感測網路是由大量感測節點所組成，主要應用於軍事或環境監控。為了滿足應用需求，故其通訊協定通常採用IEEE 802.15.4標準(低速率無線個人區域網路)。再者，感測節點配備電池做為電力來源，因此能量消耗議題至為重要。
在IEEE 802.15.4標準中，若使用超碼框結構，裝置於競爭週期(CAP)時採用載波偵測多重存取/碰撞避免(CSMA/CA)演算法來競爭通道使用權，其規範一旦通道淨空評估(clear channel assessment；CCA)失敗便立即執行隨機倒退( random backoff )程序。
然而，在CSMA/CA演算法中，未考慮每次通道淨空評估失敗所隱含的資訊而立即執行隨機倒退程序的策略將會使能量消耗提高。因此，研究通道淨空評估失敗所隱含的資訊並利用，將有助於提升能量效率。同時，考慮產能(throughput)和延遲(delay)議題也不容忽視，才能折衷出真正良好的改進方案，達到提升整體效能的目的。
本論文透過修改IEEE 802.15.4標準的CSMA/CA演算法，提出根據通道淨空評估失敗所隱含的資訊並執行額外載波偵測，以達到提升產能和能量效率的目標。

Wireless sensor networks (WSNs) whose applications are focused on military or environmental monitoring are constituted by a large number of sensor nodes. In order to satisfy the application requirements, the WSNs’ communication protocol usually adopts IEEE 802.15.4 (LR-WPAN). Furthermore, the power consumption issue is very important for WSNs that sensor nodes are equipped with batteries for the roles of power source.
In IEEE 802.15.4, the CAP (contention access period) adopts carrier sense multiple access with collision avoidance (CSMA/CA) mechanism for sensor nodes to contend channel usage. According to the standard IEEE 802.15.4 CSMA/CA mechanism, contending nodes will perform random backoff procedure immediately as CCA (clear channel assessment) failure occurred. However, this kind of FCR (fast collision resolution) method will ignore the implicit information of CCA failure and increase the power consumption because of the redundant senses. Accordingly, studying the implicit information of CCA failure and using it will benefit the power saving.
Meanwhile, the other important issues of WSNs are throughput and delay. We couldn’t prevent from considering these two issues to achieve the best solution to enhance WSNs’ performance. Therefore, this paper will focuses on three issues, which are throughput, energy consumption and delay, to improve the CSMA/CA mechanism.
Consequently, this paper suggests a modified CSMA/CA algorithm named additional carrier sensing (ACS), which has considered the implicit information of CCA failure to enhance the performance of WSNs.

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