臺灣博碩士論文加值系統

近年來隨著發光二極體 ( Light-Emitting Diode, LED ) 的製造成本越來越便宜，加上可視光通訊 ( Visible Light Communication, VLC ) 的發展越來越成熟，有越來越多關於此一領域的研究文獻。在這些文獻當中有許多系統是採用正交分頻多工 ( Orthogonal Frequency-Division Multiplexing, OFDM ) 技術。然而這些研究並無考量到 LED 本身的非線性轉換特性。另一方面，我們也發現快速傅立葉轉換 ( Fast Fourier Transform, FFT ) 與反向快速傅立葉轉換 ( Inverse FFT,IFFT )的縮放係數很顯然的對於整個系統效能會有非常大的影響。本篇論文主要在研究採用 OFDM 技術的 VLC 系統，並且將上述被忽略的問題點納入考量的範圍內。具體來說，也就是使用了直流偏壓光 ( DC-bias Optical, DCO ) 與非對稱截波光( Asymmetrically Clipped Optical, ACO )這兩種 OFDM 技術。除了這兩種技術之外我們也將較廣泛使用訊號載波調變技術加入來做不同的比較。在 VLC 系統當中，由於 LED 的強度調控 ( intensity modulation ) 的關係 OFDM 調變出來的訊號必須為實數 ( real ) 和單一極性 ( unipolar )。模擬結果顯示，當我們在設計採用 OFDM 技術的 VLC 系統時，其 LED 的非線性轉換特性是絕對必須要納入考量的範圍內。此外我們也發現對於在不同 LED 的非線性轉換特性和不同實數 OFDM 取樣 ( samples ) 的 DCO、ACO OFDM 來說，我們可以選擇其更適合的 IFFT/FFT 等效公式來達到更好的效能表現。

In the recent years, as production cost of light-emitting diode (LED) is becoming lower and visible light communication (VLC) has a great progress, the studies of VLC have drawn much interest in the literature. Many studies are investigated in the orthogonal frequency-division multiplexing (OFDM) system. However, they are explored in the absence of the nonlinear LED transfer characteristic. On the other hand, we also find that the real coefficients associated with the pair of transform functions (i.e., fast Fourier transform (FFT) and inverse FFT (IFFT)) significantly make a great impact on the system performance. In this thesis, the OFDM-based VLC system is investigated by taking account of the ignored points mentioned above. Specifically, both DC-biased optical (DCO) and asymmetrically clipped optical (ACO) OFDM are considered. We also give the VLC performance comparison between OFMD-based modulation and other widely adopted signal-carrier modulations. In VLC systems, modulated OFDM signals must be real and unipolar because of LED intensity modulation. Our simulation results show that it is definitely necessary to take the non-ideal LED transfer characteristic into account while developing the
OFMD-based VLC systems. It is also found that we can select a preferably equivalent
FFT/IFFT pair to obtain a better system performance for DCO and ACO OFDM in
various LED transfer characteristics and amplitude levels of real OFDM samples.

中文摘要 i
Abstract ii
誌 謝 iii
目 錄 iv
表目錄 v
圖目錄 vi
一、緒論 1
二、可視光通訊 4
2.1 可視光通訊系統 4
2.2 光源 4
2.3 OFDM 5
2.4 可視光通訊的應用與挑戰 7
三、系統模型與架構 8
四、研究方法 11
4.1資料調變 11
4.2 IFFT/FFT 15
4.3 LED module 17
五、模擬結果分析 19
六、結論 37
參考文獻 39

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