| 摘要: | 為解決頻譜資源日漸匱乏與使用率不佳之問題，感知無線電 （cognitive radio, CR） 技術乃因應而生，藉由次使用者 (secondary user, SU) 動態接取閒置的具執照主使用者 (primary user, PU) 之頻帶，以充分利用頻譜資源。
而欲提昇頻譜使用率，準確的頻譜偵測 (spectrum sensing) 是感知無線電的首要任務，需先判斷當前頻段是否有主使用者傳送訊號，若偵測為主使用者不存在，則次使用者可在不干擾主使用者權益下，利用此頻譜機會 (spectrum opportunity) 傳送資料；若一旦偵測到主使用者開始傳送訊號，則次使用者必須立即讓出此頻段，避免干擾主使用者系統之通訊。而能量偵測 (energy detection) 因其不需待測訊號的預先資訊以及實現複雜度低的特性，已被廣泛用於未知訊號的偵測，但另一方面，能量偵測器在低訊號雜訊比 (signal-to-noise ratio, SNR) 或遭受隱藏節點 (hidden terminal) 問題時，造成頻譜偵測效能低落。
故本篇論文基於放大轉送 (amplify-and-forward, AF) 合作式通訊 (cooperative communications) 的概念，建構出一個資料整合型 (data fusion) 合作式頻譜偵測網路，憑藉其空間分集 (spatial diversity) 增益並能延展偵測距離，提昇因低訊號雜訊比與隱藏節點而降低之頻譜偵測效能。越多次使用者組成之放大轉送合作式感知網路，頻譜偵測越準確；然而其花費的總偵測時間 (sensing time) 增加，即在資料整合中心 (fusion center, FC) 的虛耗 (overhead) 更為加重，剩下所能傳輸資料的時間隨之減少，偵測效率不佳，是為一個權衡 (tradeoff) 的問題。
本篇論文中提出頻譜偵測效率函數 (sensing efficiency function) ，分別著重在保護主使用者不被干擾以及保證次使用者使用率的兩種相對觀點下，權衡偵測花費時間、準確度與效率之間關係，以期能使用較少的偵測時間達成準確的頻譜偵測，減少次使用者感知系統原先虛耗，以具有更高資料傳送機會。由數學分析以及模擬結果驗證，利用所提出的頻譜偵測效率函數，在不同次使用者個數情況下，各具有唯一的最佳偵測時間達到最高頻譜偵測效率。
Cognitive radio (CR) was proposed to improve the utilization of spectrum resources by allowing CR users to access the idle frequency bands of licensed users dynamically.
Spectrum sensing is the key functionality for the implementation of CR, in which the CR user, known as the secondary user (SU), senses the activities of a licensed user, known as the primary user (PU), and seeks the spectrum opportunity to access. Energy detection is commonly used in CR because of its needless prior information of the PU’s signal and low complexity. However, its sensing performance would be degraded by the hidden terminal problem with low received signal-to-noise ratio.
Therefore, the cooperative spectrum sensing networks with data fusion type are constructed in this thesis based on the amplify-and-forward protocol. The cooperation among SUs improves the spectrum sensing performance by increasing its spatial diversity gain. The more SUs there are, the more accurate spectrum sensing is. However, cooperation results in sensing overhead at fusion center, in other words, redundant sensing time reduces the available time for SUs to transmit data. Accordingly, there exists a tradeoff problem in the cooperative spectrum sensing system.
In this thesis, a spectrum sensing efficiency function is proposed to formulate the tradeoff problem among sensing time, accuracy and sensing efficiency from PUs’ and SUs’ perspective, respectively. The analytical and simulation results show that there exists an optimal sensing time to achieve the best efficiency value when given the number of SUs. |
