Photodetector Characteristics in Visible Light Communication

Home , Photodetector

Dissertation by

Kang-Ting Ho

In Partial Fulfillment of the Requirements

For the Degree of

Master of Science

King Abdullah University of Science and Technology,

Thuwal, Kingdom of Saudi Arabia

Kang-Ting Ho

EXAMINATION COMMITTEE APPROVALS FORM

The dissertation of Kang-Ting Ho is approved by the examination committee:

Committee Chairperson: Dr. Jr-Hau He

Committee Member: Dr. Boon S. Ooi

Committee Member: Dr. Lance Li

Committee Member: Dr. Daniel Lau

ABSTRACT

Photodetector Characteristics in Visible Light Communication

Kang-Ting Ho

Typically, in the semiconductor industry pn heterojunctions have been used as either light-emitting diodes (LED) or photodiodes by applying forward current bias or reverse voltage bias, respectively. However, since both devices use the same structure, the light emitting and detecting properties could be combine in one single device, namely

LED-based photodetector. Therefore, by integrating LED-based photodetectors as either transmitter or receiver, optical wireless communication could be easily implemented for bidirectional visible light communication networks at low-cost. Therefore, this dissertation focus on the investigation of the photodetection characteristics of InGaN

LED-based photodetectors for visible light communication in the blue region. In this regard, we obtain external quantum efficiency of 10 % and photoresponse rise time of 71

µs at 405-nm illumination, revealing high-performance photodetection characteristics.

Furthermore, we use orthogonal frequency division multiplexing quadrature amplitude modulation codification scheme to enlarge the operational bandwidth. Consequently, the transmission rate of the communication is efficiently enhanced up to 420 Mbit/s in visible light communication. 4

ACKNOWLEDGEMENTS

Earnestly thank King Abdullah Bin Abdulaziz Al Saud, and KAUST’s proceeding and current management who realize his dream and vision. I will always cherish the tremendous experiences gained in the past one and a half years.

I sincerely thank my supervisor, Prof. Jr-Hau He, a dedicated scientist and entrepreneur with great passion and insights. His leadership and his constant support helped me accomplish this thesis dissertation. I would also like to express my appreciation to our laboratory manager, Dr. Jose Ramon Duran Retamal, who always help me push me to achieve high-performance as a top-quality engineer and scientific worker. Also thanks for all the valuable discussions and creative solutions. Special thanks to our senior lab member Dr. Cheng-Fang Kang who introduced me to this exciting field.

Thanks to Prof. Boon S. Ooi for his suggestions and discussions during my research.

He also kindly provided me with a high-technical environment to perform the experiments. Thanks also to Dr. Thien Khee Ng and Chao Shen for all the support provided during the experiments.

Thanks to Prof. Hao-Chung Kuo form National Chiao Tung University for providing me with the devices, and to Dr. Yu-Lin Tsai for giving me a lot of suggestion about conceptualizing the project.

Thanks to Prof. Gong-Ru Lin from National Taiwan University for elaborating the project design. Additional thanks to Dr. Yu-Chieh Chi, Cheng-Ting Tsai, and Huai-Yung

Wang for their explanations and lectures about OFDM measurements. 5

TABLE OF CONTENTS

Page

EXAMINATION COMMITTEE APPROVALS FORM ...... 2

ABSTRACT ...... 3

ACKNOWLEDGEMENTS ...... 4

TABLE OF CONTENTS...... 5

LIST OF ABBREVIATIONS ...... 7

LIST OF ILLUSTRATIONS...... 8

LIST OF TABLES ...... 9

1. Chapter One: Background ...... 10

1.1 Introduction...... 10 1.2 Motivation ...... 10 1.3 Multiple quantum wells InGaN/GaN LED ...... 12 1.4 LED-based photodetector ...... 13 1.4.1 Responsivity & external quantum efficiency ...... 14 1.4.2 Response time ...... 15 1.4.3 Frequency spectrum...... 16

1.5 Orthogonal Frequency Division Multiplexing ...... 17

2. Chapter Two: Measurement Setup...... 19

2.1 Current–voltage characterization ...... 19 2.2 Light source and responsivity calculation ...... 19 2.3 Light source and RC time constant measurement ...... 20 2.4 Frequency spectrum measurement ...... 21 2.5 Orthogonal Frequency Division Multiplexing ...... 22

