感知無線電媒體存取控制層之通訊協定設計及性能分析

Abstract

近年來由於無線通訊技術的進步，使得能夠分配的頻譜逐漸稀少，主要的原因來自於各個無線通訊系統為了防止彼此之間相互干擾的問題，每個系統都各自操作在特定的頻帶，且對該頻帶均有專屬的使用權，讓其他系統無法重複使用頻帶。最近的研究結果顯示，此種固定式頻率的分配方式，使得頻帶的使用效率低於35%以下，而在時間及空間上使用效率也出現極大的非對稱性，其在效率上的變化可以從15%變化到85%。為解決極低的頻譜使用效率以及改善使用效率上的對稱性，一種新的無線網路設備期待具有一定程度的智能，可以主動去感測週遭的環境，判斷可以使用的頻帶及地區，在空轉的頻帶中或在不影響該頻帶中主要使用者的前提下，動態地與他人建立連線，當該頻帶主要使用者需要傳輸時，又能及時的停止其傳輸並轉移到其他頻帶而不干擾該頻帶主要使用者，符合此種傳輸要求的設備我們命名為感知無線電。然而為了能夠動態地建立不干擾他人的連線，感知無線電有三項重要的課題需面對：(1) 頻譜偵測及識別；(2) 具有感知能力的通訊協定設計；以及(3) 頻譜換手機制。在本篇論文中，我們將逐一檢視此三種重要課題及其相對應的設計，同時，我們也發展一套跨層的分析模型，得以分析感知無線電網路中的系統效能及傳輸延遲。 在第一個課題中，由於現今的頻譜偵測及識別技術均需要較為複雜的訊號處理及統計分析，使得硬體複雜度提高且造成多餘的能量消耗，對此若能藉由節點的位置資訊事先判斷出其所在的位置是否容易造成主要使用者的干擾，如此一來，只有當節點進入到會造成干擾的區域時，才區要執行頻譜偵測及識別的動作，若否則可直接與接收端建立連線，在此我們驗證此種想法的可行性，在我們的結果中顯示出，在一個主要使用者基地台的覆蓋範圍中，有高達45%的區域可以滿足此種要求，讓感知無線電在不需偵測頻譜的前提下即可與其接收端建立連線，而同時又不影響原先主要使用者連線的品質，其頻譜的使用效率在此種網路架構下，更較原先系統提高到145%。 針對第二個感知通訊協定設計的課題中，由於感知無線電使用者被允許使用的時間只有在通道處在空轉的情況下，才能與接收端建立連線，可預期的是空轉的時間相當有限，在此，最大的問題在於要如何讓感知無線電在不干擾主要使用者的前提下，又能快速的和接收端建立連線，同時又能滿足連線品質的要求。對此，我們提出一套藉由觀察使用者鄰居的傳輸狀況，搭配可以根據鄰居傳輸資料而動態調整的競爭解決機制，避免干擾主要使用者並降低通訊延遲及碰撞，再加上邀請式保留機制，來滿足通訊服務對於連線品質的要求。從我們的模擬結果來看，我們所設計出的通訊協定可以完整保證不造成主要使用者的干擾，且頻譜使用效率較傳統的載波偵測多重存取既干擾避免機制的通訊協定提高至少50%以上，同時存取時間上的公平性也大幅降低五倍以上。 最後一個頻譜換手的課題，也是感知無線電最獨特且亟需解決的問題，此種換手機制與傳統水平或垂直換手機制最大的不同在於，使用者必須被迫在傳輸封包的過程中轉換到另外一個頻帶，而傳統的換手機制可以等待使用者傳輸完一個完整的封包後再行轉換。在此，我們針對三種感知無線電可能的傳輸劇本：無頻譜換手機制；事先定義通道的頻譜換手以及頻譜偵測後的頻譜換手機制作性能分析，從我們得到的數值分析來看，頻譜偵測後的頻譜換手機制由於可以得到較為正確的標的通道，使其有較好的平均傳輸速率，同時，我們也藉由分析，提供設計者在給定的連線維持機率與平均傳輸速率的要求下，在兩種頻譜換手機制中各項系統參數最大忍受的設定值。 在本論文中最後一部份，我們提出一套跨層分析模型，將實體層因多重路徑衰減，屏蔽效應所造成傳輸失敗，或因擷取效應，多重通道傳輸以及方向性天線對訊號品質改善，而間接對於媒體存取控制層中競爭窗口大小改變的影響，一同於模型中考慮，藉此，我們得以準確的估計感知無線電於實際無線通道中的性能，同時藉由此模型，可以給設計者要如何有效提升性能提供一個明確的方向。

