same MBS content synchronously, and MSs do not need to re-register and re-establish the multicast connection during handover between BSs in the same MBS zone. However, when the serving BS and the target BS belong to different MBS zones, the inter-MBS zone handover is performed. The longer inter-MBS zone handover delay and frame offsets between adjacent MBS zones will cause large packet loss and make the QoS of real-time applications infeasible. But the problem of huge packet loss caused by inter-MBS zone handover is little studied. In this paper, we propose an improvement on the inter-MBS zone handover procedure to greatly reduce the handover delay. Moreover, we also propose a data recovery scheme for inter-MBS zone handover by using extra multicast connections as recovery channels for MBS services to minimize the packet loss. Simulation results show that our proposed schemes achieve almost zero packet loss with relatively small number of recovery channels compared to the number of MBS sessions.

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