次級異質網路在頻譜共享中的共存:個案研究

Abstract

近年來，由於設備的廣泛使用，對無線通信的需求呈指數增長。分配額外的頻譜是網絡容量增加的主要解決方案。但是，主要用戶(有執照的使用者)並不總是佔用他們的頻道，進而產生一些無人使用的時間空隙。研究人員已經表明頻譜共享是一種有效的解決方案。次要用戶(沒有執照的使用者)藉由使用時間空隙，與主要用戶共存來增加頻譜的使用率。其中，又以VHF/UHF頻段的電視頻道討論最多。電視白頻道意旨那些很少被主要用戶利用的電視頻道。 美國FCC要求主要用戶存取電視白頻道時必須依靠地理位置數據庫。當主要用戶開始訪問他們的頻道時，他們將註冊到數據庫。對於次要用戶，如果他們想要訪問該頻道，他們需先在數據庫中查詢頻道狀態。這是主要用戶和次要用戶的共存方式。針對次用戶間的共存，IEEE 802.19.1提出了相同於地理位置數據庫的實作方法。因此，集中式機制被視為是解決共存的一種趨勢。但是，當可用電視白頻道不足時，真正的挑戰就發生了。 IEEE 802.19.1帶來了一種時間共享的概念，即在一個頻道上為不同的網路調度傳輸週期，這是一個集中式機制。至於分佈式機制則依賴各網路的MAC層行為。我們關注TDMA和CSMA標準之間的共存。基本上，TDMA標準具有頻道使用的優勢，因為它沒有先聽後送的機制。 本文中，我們考慮802.22和802.11af之間的共存。802.22和802.11af之間的共存包含兩個問題。首先，802.22設備即使在802.11af設備正在傳輸時也會開始傳輸。其次，有主要用戶沒有聯網功能，因此他們不會註冊到地理位置數據庫，像是麥克風。802.22設備支援偵測主要用戶的功能，一段時間後每個802.22設備停止傳輸並偵測頻道狀況。此時如果802.11af設備正在傳輸，802.22設備會誤檢麥克風的存在並跳到另一個頻道。因此，我們提出了一個兩階段檢測機制來區分兩種情況：麥克風的存在或是只存在802.11af網路。

In recent years, demand for wireless communication has been increasing exponentially because of widespread usage of devices. Assigning additional spectrum is the major solution for network capacity increase. However, licensed users do not always occupy their channels, which generates some time slots that no one uses. Spectrum sharing becomes another solution. Researchers have shown that the licensed spectrum that is owned by primary users is underutilized and TV channels in the VHF/UHF band are discussed most. Hence, a more efficient use of this spectrum is to permit unlicensed users to coexist with primary users. Moreover, TV white spaces (TVWS) refer to channels not used frequently. The unlicensed user is named as the secondary user. The FCC ruling requires that TV white spaces devices must rely on a geo-location database. When primary users start to access their channels, they will register to the database. For secondary users, they need to query the database for information of a channel status if they want to access the channel. It is the coexistence method for primary users and secondary users. As for the coexistence method among secondary users, IEEE 802.19.1 is proposed. It utilizes databases as a controller to assign channels to different secondary users. Therefore, centralized mechanism is considered as a trend. However, the real challenge happens when the available TVWS spectrum is insufficient. IEEE 802.19.1 brings a time sharing concept that schedules transmission periods for different secondary networks on one channel. This is a centralized solution. As for the distributed way, it depends on MAC layer and we focus on the coexistence between the TDMA-based and the CSMA-based standard. The TDMA-based standard with duty cycle is a distributed solution for coexistence with the CSMA-based standard. A TDMA-based base station ceases some frame transmissions periodically and a CSMA-based device senses the channel as idle before accessing. In this thesis, we consider the coexistence between 802.22 and 802.11af. 802.22 is a TDMA-based standard and 802.11af is a CSMA-based standard. The coexistence between 802.22 and 802.11af contains two problems. First, 802.22 devices start to transmit even when 802.11af devices are transmitting due to the none listen before talk mechanism of 802.22. Second, 802.22 devices support the sensing detection of microphones because they may not register to the TVWS database. In the sensing period, every 802.22 device stops transmitting and listens to the channel. If 802.11af devices are transmitting at this time, 802.22 devices do the misdetection that microphones exist and hop to another channel. Therefore, we propose a two stage detection mechanism used to distinguish two scenarios: the existence of microphones or only the existence of 802.11af networks.