電視頻帶白空間雲端感知無線網路媒體存取層演算法之設計與實作

Abstract

根據聯合國所屬之國際電信聯盟(International Telecommunication Union，ITU)在2006年對新世代行動寬頻通訊標準(International Mobile Telecommunications，IMT) IMT-2000 及IMT-Advanced所做的頻寬需求調查顯示，行動通訊所需之頻寬在2015年將增為840MHz、2020年更將增至1.720MHz 。要在既有可得且適用於行動通訊之頻率中，提撥出如此寬之頻帶是極其困難的。可行的方式之一是對可回收之頻帶做積極且前瞻之規劃與利用。 近年來由於數位電視技術的發展，使所需頻寬較原本使用類比電視所需的頻寬減少約75%左右。藉由此類比電視轉為數位電視的契機，各國都在思考如何利用此頻譜重新分配的時機，建立一個前瞻、彈性且有效率的頻譜使用機制。由於既有電視頻帶700MHz之頻譜特性良好，極適合於行動通訊，因此提撥此電視信號數位化後空出的頻段(泛稱電視頻帶白空間，TV White Space)作為未來行動寬頻通訊的使用成為一具體可行之方案。 然而，美國聯邦通訊委員會(Federal Communications Commission，FCC) 於2002年公佈一份調查報告指出，已發放執照的頻段使用率隨時間與地點有15%~85%的極大變異。且美國國防部高等研究計劃局(Defense Advanced Research Projects Agency，DARPA)亦發現在同一時間點，頻譜的使用率竟然只有2%左右。這些研究調查促使FCC決定修改過往的頻譜政策，開放非執照使用者，可以在優先權使用者不使用的頻段與區域，暫時利用此頻段，以改善頻譜利用率。故在TV White Space上使用感知無線技術，提供彈性及有效的頻譜資源分享與管理已然是最受矚目的技術選項。2009年六月，美國數位無線電視標準法案正式實施，全面停止類比無線電視訊號，改由用進階電視系統委員會(The Advanced Television System Committee，ATSC) 所建議的數位電視標準來進行播出，並將空出的頻段開放給感知無線電進行試驗；若測試結果良好，未來將更進一步開放其它的頻段。於此同時，英國的通訊管理局(Office of Communications，Ofcom)也在2009年7月公布了有關於感知無線電測試的規定。與FCC的規定相比，Ofcom定義的感測功率更加嚴格，地理要求則較寬鬆。除了美國和英國外，加拿大也開放部分地區測試，歐盟也開始研擬感知無線電的測試標準。 由於700MHz並非無條件釋放給沒有使用執照的裝置使用，故FCC擬定了一系列規範，以保障執照使用者的頻段使用權力。以IEEE802.22為例，感知無線系統必須能藉由非執照使用者所回報之頻譜感測資料與地理位置，建構執照使用者之頻譜資料庫(Spectrum Database) ，用以確保無執照使用者之訊號不致干擾執照使用者的訊號品質。此外，為加強頻譜資源之管理與使用效率，感知無線基地台亦需藉由一控制中心遂行頻譜資源之分配與負載平衡。考量不同感知無線電系統業者將共存於電視空白頻段，此控制中心亦將擔綱協調系統業者間之頻譜資源分配。由此可知感知無線系統之實現有賴於一強大的網路運算中心，以支援大量且動態之頻譜感測資料更新，即時之頻譜地圖建立與查詢，及頻譜資源擷取與管理。 此計畫之研究內容為電視頻帶白空間感知無線網路媒體存取層之協定設計與實作，第一年之規劃 為設計並實作一套適用於infrastructure-based感知無線網路非執照使用者之頻道存取使用機制；第二年之規畫是設計並實作一套兩階層式頻道資源管理配置演算法。我們計畫以雲端作為一個中央調控的平台，透過設計的媒體層協定，蒐集佈建於感知無線網路中的感測裝置(sensing devices, SDs)所回報的地理資訊與頻道資訊，進一步將收集到的資訊傳送到雲端資料庫，供已執行於雲端的頻譜估測演算法建立出頻譜資料庫。透過查詢頻譜地圖資料庫及雲端平台的協調，我們設計的兩階層式架構的頻道資源管理分配機制，能給予優先權使用者更為嚴謹的保護，並提升無線通訊系統頻譜資源分配的效能。

According to the survey conducted by the International Telecommunication Union, (ITU) for the next generation broadband mobile wireless standards: International Mobile Telecommunications (IMT) 2000 and IMT-Advanced, the bandwidth requirement for mobile wireless applications will increase to 840MHz by 2015, and even to 1720MHz by 2020. Allocating extra bandwidths from the available unlicensed spectrums to meet such wideband demands is apparently very difficult. A feasible solution to resolve this issue is to reallocate early-on the spectrums whose licenses are going to expire in the near furture and intergrate the fragmented available spectrums for access in a smarter and cognitive manner. Owing to the recent advances in digital TV technologies, the bandwide requirement for TV broadcasting can be drastically reduced by 75% once using digital broadcasting technologies. Taking this opportunity of TV digitalization, nationals around the globe are embarking on the reallocations of the spectrum to be freed from the legacy TV band. Since the 700 MHz frequency of TV radio is suitable for mobile applications, allocating the spectrum of TV whit space (TVWS) for the next-generation broadband wireless applications becomes a feasible solution to meet the bandwidth demands. Despite the scarce wireless resource, a reseach report conducted by the Fedral Communications Commision (FCC) in 2002 shows that the usage of the licensed spectrum can vary between 15% and 85% in different areas and time. Beside, another report released by the Defense Advanced Research Agency also shows that the spectrum usage is only about 2% in the same time. To improve spectrum usages, the FCC releases some spectrums in the TVWS for the testing of the cognitive radio network. The technology of cognitive wireless network is gaining popularity and getting matured in recent years. In July 2010, the FCC has discussed whether to release the 1675-1710 MHz for the testing of cognitive wireless network. Same wise, the OFCOM from England also released the regularity of the testing of cognitive wireless network. These are strong evidences that the cognitive wireless networks will become a major trend of wireless network and dominates the future wireless industry. In view of the importance of cognitive radio technology on TVWS, in this 2-year project, we aim at designing and prototyping a medium access control (MAC) mechanism in TVWS cognitive radio networks. In the first year, we will develop a MAC protocol for infrastructure-based cognitive radio networks; the second year will focus on designing a two-tier resource management protocol. Specifically, the Cloud plays the role as a central controller of the cognitive wireless network and is responsible for the collections of the sensing information from all the secondary users. Our designed MAC protocol assists the Cloud to construct the Spectrum Database. Besides, the Cloud can serve as the central resource managers of multiple cognitive wireless networks and manage the overall spectrum utilization in a more efficient manner, while ensuring the protection to the primary users (PUs) of all the licensed spectrums. Through querying the Spectrum Database, Cognitive Radio Access Points (CR-APs) are able to coordinate transmissions among Secondary Users (SUs).