3. Chapter Three: Devices Characterization ...... 23

3.1 External Quantum Efficiency ...... 23 3.2 Response Time ...... 24 3.3 Frequency Spectrum ...... 25 3.4 Orthogonal Frequency Division Multiplexing ...... 26 3.4.1 Bias Voltage Depend ...... 26 3.4.2 Start Frequency ...... 27 6

3.4.3 Bandwidth ...... 28 3.4.4 Summary ...... 30

4. Chapter Four: Conclusion...... 31

BIBLIOGRAPHY……………………………………………………………………………………………………….. 32

LIST OF ABBREVIATIONS

VLC Visible light communication

OWC Optical wireless communication

RF Radio frequency

Li-Fi Light Fidelity

LED Light-emitting diode

PD Photodetector

SSL Solid-state lighting

OOK On-off keying

YAG Y3Al5O12:Ce

MQWs Multiple quantum wells

OOK On-off keying

FWHM Full width at half maximum

OFDM Orthogonal frequency-division multiplexing

FDM Frequency division multiplexing

BER Bit error rate

SNR Signal-to-noise ratio

4-QAM Quadrature phase shift keying

FEC Forward error correction

LIST OF ILLUSTRATIONS

Figure 1. White light spectrum of a coating YAG phosphor illuminated by blue light LED……………………………………………………...... 11

Figure 2. The band diagram of AlGaN/GaN/InGaN MQWs structure...... 13

Figure 3. Output wave forms of the LED-based PD...... 14

Figure 4. (a) The current-voltage characteristics of perovskite solution-processed hybrid perovskite PDs…………...... 15

Figure 5. The relation between the optical signal and electrical response of PD…...... 16

Figure 6. The spectrum efficiency is improved by OFDM modulation...... 18

Figure 7. The different bit-to-symbol mapping for OFDM…...... 18

Figure 8. The profile shows the size of light spot and power intensity distribution which is uniform that we can calculate the power illuminated on devices...... 19

Figure 9. The profile shows the size of laser spot and power intensity distribution which is Gaussian beam that we can calculate the power illuminated on devices…...... 21

Figure 10. The LED is used gold wire bonding bonded on a designed mount…...... 22

Figure 11. The structure of InGaN/GaN MQW LED…...... 23

Figure 12. The photonics-electronics performance of LED-based PD...... 23

Figure 13. The time response measurement illuminated 405 nm laser diode which maximum power is 900 mW……………………………………………...... 24

Figure 14. The temporal characteristics of LED-based PD...... 25

Figure 15. The frequency spectrum of LED-based PD is applied in different bias...... 26

Figure 16. The OFDM testing depends on different bias voltage……………………...... 26

Figure 17. The BER and SNR plot shows start frequency only 2.34 GHz pass the FEC limit………………………………………………………………………………………………………………...... 28

Figure 18. The start frequency of bandwidth testing is 2.34 GHz…………………...... 29

LIST OF TABLES

Table 1. The BER and SNR value is measured in different bias voltage. All the reverse bias pass the FEC certification...... 27

Table 2. The value of BER and SNR with different start frequency…...... 28

Table 3. The bandwidth correspond to different sub-carrier and data rate…...... 29

1. Chapter One: Background

1.1 Introduction

Traditional wireless communication is based on radio frequency or microwave communication, however these technologies suffer of low bandwidth, and thus new technologies need to overcome these limitations in order to supply enough bandwidth for the high-demand of data from global networks. For this reason, in the recent years a new type of optical wireless communication has been developed based on visible light, thus named visible light communication (VLC). VLC offers not only high bandwidth due to the high frequency of the carrier but also high-security, low-cost energy, electromagnetic interference immunity, and worldwide availability, among others. In fact, VLC has been proposed as a 100 Gbit/s visible wireless access network for indoor applications. [1]

Moreover, VLC technology finds application into many areas such as indoor building [2], hospital [3], traffic-signal [4], mining[5], and in-flight [6], or into different environments such as underwater [7].

1.2 Motivation

Since the first visible light light-emitting diode (LED) was developed by Holonyak in

1962 [8], the solid-state lighting (SSL) technology has been continuously developed. But, it was not-fully developed due to the missing blue-LED, required to generate white light.

Therefore, white light illumination technology was not fully developed until the first high- brightness blue LED was demonstrated by Nakamura in 1994 [9, 10]. Accordingly, the 11 white light LED developed quickly either by mixing red-green-blue LEDs or by coating blue

LED with Y3Al5O12:Ce (YAG) phosphor as shown in figure.1 [11]. Afterwards, traditional lighting devices such as incandescent light or light bulb were replaced by LEDs due to the higher brightness, lower cost, and lower consumption.

