無線感測網路(wireless sensor networks; WSNs) 基於 IEEE 802.15.4標準是能夠實現低功耗低傳輸率和短距離無線個人區域網路 (wireless personal networks; WPAN)，是由大量的感測節點(sensor nodes)所組成的網路。感測節點的體積通常很小，且有能量來源上的限制，為了適用於不同的領域需求，大部分的感測節點以配備電池做為主要的能量來源。因此，在無線感測網路中，能量消耗的分析是一個重要的效能測量議題。
在IEEE 802.15.4中，主要是使用超碼框架構(superframe structure)來做為控制資料的傳輸並取得感測節點間的同步。在此架構中競爭存取週期(contention access period; CAP)使用時槽化的載波偵測多重存取/碰撞避免(carrier sense multiple access with collision avoidance; CSMA/CA)競爭機制，其隨機後退時間及資料間的碰撞將顯著的影響通道的使用率。
依據CSMA/CA機制，當感測器節點通道淨空評估 (clear channel assessment; CCA)的通道檢測結果是忙碌時便立即執行隨機倒退(random backoff)程序。它可能忽略了通道淨空評估失敗所隱含的資訊，將會使能量消耗提高，並進一步導致多餘的感覺。在時槽化CSMA/CA中，盲目的隨機倒退程序將導致較低的通道利用率。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。
本文提出了一種基於IEEE 802.15.4的額外載波偵測(additional carrier sensing; ACS)演算法用來提高原始時槽化CSMA/CA的載波偵測機制。我們使用一個分析馬可夫鏈(Markov chain)的模型，來評估ACS演算法的表現。不管分析和模擬結果都顯示本文所提出的演算法的效能優於IEEE 802.15.4標準，提高了產能、平均媒介存取控制 (medium access control; MAC) 延遲和檢測CCA時因所需次數減少而降低能量消耗。

Wireless sensor networks (WSNs) based on the IEEE 802.15.4 standard is able to achieve low-power transmissions in low-rate and short-distance wireless personal area network (WPAN). It is comprised of a large number of sensor nodes. Sensor nodes are normally small in size and have restricted power sources. In order to match the applications of different fields, most sensor nodes are equipped with batteries for the roles of main energy source. Thus, the power consumption is an important performance measure issue in WSNs.
In IEEE 802.15.4, the superframe structure is used to control packet transmission and attain synchronization of sensor nodes. In the contention access period (CAP), the slotted carrier sense multiple access with collision avoidance (CSMA/CA) is used for contention mechanism. The channel utilization is significantly affected by idle backoff time and collision due to the use of the CSMA/CA algorithm.
According to the CSMA/CA mechanism, sensor nodes perform backoff process immediately when the clear channel assessment (CCA) detecting busy channel. It may neglect the implicit information of the failed CCA detection and increase the power consumption because of the redundant senses. The blind backoff process in the slotted CSMA/CA will cause lower channel utilization. Therefore, studying the implicit information of CCA failure and using it will reduce the power consumption.
This thesis proposes an additional carrier sensing (ACS) algorithm based on IEEE 802.15.4 to enhance the carrier sensing mechanism for the original slotted CSMA/CA. An analytical Markov chain model is developed to evaluate the performance of the ACS algorithm. Both analytical and simulation results show that the proposed algorithm performs better than IEEE 802.15.4, which improves throughput, average medium access control (MAC) delay and due to the less number needed of CCA detecting to reduce power consumption.

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