正交分頻多工系統中利用不同導航符號之載波頻率飄移估測

Various Pilot-aided Estimation Schemes for Carrier Frequency Offset in OFDM Systems

張餘寬(Yu-Kuan Chang)

指導教授 ： 林容杉

國立暨南國際大學/科技學院/通訊工程研究所/碩士(2010年)

https://doi.org/10.6837/NCNU.2010.00147

摘要

在這最近數十年期間，正交分頻多工(OFDM)技術已是無線通訊一個吸引人的傳輸方式。正交分頻多工技術的基本原理在於將原本較高速傳輸速度的資料分成數個低速傳輸速度的資料，並且使用數個彼此正交的子載波同時傳送。換句話說，正交分頻多工技術中每個子載波只佔頻寬中一小部分，且因每個子載波因正交來達到節省頻寬，故正交分頻多工技術可看成多載波傳輸的一個特例。同步的影響在正交分頻多工系統中是一個被廣泛的研究議題，包含了時間同步和頻率飄移。若同步錯誤沒妥善估測補償會產生符元間干擾與載波間干擾，造成通訊訊號的破壞與系統效能的下降。通常頻率同步在接收端是最主要被完成的工作，因此要如何估測正確的頻率偏差在正交分頻多工系統中是一項重要的問題。

在本論文中，以假設時間同步在完美的情況下做頻率飄移估測。一開始將介紹一些傳統上頻率飄移估測方法。在本篇論文中，將提出三個前置訊號的排列方法使其具有降低複雜度和改善系統效能。第一個方法是將補零訊號插入，以降低運算的複雜度。因此當補零訊號選擇在一段適當長度時，系統的效能並沒有降低的非常嚴重但運算量也能降低。第二個前置訊號的排列是在頻率端也就是在反快速傅立葉轉換(IFFT)前不做任何排列，經過反快速傅立葉轉換之後會有一個特殊的序列，我們再利用這特殊的序列來作頻率飄移估測。第三個方法是結合整數和小數估測方法，在複雜度上跟傳統方法較低。最後我們把這三個方法和傳統方法作性能及複雜度比較。

關鍵字

正交分頻多工系統 ； 載波頻率飄移 ； 前置訊號配置

並列摘要

During the last decades, orthogonal frequency division multiplexing (OFDM) technology has become the most attractive transmission scheme for wireless communications. OFDM systems are very sensitive to synchronization. The synchronization errors usually result in both inter-symbol interference (ISI) and inter-carrier interference (ICI) to distort transmission signal and degrade system performance. In general, frequency synchronization is the major task to be completed in the receiver end. Therefore, how to correctly estimate the error of frequency offset in OFDM systems is crucial and significant.
In this thesis, the time synchronization is assumed to have been well performed already and here only the compensation of carrier frequency offset (CFO) is focused on. Three proposed preamble patterns are given for reducing the computational complexity and improving the system performance. The modified Park's preamble is proposed to reduce operational complexity with insertion of zero padding to estimate the frequency offset. In addition, a special sequence preamble pattern is proposed to estimate frequency offset. This preamble pattern can be easily permuted in frequency domain and after inverse fast Fourier transform (IFFT) have a special sequence for frequency synchronization. Third method combines integral frequency offset (IFO) and fractional frequency offset (FFO) estimation, and computational complexity is less than conventional methods. Finally, the performance comparison between our proposed methods and several traditional schemes is given in this thesis.

並列關鍵字

OFDM systems ； carrier frequency offset ； preamble pattern

參考文獻

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