Figure 1. White light spectrum of a coating YAG phosphor illuminated by blue light LED.

In VLC, LEDs represent a great advantage because they can be used not only as a white light source for illumination but also as signal carriers emitters for communication.

Additionally, since LEDs and photodetectors are based on the same device structure, which is pn heterojunction, LEDs can be used as PDs as well, namely LED-based PD.

Therefore, by using LED-based PD as emitter and receiver at the same time, VLC networks could be quickly developed with a low-cost integration method. However, the main constraint of LEDs is the low modulation bandwidth, typically in the order of tens of MHz

[12]. This is the reason because we make use of a more efficient modulation technique, 12 rather than the basic ON-OFF keying (OOK) modulation. In conclusion, the motivation of this dissertation is to investigate the performance of commercial blue-LED-based PD under high efficient bandwidth modulation scheme for VLC application.

1.3 Multiple quantum wells InGaN/GaN LED

Gallium nitride (GaN) and indium nitride (InN) are binary III/V semiconductors with direct band gap of 3.4 eV and 0.69 eV at room temperature, respectively. Thus GaN- and

InN-based LEDs are commonly used as near ultraviolet (UV) and near infrared (IR) light sources, respectively. Alternatively, indium gallium nitride (InGaN, InxGa1-xN) is an alloy semiconductor material mixing GaN and InN in which the bandgap can be tuned along the visible region by modulating the indium to gallium ratio.

In order to produce high-efficient LED, scientist have designed multiple quantum wells (MQWs) structure in which thin emitting layers are sandwiched between high potential barrier layers. By using this MQWs structure electron-hole pairs are confined in quantum well to increase the probability of recombination, and thus improving the energy conversion efficiency: ratio of power input to light output. To further improve the efficiency, an electron blocking layer can be used to block electrons and confine them into the MQW active region as shown in Figure.2. However, this blocking layer limits the separation of photogenerated electron-hole pairs in the LED-based PD, reducing the power conversion efficiency of the PD.

Figure 2. The band diagram of AlGaN/GaN/InGaN MQWs structure.[13] The electron- hole pairs are confined in the MQWs layer that improve the emission efficiency.

1.4 LED-based photodetector

PN-heterojunction can be used as either LED or PD. Therefore, by using the same structure both LED and PD can be used in the same device, thus forming what it is called

LED-based PD[14, 15]. In LEDs-based PD, the driving force for lighting is forward current bias, while for photodetection is reverse voltage bias. LED-based PDs were investigated as high-speed and wavelength selective PD by Miyazaki and coworkers in 1998.[16] They observed that the full width at half maximum (FWHM) pulse width of two commercial blue (450-nm) and red (660-nm) AlGaAs-based LEDs after a 2 ns light excitation impulse reached 3.3 and 7.7 ns, respectively, as shown in Figure. 3. Due to the lack of research in

InGaN blue LED-based PD, one of the goals of this dissertation is to investigate the photoresponse time characteristics of this LEDs. Additionally, we will also investigate another critical PD parameters such as responsivity, response time, and frequency spectrum. 14

Figure 3. Output wave forms of the LED-based PD were observed by oscilloscope for the impulse incidence.[16] (a) Blue-LED and (c) Red-LED without bias. (b) Blue-LED and (d) Red-LED with the reverse bias of 75 V.

1.4.1 Responsivity & External Quantum Efficiency

Responsivity is the ratio of photocurrent to the incident light power at a given wavelength. External quantum efficiency (EQE) is the number of carriers produced by the

PD to the number of photons of incident light. EQE is calculated by responsivity:

퐼푝ℎ표푡표 퐴 푞 휆 휇푚 R = ( ) = 휂 ≈ 휂 ( ) 푃𝑖푙푙푢푚𝑖푛푎푡푒푑 푊 ℎ푓 1.23985 휇푚 × 푊/퐴

where R is responsivity, 퐼푝ℎ표푡표 is photocurrent, 푃𝑖푙푙푢푚𝑖푛푎푡푒푑 is power illuminated on device, 휂 is EQE, 푞 is electron charge, h is Planck’s constant, f is frequency of light, and 휆 is wavelength of light. For example, the photocurrent can be calculated from the current- 15 voltage (I-V) characteristics at specific voltage bias as shown in Fig. 4(a). Meanwhile, Fig.

4(b) shows the wavelength-dependent as-calculated EQE and detectivity.

