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標題: 無線區域網路802.11b之 FIR濾波器與等化器之效能分析研究
Performance Evaluation of FIR Filter and Equalization for 802.11b WLAN
作者: 呂明和
Lu, Me-Ho
關鍵字: 802.11b;802.11b;LMS equalization;SQRT raised cosine Filter;ISI;LMS 等化器;平方根升餘弦濾波器;ISI效應
出版社: 電機工程學系
近年來在通訊領域中無線區域網路(Wireless Local Area Network)已經形成了一種風潮,從802.11到802.11b以及802.11a和802.11g的發展,我們發現到適合各種環境及傳輸速度要求的成品及技術已更趨完善。其中採用不同的調變技術如貝克(Barker)、CCK和OFDM等無非是要我們經過通道的各種傳輸錯誤率更小。在無線傳輸上,多重路徑(multi-path)所引起的ISI效應一直是一個主要需要克服的議題。
在802.11b中ISI也是個重要的議題。在取樣上我們採Over-sampling的方式,由於在頻域上每個chip的高頻成份會互相干擾,於是在基頻(baseband)傳送端及接收端分別加上一個Band limited 的 sqrt- raised-cosine Filter用以解決chip間的ISI問題,因為 Raised cosine filter 的取樣點sidelobe 的部分剛好落在ISI 等於零的點上面。
而在對付multi-path的ISI問題上,在高速傳輸上(CCK5.5Mbps及CCK11Mbps)我們採用通道等化器來解決。至於等化器採用的演算法,由於我們有preamble這訊號做為training sequence的依據於是我們採用了適應性LMS演算法。
在實現過程中我們發現接收端的sqrt-root-raised-cosine Filter的Tap值是固定的(Fixed)而(LMS)等化器的Tap 是適應性的(Adaptive),而他們的共通性都具有對付ISI效應,於是我提出了等化器的新架構,將濾波器和等化器合併成一個元件,這個新架構除了可以省掉一個元件外並能減化硬體的複雜度及加快等化器的收斂速度

Wireless Local Area Network communications become more and more popular part in communication area. Under the development from 802.11 to 802.11b and 802.11a to 802.11g, we find that products and technologies that meet the requirement for varies circumstances and varies distance for multi-rate transmission .In the IEEE standards, the purpose that adopting various kind of modulation such as Barker code CCK and OFDM is to reduce the bit error rate when signals is transferred through the channel. In wireless communication the ISI caused by multi-path effect that always is an important issue.
ISI is also an important issue in 802.11b. we adopt the way of over-sampling ,our data spectrum must meet the radio spectrum requirement for band limited transmitter ,we adopted a couple of low pass filter pair —SQRT raised cosine filter in the transmitter and receiver design because of the characteristics of raised cosine filter, it can sampling at the point of zero ISI in the side-lobe spectrum.
We adopt equalization to eliminate ISI problem in high data rated .The Algorithm using for our equalization, we adopt the adaptive LMS method because the transmitter preamble can used to be training sequence for our received data , and LMS is a popular and much easier realization algorithm.
In the realization process, we find that the taps of the SQRT raised cosine Filter is fixed and the tap of the LMS equalizations is adaptive, and both of them have the same property to overcome the ISI effects. We address a new structure to merge both the raised cosine filter and the equalization in this thesis. Our new structure has the benefits of saving hardware complexity and fast convergence.
Appears in Collections:電機工程學系所

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