正交分頻多工無線區域網路系統訊框偵測器及頻率估測器之演算法及效能評估

Abstract

隨著可攜式電腦與網際網路的進步，對於無線多媒體通訊系統的要求急速地增加。傳輸率達20 Mbps以上無線區域網路系統被廣泛地討論。在高速傳輸之無線通訊下，通道波散所造成之符際干擾(ISI)將造成系統效能嚴重衰退。正交分頻多工(OFDM)為一對付ISI之有效技術。近年來，OFDM技術亦引進為下一代無線系統標準中，如IEEE 802.11a。基於IEEE 802.11a標準下之無線通訊系統，在未經排程下將已包裝之OFDM訊號傳送出。因此，接收機之首要工作為偵測所收到包封之起始。在本論文中，吾人所提出之訊框偵測器可偵測且結合長及短之前置訊號以得到更精確之訊框偵測。模擬結果顯示出吾人所提出之訊框偵測器在低訊雜比之環境下仍可正常運作。在訊框偵測完成之後，接收機將由短前置訊號估測與更正載波頻率飄移。在本論文中，吾人將結合兩種頻率估測器藉以增加可估測範圍並獲得更精準的估測結果。

The demand for wireless multimedia communications is rapidly increasing with advances in portable computer hardware and Internet services. High data rate wireless local area network (LAN) systems with data rate exceeding 20 Mbits/s are discussed all over the world. In high data rate wireless communications, intersymbol interference (ISI) generated by channel dispersion will cause severe degradation of the system performance. Orthogonal frequency division multiplexing (OFDM) is a very promising technology for combating the ISI. Recently, the OFDM modulation scheme has been introduced in the next-generation wireless system standards such as IEEE 802.11a (US). In a wireless LAN system based on the IEEE 802.11a standard, packetized OFDM signals are transmitted without scheduling. Therefore, the first task of the receiver is to detect the start of an incoming packet. In this thesis, a frame detector is proposed which can detect and combine the short and long preambles to gain more accurate frame detection. Simulations show that the proposed frame detector works well in the low SNR environment. After frame detection, the receiver estimates and corrects the carrier frequency error by using the short preamble. In this thesis, we combine the two frequency estimators to increase the estimation range and gain more accurate estimation result.