在合作式通訊中添加中繼站而運用空間多樣性的變化可以為單一傳輸路徑提供額外的選擇性，目前對於中繼站的研究莫過於放大前傳及解碼前傳，在此兩種模式下，許多文獻已提出增加中繼站於無線系統當中對於可靠度將會有顯著的提升；但隨著中繼站的增加伴隨而來的影響亦會反應於傳輸延遲上因而降低傳輸速率。

在多用戶的無線傳輸系統中，基於訊號皆為廣播形式，因此利用中繼站與收受
端皆能同時接收多點訊號源的特性再加上媒體存取控制(MAC) 層中的網路編碼，將這些廣播訊息集合起來進而提升整體的系統吞吐量，雖然是需要額外的記憶體來儲存這些廣播訊息，不過整體而言，能充份利用無線傳輸的廣播特性對於使用者或是整體系統來說仍是有利的。

本論文是假定在中繼站已存在網路中的條件下，提出混合式網路編碼的中繼站
協定方式，此協定方式考慮使用者所需要的訊息而進行條件解碼前傳，這與無中繼站協定系統相比，不僅僅能夠降低系統的錯誤率亦能無損於傳輸延遲，模擬結果與理論結果相當匹配。添加中繼站在往後將是一項必然的應用，發展出高效率的網路接取協定來收集散失的廣播訊息、且搭配網路編碼將能比原本無中繼站的協定方式更可靠以及更快速。

Spatial diversity can be used to mitigate burst errors due to fading channels. One method to implement spatial diversity is through the placement of relays in the network.
However, an inherent problem is the extra delay introduced by the relaying process. In addition to the source transmission, certain time needs to be reserved for the relay transmission. As a result, it is necessary to determine when the cooperative relay is needed and how the relays should cooperate to improve the bandwidth efficiency.

This thesis proposes a hybrid relay switching protocol (HRSP) which jointly applies the hybrid automatic repeat-request (HARQ) scheme and network coding mechanism at the MAC layer. The proposed hybrid scheme adaptively switches between different transmission modes depending on the reception results of the relays and destinations. By combining the HARQ with network coding, it is expected that the cooperative relaying can be performed more efficiently. Both numerical and simulation results show that the proposed protocol remarkably outperforms the conventional point-to-point (PP) transmission and network-coded relaying in terms of outage probability and delay.

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