感知無線電網路之動態頻譜存取之延遲時間分析

Abstract

在感知無線電(Cognitive Radio)網路中，動態頻率存取是一個非常重要的議題，它也對媒體存取控制(Medium Access Control)的設計產生許多挑戰，其中一個非常重要的挑戰為在感知無線電系統中，為了解決註冊頻段沒有有效利用的狀況，會允許非註冊頻段使用者可以短暫的借用註冊頻段，當註冊頻段使用者想使用的時候，非註冊頻段使用者必需交回該頻段使用權。因此，和一般著重在平均資料傳輸量的多頻道媒體存取控制不同的地方，在感知無線電網路中，應該特別著重的是非註冊頻段使用者能多快利用到註冊頻段。 在這篇論文中，我們主要研究感知無線電網路中的動態頻段存取延遲時間的問題，對於感知無線電媒體存取控制的控制頻道設計，目前存在兩種方法: 預設式、嵌入式。預設式的設計方式是目前的主要方法，但是目前普遍使用的IEEE 802.11媒體存取控制卻不是使用預設式的設計方式且沒有頻段搜尋的功能，因此，我們想了解目前大量使用的IEEE 802.11媒體存取控制是否值得加入頻段搜尋的功能而成為感知無線電媒體存取控制的候選人。並且，此時存取延遲時間仍是一個未被深入討論的議題，因此在我們論文中，我們比較兩種不同感知無線電媒體存取控制的控制頻道設計下的存取延遲時間分析。 本篇論文將同時考慮針對一個封包和全部封包下的存取延遲時間。我們提出一個分析方法來針對預設式、嵌入式控制頻道設計方法下計算一個封包和全部封包下的存取延遲時間，根據我們的結果，從一個封包和全部封包存取延遲時間觀點來看，目前普遍使用的IEEE 802.11嵌入式的設計方法在某些系統的情況下是值得加入頻段搜尋的功能而成為感知無線電媒體存取控制。結果可以整理如下，當系統中人數多時，使用預設式控制頻道設計方法比較好，而在當系統中人數不多時，使用嵌入式控制頻道設計方法比較好。特別值得注意的是，以全部封包延遲時間的觀點來看的話，當資料傳送的長度不大時，我們建議使用嵌入式控制頻道的設計方法來設計媒體存取控制，即使當時在系統中的人數是比較偏多的情況。

Dynamic spectrum access (DSA) is the key feature of cognitive radio (CR) networks, but it also poses many new challenges on the medium access control (MAC) design. One of key challenges is the fact that the secondary CR users can only borrow the licensed spectrum from the primary users for a short period of time, and will lose the channel access privilege once any primary user appears. Hence, unlike many available multi-channel MAC protocols for ad hoc networks where throughput is the main performance issue, the DSA protocol of CR networks shall place more emphases on access latency. Recently, there exist two control channel design methods for CR MAC: dedicated and embedded control channel method. The most design methods for the control channel in CR MAC is dedicated control channel. However, the popular IEEE 802.11 MAC without channel search capability uses embedded control channel method. Thus, we would like to investigate whether the legacy IEEE 802.11 CSMA/CA MAC protocol with embedded control channel can be the CR MAC candidate by adding a channel search capability. Furthermore, the access delay analysis for dedicated case with control and data channel saturation problem and embedded case with channel mismatch problem are still less discussed. Thus, in this thesis, we compare two different control channel design methods ( the dedicated CR MAC and the legacy IEEE 802.11 MAC with channel search capability) by analyzing their access delay performance. In this thesis, we consider not only the per packet access delay but also the total packets delay when comparing the CR MAC with dedicated and embedded control channels. We develop an analytical model to calculate the per packet and the total packets dynamic spectrum access latency for CR MAC with dedicated and embedded control channels. From the results, we find that the legacy IEEE 802.11 CSMA/CA MAC protocol with embedded control channel is worth extending to CR MAC by adding a channel search capability for some specific system characteristics. From our results, we conclude that the CR MAC can dedicate a certain portion of spectrum for sending control frames if number of users is large. Otherwise, the embedded control channel shall be considered as long as the number of users is not large. Furthermore, from the total packets delay viewpoint, when the data length is small, the embedded control channel method is suggested to be used even though the number of users is large.