微波存取全球互通系統帶外功率降低之研究

Studies of Out-of-Band Power Reduction in WiMAX System

袁碩蔓(Shuo-Man Yuan)

指導教授 ： 許超雲

大同大學/電機資訊學院/通訊工程研究所/碩士(2008年)

摘要

近幾年，高資料傳輸率的無線通訊設備已很普遍地在使用中。其中OFDM系統能夠更有效率地使用無線頻譜，已被廣為人知的WLAN、DVB、DAB和WiMAX等等的系統採用。
由於OFDM系統在訊號待傳送之前加入時間保護區間，因此資料子載波之間彼此失去了正交性，對於鄰頻而言，主頻的副瓣延伸到帶外會被鄰頻視為干擾。所以，如何有效地降低OFDM系統頻帶間或鄰近頻帶的互相干擾，提供更佳的無線通道環境，已成為一個很重要的研究議題。尤其，Mobile WiMAX系統已在台灣進行佈署，然而在頻譜的規劃上，並無規劃保護頻帶，僅靠使用頻帶間的零子載波作為保護頻帶是無法滿足有效抑制頻帶外功率。
在這篇論文中說明兩種現有的方法，其應用資料子載波兩旁的零子載波，將其加入可以有效抑制頻帶外功率的參數，來達到降低頻帶外功率。此方法運用符元邊界平滑的觀念，保持OFDM系統在時域上符元與符元間的邊界連續性，使得訊號在頻域上的表現更為收斂，來達到有效地抑制頻帶外功率。
另外，由德國航太中心所開發出來，其應用在頻域上訊號的疊加特性，在資料子載波兩旁的零子載波上加入經過事先運算所得到的複數加權因子，稱為插入消除載波法。因此，消除載波的的副瓣可消除資料子載波的副瓣，達到抑制頻帶外功率。
我們以模擬方式分別將上述方法導入Fixed WiMAX系統中，其擁有256個子載波。另外採用創新的構想，將這上述的方法結合，先執行插入消除載波法，再執行強迫為零平滑處理法或是尾隨平滑處理法，比較其差異點。應用強迫為零或尾隨平滑處理法，由於訊號在頻域上的表現愈來愈收斂，所以對帶外功率的抑制有明顯的成效。應用插入消除載波法，消除載波的副瓣離主瓣愈遠振幅愈小，所以對衰減資料載波的副瓣的效果會愈來愈輕微，但在靠近訊號的帶外功率有顯著的抑制效果。然而，採用結合的方式可將兩種方式的優點表現出來，在靠近主頻訊號的帶外功率即鄰頻的位置有顯著的抑制，在遠離訊號的帶外功率即下一個鄰頻的位置也會有愈來愈顯著的抑制效果。

關鍵字

帶外功率降低 ； 微波存取全球互通

並列摘要

Duo to the growth of wireless communication with high data rate, the radio frequency spectrums should be utilized efficiently. The technology of OFDM system that utilizes spectrums more efficient than the traditional single carrier system is the best choice for the modern wireless communications. Therefore, the OFDM technology has been introduced into the digital wireless communication systems, such as DAB、DVB、DMB、 ISDB-T、HD Radio (IBOC)、DRM、DVB-H、HDTV and IEEE 802.11a、 802. 11g、802.16d、806.16e、802.20 and so on.
Since OFDM system suffers from an intrinsic high out-of-band power emission, it gives us the motivation to research and develop new technologies that can overcome this problem. Mobile WiMAX is set up in Taiwan presently, but channels have no guard band to separate from each other. It uses null subcarriers to treat as guard band.
In this thesis, we utilized the concept of “Symbol Boundary Smoothing” and proposed two approaches to suppress the out-of-band power emission in WiMAX system. The new approaches can overcome this problem by inserting auxiliary parameters into the original null subcarriers at both sides of OFDM spectrum, then the boundary of successive symbols become continuous. Besides, another approach proposed by German Aerospace Center also utilizes the null subcarriers. The null subcarriers are modulated with will-defined complex weighting factors, that is, these special subcarriers named Cancellation Carriers whose sidelobes can cancel the sidelobes of data carriers mutually according to superposition property. In this way, the reduction of out-of-band power emission is archived. The approach Insertion of Cancellation Carriers has excellent out-of-band power reduction at adjacent channel, and the approaches Symbol Boundary Smoothing have excellent out-of-band power reduction at alternate channels. We combined these approaches, the simulation results show that proposed approaches have significant reduction of out-of-band power emission.

並列關鍵字

WiMAX ； Out-of-Band Power Reduction

參考文獻

[1] Ronald N. Bracewell, The Fourier Transform and Its Applications, 3rd edition. McGraw Hill, 1999.

[3] Sinja Brandes, Ivan Cosovic, and Michael Schnell,“Reduction of Out-of-Band Radiation in OFDM Systems by Insertion of Cancellation Carriers,”IEEE Communication Letters, vol. 10, no. 6, pp. 420-422, June 2006.

[5] Richard Van Nee and Ramjee Prasad, OFDM wireless Multimedia Communication. Artech House, 2000.

[7] Jeffrey G. Andrews, Arunabha Ghosh, and Rias Muhamed, Fundamentals of WiMAX. Prentice Hall PTR, 2007.

[9] WiMAX Forum, Mobile System Profile Release 1.0 Approved Specification (Revision 1.4), May 2007.