OFDM系統時間同步之研究

A Study of timing synchronization of OFDM

汪順(Shung Wan)

指導教授 ： 鄭立德

中原大學/工學院/電子工程研究所/碩士(2006年)

https://doi.org/10.6840/CYCU.2006.00499

摘要

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(DAB)、數位音訊廣播(DVB)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，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。
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。

關鍵字

定時同步 ； 正交分頻多工

並列摘要

Nowadays, OFDM is the major modulation technique for the fourth mobile communication systems. Recently, OFDM has been widely used in DA B, DVB, IEEE 802.11a and HIPERLAN-2. OFDM is a multicarrier transmission system which divides high speed transmission data into several low speed data packets. It has the advantages of bandwidth efficiency, noise reducing, cryptogram imporving and insensitive to multipath fading. The received OFDM signal have the problem of frequency offset which causes interference in
subcarriers and destroys the orthogonality among subcarriers. Therefore, synchronization in timing
and frequency is the important processing procedure for OFDM to demodulate the signal correctly.
Based on the foundation of WLAN 802.11a, in this thesis, we prsent several estimating algorithms and propose a synchronization structure for OFMD. This structure includes estimation in the start point of signal frame in the time domain, estimaion in the edge of signal frame, frequency offset estimaion, and the compensation of reminding timing sequence and phase offset in the frequency domain. For practical consideration, we adopt dynamical simulations to compare the system performance in every situations.We made the constraints on bandwidth, amplitude modulation and demodulation, low pass filter and sampling in continuous time. The performance of our system is investigated by computer simulations.The results show that our proposed synchronization method peforms well in AWGN and Rayleigh fading channels.

並列關鍵字

OFDM ； timing synchronization

參考文獻

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