基於WiFi之異質網路效能分析

Abstract

近年來，隨著網路使用者不斷的增加以及上網裝置的數量也不斷的提升，導致頻譜資源逐漸不足。為了解決這方面的問題，許多人開始提倡利用感知無線電的技術來使用電視白頻譜，藉此改善頻譜資源不足的問題。 在感知無線電的技術中，非授權用戶可以透過頻譜感知來使用頻帶以增加頻帶使用率，但同時要避免干擾到授權用戶的使用，因此關鍵是精確的頻譜感測以及有效的避免對授權用戶的干擾。 在我們的設計中我們利用IEEE 802.11中的PCF模式來達成CR控制通道的實現，在PCF的期間，CR AP可以收集感測裝置的感測結果，收集CR用戶的資源請求，以及通知資源分配結果。透過正確的設置DCF與PCF模式的週期，我們可以即時的更新頻譜資料庫，增加TVB的頻道使用率。 由於我們將WiFi的時間軸切割成DCF與PCF，雖然較長的PCF周期時間可以改善頻譜資料庫的正確性與TVB的使用率，但也會減少DCF的週期時間並影響到傳統WiFi用戶的效能。 為了研究這個問題的權衡，我們探討此異質網路，並分別推導出用於收集感測回報與資源需求的適當時間，並且依照頻譜估測準確度與頻譜配置結果探討TVB網路效能，同時討論PCF時間長度對WiFi網路效能的影響。

In recent years, with the increases of users of network and the devices connecting network, the available resources of the spectrum have been declining. It has been proposed that to apply the technology of Cognitive Radio technology to used under-utilized TV channels (TV white space), to solve the present scarcity of spectrum. The technology of the Cognitive Radio can help unlicensed users utilize spectrum more easily. However, it is very important to avoid interfering with the licensed users. And the key technology to avoid interfering with the licensed users is the precise spectrum sensing. In our design, the model of Point Coordination Function (PCF) of IEEE 802.11 is used to realized the Cognitive Radio Control Channel. In the PCF duration, CR AP functions as a coordinator, collecting and analyzing data passing through the sensors and request from users of CR, and provides the results of resources distribution. The data base of spectrum could be updated in real time, which increases the usage of TVB channel by the precisely setting of modes/intervals of Distributed Coordination Function (DCF). The time horizontal of WiFi is usually separate into DCF and PCF, and the interval of PCF is longer than that of DCF, which could improve the precise of spectrum data base and the utilization of TVB channel, but influence the efficiency and quality of traditional WiFi system in parallel. In order to better the disadvantage, we analyzed the heterogeneous network and separately estimated the adequate timings for Collection Period and Resource Period. We also evaluated the efficiency of TVB network according to the accuracy and allocations of estimated spectrum, and analyzed the impact of the interval of PCF on the transmission efficiency of WiFi network.