A Fair Channel Access Protocol for IEEE 802.11af

近年來，物聯網有著顯著的發展，許多新興的應用爆炸性的出現。隨之而來的就是大量頻譜的需求。但以現行頻譜的使用情形來說，免費的頻帶皆已十分的壅擠，需付費使用的頻帶又不是物聯網這種新興小應用可負擔的。好消息是，在甚高頻及特高頻的地方，雖然目前已被授權給電視訊號使用，但實際電視訓好使用的情形卻非常的低。若是能將此頻段拿來再利用，可解決目前頻譜使用不足的問題。 因為在甚高頻及特高頻的特性，使得裝置能傳送的範圍更廣。但隨之而來的是更嚴重的不公平性的問題。這篇論文一開始會先探討這些不公平性產生的原因及做簡單的分析。之後，提出一個新的協定來達到公平性。在這個協定中，不同於原始使用載波偵聽多路存取的方始需要不斷加倍自己的視窗大小，我們提出的協定會選取一個固定大小的視窗。這個固定大小的視窗是每個裝置分散式的算出，不須經由分享器做集中式的運算。因此，每個裝置需由獨自偵測問題的機制。偵測之後，裝置間互相溝通的方式也會在本論文中提出。另一個不同的地方是，在有些情況，就算偵測到通道是用忙碌的，仍然會繼續倒數計數器。在我們的模擬及分析的結果中，不論在什麼樣子的情形，平均而言，幾乎可達百分之九十五的公平性。The explosive growth of Internet of Things (IoT) applications requires many wireless spectrum resources to support connections of billions of sensors and actuators. Unfortunately, the licensed bands such as 2/3/4G networks are very expensive while the unlicensed 2.4GHz/5.8GHz bands are very crowded. The good news is that although many of the licensed bands are already assigned, the actual usage is low, especially in the very-high frequency (VHF) and the ultra-high frequency (UHF) television (TV) bands. In views of this fact, IEEE 802.11af PHY/MAC standard, an emerging technology in the IEEE 802.11 standard family, is developed so that wireless devices can operate in these bands, provided that no harmful interference is caused to the TV broadcasters and the wireless microphones. Thanks to the low-frequency operation, the IEEE 802.11af protocol has excellent propagation/penetration characteristics and thus, a larger coverage. These characteristics make the IEEE 802.11af standard a valuable solution for various applications, but also cause much serious unfairness problems due to the capture effect, the hidden terminal problem, and designs of IEEE 802.11 MAC protocol. In this thesis, we first evaluate the three unfairness issues in an IEEE 802.11af network, and then propose a distributed-based scheme. In our scheme, every station in a basic service set (BSS) selects a fixed contention window (CW) size rather than doubling it after a failed transmission. Stations choose their CW sizes based on the number of captured and hidden stations. In order to detect the number of captured and hidden stations, some mechanisms are implemented. In addition, rather than freezing the backoff slots when the channel is busy, stations continuously count down the backoff slots. With our design, all stations are guaranteed to successfully transmit the data/control frames with an equal probability. The performance of our solution is evaluated by Opnet-based simulations. Both the analytical and simulation results show that the proposed scheme leads to 95% fairness in terms of the probability of the successful transmissions