As the advance of wireless communication technology, the spectrum scarcity becomes a severe problem due to the designated frequency band for each legacy system to avoid the interference. However, current experiments have showed that the overall spectrum efficiency over the 0$\sim$3 GHz frequency range is less than 35\%. In addition, the temporal and geographical variations in the utilization of the assigned frequency band range from 15\% to 85\%. Thus, it is necessary to develop a smart device to have the flexibility to access in various frequency bands while not obstructing the licensed user's transmissions in the legacy system. Cognitive radio (CR) is designed to be such an intelligent device, which can sense its surrounding environment, dynamically access the idle channel(s) for a temporary data transmission and return the occupied channel(s) to the licensed user's transmissions. However, to set up a temporary connection without interfering the legacy system, three essential functions are required to be solved: (1) the spectrum sensing over a wide frequency range, (2) an intelligent cognitive medium access control (MAC) protocol, and (3) the link maintenance mechanism to restore the CR user's link. In this dissertation, we will inspect the above three functions to design the MAC protocol and evaluate the performances of the CR networks.

In the wideband spectrum sensing issue, CR devices have to recognize when and which channel(s) the licensed users occupy and to identify the opportunity of reusing the frequency of legacy systems. However, the wideband spectrum sensing is a time and energy consuming processes. In this part, we aim to alleviate the burden of spectrum scanning in a CR device by means of location awareness. We investigate to what extent a CR system with location awareness capability can establish a scanning-free region where an ad hoc connection of the CR users can coexist with an infrastructure-based connection of the primary user. It has been shown that the frequency band of the legacy system can be reused up to 45\% by the overlaying cognitive ad hoc network if certain positioning techniques help CR users to locate primary and other secondary users.

Then, we put our effort on the cognitive MAC protocol design, in which a CR device can quickly establish a temporary connection without interfering the licensed user. To establish an overlaying ad hoc network, the cognitive MAC protocol shall not only achieve high spectrum utilization as well as the QoS satisfactory, but also establish the link in a short time without introducing interference to primary users. To this end, we propose four enhanced mechanisms for the CSMA/CA MAC protocol: (1) a neighbor list establishment mechanism for recognizing spectrum usage opportunities, (2) a set of contention resolution methods to reduce collisions as well as the access delay and its variance, (3) an invited reservation procedure for meeting the delay requirements of real-time traffic, and (4) a distributed frame synchronization mechanism to coordinate transmission without central controllers. Compared to the legacy IEEE 802.11 WLAN, the enhanced CSMA/CA MAC protocol can improve the system throughput by 50\% through analysis and NS-2 simulations, while keeping the dropping rate lower than 2\% for delay-sensitive traffic. Furthermore, the standard deviation of the access delay is reduced by five times.

Next, we study three link maintenance mechanisms for CR devices when the licensed user returns to the occupied channel(s): (1) non-spectrum handoff method, (2) the pre-determined channel list spectrum handoff and (3) the spectrum handoff with radio sensing scheme. In the non-spectrum-handoff scenario, the CR device stays in the original channel and continues the transmission after the licensed user leaves the channel. In the pre-determined channel list spectrum handoff, the CR device lists the target channels before setting up the link. As for the spectrum handoff with radio sensing scheme, the CR device senses the spectrum and then chooses the target channel when the licensed user returns back. In this part, we study the link maintenance probability and the effective data rate in these three mechanisms. Our numerical results are shown that the erroneous channel selection probability, the radio sensing time and the number of handoff trials are the three important system parameters for the two spectrum handoff schemes. We also provide the design guide of the system parameters for the given effective data rate and the link maintenance probability.

At last, we develop a physical(PHY)/MAC cross-layer analytical model to investigate the performances of the single and multiple channels CSMA/CA MAC protocols in a lossy wireless environment. From the physical layer perspective, the developed model incorporates the effects of capture and directional antennas, while from the MAC layer perspective, our approach takes into account of the contentions in single and multi-channel environments and the effect of the binary exponential backoff process. The proposed cross-layer analytical model not only provides insights into the physical layer impacts on the throughput of the CSMA/CA MAC protocol, but also indicates to how directional antennas can improve the CSMA/CA based system in terms of antenna beamwidth and the number of radio transceivers.

In summery, we have explored the three important issues for CR devices in this dissertation: (1) the wideband spectrum sensing, (2) the cognitive MAC protocol and (3) the link maintenance mechanism. Finally, we develop a PHY/MAC cross-layer analytical model to investigate the performances of the single and multiple channels CSMA/CA MAC protocols.