感知無線電被提議在無線終端下，次要感知用戶可以伺機登入主要授權用戶頻段來提升頻譜利用率。當次要用戶偵測通道為空閒且使用通道來進行傳輸時，如果主要用戶返回使用通道，由於彼此之間的訊號互相干擾，造成主要與此要用戶傳輸錯誤。為了解決這個問題，論文中我們提出利用分散式時空編碼來減緩碰撞對主要用戶及次要用戶所造成的影響。透過兩個次要用戶之間的合作，在同一通道同時傳輸不同的訊號，可提高傳輸速率及降低占用通道的時間，進而減少次要用戶對主要用戶的干擾。經由模擬的結果並和不同的傳輸方式相比較以來呈現我們所提出的方法的優劣。

Cognitive radio has been proposed to improve spectrum efficiency by allowing wireless terminals acting as secondary users (SUs) to opportunistically access licensed frequency bands, owned by primary users (PU). When a SU accesses and transmits on a channel (i.e., frequency band) that is sensed to be idle , a collision may occur if the PU returns to the same channel. Consequently, both PUs and SUs may not be able to decode due to cochannel interference. In this thesis, we propose to mitigate the collision between PUs and SUs by distributed space time coding (DSTC). Based on DSTC, two SUs can transmit different signals at the same time to maintain full-rate transmission while reducing their channel occupancy time that in turn mitigate the harmful effect of collision. Simulation results and comparisons with different transmission modes are presented to identify the merits of our proposed scheme.

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