| 摘要: | OFDM技術以它在多重路徑和窄頻干擾方面的良好性能以及高頻譜利用率得到廣泛應用,並成為第四代移動通信的核心技術,但是OFDM對同步非常敏感。研究了各種同步因素對系統的影響，透過模擬設計了一種基於IEEE 802.11a的OFDM寬頻資料通信同步系統的改善方案。 正交分頻多工(OFDM)是現今第四代行動通訊系統中最受矚目的調變技術，近幾年來，OFDM技術已廣泛的被使用於數位視訊廣播(DVB)、數位音訊廣播(DAB)及無線區域網路標準IEEE 802.11a與HIPERLAN-2。OFDM系統為多重載波傳輸技術，可以將高速傳輸的每一筆資料分割細分為多個低速的資料封包型態，並將這些低速的資料封包以個別的載波傳輸，可以有效的利用頻寬、降低雜訊、提高保密與降低多路徑衰減等通訊障礙的優異性。在OFDM所接收的訊號會有頻率偏移的現象出現，此頻率偏移會造成各子載波間的干擾。因為頻率偏移會使載波間的正交性受到破壞，造成相位雜訊，所以時序與頻率的同步是OFDM接收端能正確解調訊號的重要處理工作。 由於其頻寬效率及對抗頻率選性衰減上有很好的效能，近年來已被廣泛的應用在各種無線傳輸技術 上，特別是像WLAN、DAB、DVB以及數位無線中頻廣播DRM等等。OFDM對頻率偏移效應相當敏感，這是由於接收端與發射端間頻率誤差的結果；它會引起內部載波干擾，且會破壞OFDM信號組的正交性。其他如取樣頻率偏移，以及多路徑通道效應，同樣會造成系統效能的下降。因此，在接收端需要有一個良好的估測系統來還原信號。 在本篇論文中，我們基於802.11a無線區域網路的基礎上，將展現幾種估測演算法，並提出一個可行的OFDM系統同步架構。這個架構包括時域上的訊框起始點檢測，訊框的邊緣估測與頻率偏移估測 ，以及頻域上的殘餘時序與相位偏移補償。為了配合實際環境的需要與實驗的真實性，我們採用了動態模擬來幫助我們了解實際可能遭遇之狀況；因此，我們加入頻寬限制，振幅調變與解調，低通濾波，連續時間之取樣等實驗流程，並在加成白色高斯雜訊以及雷利衰退通道下進行電腦動態模擬。根據模擬，我們所提出的系統在802.11a的IEEE通道模型下可以獲得良好的同步效果。我們嘗試了幾種在無線通訊系統上的時間同步方法，期望對接收器的時間同步做通盤深入的探討。 OFDM has been widely used in communication systems and is the key technology of 4G in the field of mobile communication, because of its excellent performance in overcoming multi-path channel and narrow band interfering. However the principle weakness of OFDM is sensitivity to synchronization. So synchronization is very important to OFDM systems. By researching various of the synchronization factors, according to simulation we introduce a method of improving the synchronization system for broad band data communication based on 802. 11a. OFDM is the most famous modulating technology of the fourth mobile communication system nowadays.Recently years, OFDM technology has widely used in DAB、DVB、IEEE 802.11aandHIPERLAN-2. OFDM system is the multi-carrier transmission system, it can be divided every high speed transmission data into several low speed data packet style. It can use wideband efficiently, reducing noise, improving cryptogram and reducing multi-path fading etc. has the disparity in communication the signal in OFDM receiver will appear frequency offset, the frequency offset will affect interference in every sub carriers. Because in every sub carrier frequency offset will destruct the orthogonality, producing phase noise, so that synchronization in timing and frequency is the main processing procedure in OFDM receiver to demodulate signal correctly. Due to wideband efficiency and deal with frequency selecting fading has the well efficiency, recently years had widely used in various wireless transmission technology, especially in WLAN, DAB, DVB and DRM etc. OFDM is very sensitive in frequency offset effect, this is because they have frequency error between TX and RX, so that it produces ISI (Inter-Symbol-Interference), destruct orthogonality in OFDM symbols. Others, frequency sampling offset, multi path effect, will also reduce system efficiency. So that, in RX needs a well estimator to reconstruct original signal. In the thesis, we focus on the foundation of WLAN 802.11a, it will appear several estimating algorithm, and proposing a valid synchronization structure in OFMD system. This structure including the estimation in the start point of signal frame in time domain, estimating the edge of signal frame and estimating frequency offset, and the completion of reminding timing sequence and phase offset in frequency domain. To deal with the need in real environment and the reality in practice, we adopt the dynamical simulation to help us to understand the reality in every situation. So that, we add the limitation of wideband, amplitude modulation and demodulation, low pass filter, sampling in continuous time, simulating in AWGN and Rayleigh fading channel. According to the simulation, our proposed method in the IEEE channel model of 802.11a get a well done effect with synchronization. We conduct extensive experiments to examine several synchronization timing methods in depth exploration into wireless communications systems. |
