在本篇論文中，我們實現基於直流偏置光正交分頻多工(DCO-OFDM)的可見光通訊(Visible Light Communication)收發機。整合收發機電路與紅色發光二極體(LED)後，可以在15公分的傳輸距離達到傳輸速率每秒24百萬位元。
可見光通訊系統是由我們與工研院合作完成，工研院主要負責前端模組、乙太網路、數位類比轉換器(ADC/DAC)，而我們主要負責基頻收發機電路。根據工研院所提供的前端電路，我們共同擬訂適合的系統規格。在此系統中，我們使用16正交振幅調變技術(16-QAM)與1024個子載波。為了克服多路徑效應造成的影響，我們的系統採用1/16符元長度的循環字首(Cyclic Prefix)。在傳送端中，子載波會間隔地放置領航信號(Pilot Signal)。在接收端中，我們可以利用領航信號估測訊號的相位旋轉，補償取樣時脈偏移(Sampling Clock Offset)造成的影響。在傳送端中我們加入前置符元(Preamble)，並且在接收機中基於Park演算法完成時間同步。另外，為了補償通道效應，我們在接收機中採用一階等化器。最後，我們採用部分傳輸序列(Partial Transmit Sequence)結合限幅技術，並且降低系統峰均能量比約4分貝。
前端電路的通道訊雜比(Signal-to-noise Ratio)約為22分貝，可提供頻寬為20百萬赫茲。整合前端電路後，在傳輸距離15公分的情況下為無誤碼，並且頻譜使用效率為1.2(位元/秒/赫茲)。我們利用工研院提供的封包產生器驗證收發機至少可達每秒24百萬位元的傳輸速率。最後，我們可以利用此可見光系統傳遞並播放720p高解析影片。透過紅色光傳輸，影片可以穩定且無遺失封包地播放。

In this thesis, we have implemented a DC-bias optical orthogonal frequency division multiplexing (DCO-OFDM) transceiver for visible light communications (VLC). Based on red light-emitting diode (LED), the data rate has reached 24 Mbps under 15 cm transmission distance.

We have collaborated with the Industrial Technology Research Institute (ITRI). ITRI members have implemented the front-end modules, the Ethernet, and the analog-to-digital converter/digital-to-analog converter (ADC/DAC), and we have implemented the baseband transceiver circuits. According to the front-end circuits provided by ITRI, we worked out the appropriate system specifications. In this system, we adopted 16 quadrature amplitude modulation (16-QAM) and 1024 subcarriers. To overcome the effects of multipath, we used the cyclic prefix (CP) which is 1/16 the length of the symbol time. At the receiver, we used the pilot signals to estimate the phase rotation of the signals and compensated the effects of sampling clock offset (SCO). We added preamble at the transmitter and completed time synchronization based on Park algorithm. In addition, in order to compensate the channel effect, we used the one tap equalizer at the receiver. Finally, we used partial transmit sequence (PTS) technique combined with clipping method and reduced the maximum peak-to-average power ratio (PAPR) of the system by about 4 dB.

The channel signal-to-noise ratio (SNR) of the front-end circuits is about 22 dB, and the bandwidth is 20 MHz. After integrating the front-end circuits, there is no error bit in the system under 15 cm transmission distance, and the spectral efficiency is 1.2 bit/sec/Hz. We use the packet generator to verify that this transceiver can achieve transmission rates of 24 Mbps. Finally, we can use this VLC system to transmit and play 720p high definition video. Through red LED transmission, video can be played steadily without loss of packets.

Abstract i
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Main Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 System Description 5
2.1 Overview of Indoor VLC System . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Related Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 OFDM System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Modulation Schemes for VLC . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4.1 On-off Keying (OOK) . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4.2 Pulse Amplitude Modulation (PAM) . . . . . . . . . . . . . . . . . . 11
2.4.3 Variable Pulse Position Modulation (VPPM) . . . . . . . . . . . . . 12
2.4.4 DC-biased Optical OFDM . . . . . . . . . . . . . . . . . . . . . . . 13
2.4.5 Asymmetrically Clipped Optical OFDM . . . . . . . . . . . . . . . . 15
2.5 Optoelectronic Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5.1 Electrical-optical Transducers . . . . . . . . . . . . . . . . . . . . . 18
2.5.2 Photosensitive Components . . . . . . . . . . . . . . . . . . . . . . 18
3 System Design 21
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2 Design and Verification Flow . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3 System Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.4 DCO-OFDM System Model . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.4.1 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.4.2 Subcarrier Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.4.3 Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform
(FFT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.4.4 Cyclic Prefix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4.5 Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.4.6 Channel Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.5 PAPR Reduction Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.5.1 Signal Distortion Techniques . . . . . . . . . . . . . . . . . . . . . . 39
3.5.2 Signal Scrambling Techniques . . . . . . . . . . . . . . . . . . . . . 40
3.5.3 Combination of PAPR Reduction Technique . . . . . . . . . . . . . . 43
3.6 Data Rate Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4 Architecture Design 47
4.1 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.2 Subcarrier Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.3 Fast Fourier Transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.4 Timing Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.5 Sampling Clock Offset Estimation and Compensation . . . . . . . . . . . . . 51
4.6 PAPR Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5 Implementation Results 55
5.1 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.1 PAPR Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.2 Timing Synchronization . . . . . . . . . . . . . . . . . . . . . . . . 58
5.1.3 Sampling Clock Offset Compensation . . . . . . . . . . . . . . . . . 59
5.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.3 Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.4 Bit Error Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.5 Performance Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6 Conclusions and Future Works 69
6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.2 Future Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
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