利用相關值輸出之互補編碼調變決策迴授等化器

Abstract

IEEE 802.11b無線區域網路系統採用互補編碼調變(CCK)來當作傳輸速率5.5Mbps以及11Mbps高速無線傳輸的調變方法。CCK藉由其調變編碼的方式，使得同一個symbol裡的八個chip有一定的相關性，在AWGN通道的影響下，使用此種調變並利用相關器解調會比傳統的QPSK有更好的表現。由於無線傳輸的環境裡，訊號經過多路徑通道後會有時間上的擴散導致inter-symbol interference (ISI)干擾的產生，故需要用一些方法來克服。在論文中我們討論用等化器(equalizer)來消除ISI的干擾，而決策迴授等化器(Decision-Feedback Equalizer, DFE)由於效能佳且易於硬體實現，所以是通訊系統很常用的等化器。 如果用在CCK 的調變系統上， 當DFE的slicer決策錯誤時會破壞一個CCK symbol裡chip 之間的相關性，導致整個symbol解調錯誤的可能。所以我們提出一個方法，利用CCK解調之相關器輸出的symbol迴授到DFE的feedback filter。也就是說，是對symbol做決策而不是對chip做決策，這樣可以減低決策錯誤的發生率。 由模擬結果可以看出此correlation-aided DFE可以有效減低決策錯誤，並且比一般的DFE表現的更好。

Complementary code Keying (CCK) is employed as the modulation method for the high-rate scheme of the IEEE 802.11b WLAN system which provides 5.5Mbps and 11Mbps for high data rate transmission. The 8 chips within a CCK symbol have certain correlation properties and thus CCK performs better than conventional QPSK modulation in a AWGN channel. The WLAN system is operated in a wireless environment which will introduce time dispersion of the signal, thus techniques that combat the multipath interference are needed. In this thesis we discuss the equalizers that combat the intersymbol interference (ISI) and the decision-feedback equalizer (DFE) is commonly used in communication systems because it performs well and is simple for implementation. However, when applied in CCK scheme the silcer or hard-decision device in the DFE will destroy the correlation between chips in the same CCK symbol when the chip decision is not correct. Thus, a correlation-aided DFE is proposed that utilizes the detected symbols from the modified FWT block as the input of the feedback filter. That is, detection is done for symbols instead of chips to mitigate the detection error. Simulation results show that the modified DFE mitigates the detection error and thus perform better than the conventional DFE.