Figure 4. (a) The current-voltage characteristics of perovskite solution-processed hybrid perovskite PDs under dark and 550-nm illumination. Three blocking layer were used to decrease the dark current. (b) The EQE and detectivity of perovskite-based PD. Figures adapted from reference [17].

1.4.2 Response time

The photodiode speed is determined by the photovoltage response time which is determined at the same time by the drift velocity of the photogenerated electron-hole pairs in the depletion layer. In particular, the rise time of the photoresponse is dominated by the diffusion time and capacitance. Therefore, faster rise times are obtained with smaller diffused area photodiodes and higher applied reverse bias. In order to understand the the relation between the optical input and the electrical response of PD, Figure. 5 depicts the evolution of the response to a rectangular input pulse. Fig.5 (a) is the optical signal modulated in OOK. Fig. 5(b) and 5(c) represent the effects of small and large 16 capacitance, showing the fast and slow PD photoresponse, respectively. For a large capacitance PD, the device spends more time charging until saturation is reached.

Meanwhile for small capacitance PD, the carriers diffusion time could become longer, leading to a tail at the end of photoresponse as shown in Fig.5 (d).

Figure 5. The relation between the optical signal and electrical response of PD. (a) Optical input modulated OOK modulation. (b) High speed PD performance which is not confined by the diffusion carriers. (c) PD has large capacitance. (d) PD has large diffusion component.

1.4.3 Frequency spectrum

The frequency spectrum is based on the measurement of the amplitude of photoresponse at different frequency. The cut-off frequency is the operating frequency of the PD at which amplitude of the photoresponse is reduced -3dB, and it is evaluated by the rise time.

1.5 Orthogonal Frequency Division Multiplexing

Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme in which the single-carrier bandwidth is split into several subcarriers.[18-23] As the repetition rate of each subcarrier increases, the symbol duration enlarges and the interference caused by multi-path decay decreases. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions. Each subcarrier equalization is orthogonal that will not interfered and simplified because OFDM is viewed as using many simple narrowband signals rather than one complicated wideband signal.

Compared with frequency division multiplexing (FDM), OFDM has higher bandwidth efficiency. Subcarriers in OFDM can be superposed with other channel. Even some of signal is lost during transmission, the lost signal is fixed by the forward error correction

(FEC) codification method. Figure.6 (a) and 6(b) show an example of a signal sent to receiver and how the PD frequency response will be. Fig.6(c) shows more in detail how a small set of subcarriers are lost in fades and that this lost can be fixed by FEC.

FEC is a technique used for controlling errors in data transmission and retrieving the error. The amount of bit error rate (BER) and signal-to-noise ratio (SNR) limit the retrieve level of FEC. BER is the number of bit errors per unit time. SNR means how much noise is in the output of a device. The certification of BER should be lower than 3.8×10-3 and SNR should be higher than 8.5 dB that the FEC coding can fix the error in transmittance. 18

Figure 6. The spectrum efficiency is improved by OFDM modulation.[21] (a) The idea PD frequency spectrum is flat that it can receive full spectrum of signal. (b) The real frequency spectrum in device. There are two deep fade frequency in the whole spectrum. (c) OFDM divides the signal to many channels that the lost channels in the fades will not affect the result.

Quadrature amplitude modulation (QAM) consists of both an analog and a digital modulation scheme. QAM is an important modulation scheme because of its widespread adoption in current technologies. 4-Quadrature amplitude modulation (4-QAM) transmits four unique combinations of phase and amplitude that each symbol has a unique two bits digital pattern. Figure. 7 shows the bit-to-symbol mapping for OFDM. In this experiment,

4-QAM is used in the OFDM modulation and the corresponding data rate is 2 bit/s.

Figure 7. The different bit-to-symbol mapping for OFDM. Binary phase-shift keying (BPSK or 2-PSK) uses two opposite signal phases (0 and 180 degrees).[24] QPSK uses four combinations of phases and amplitude pattern. 19

2. Chapter Two: Measurement Setup

2.1 Current–voltage characterization

The current-voltage characteristics is measured by a Keithley 4200-SCS parameter analyzer with Keithley 4210-PA and a Cascade CM300 probe station at room temperature.

2.2 Light source and responsivity calculation

Below is the system diagram for responsivity measurement:

The light source is a Newport Xenon Lamp 6692 connected to a Newport mono- chromator with grating model 74166. To confirm the spot size and power intensity distribution, the optical profiles were measured by beam profiling camera Ophir Spiricon silicon CCD cameras SP620U.

Figure 8. The profile shows the size of light spot and power intensity distribution which is uniform that we can calculate the power illuminated on devices. 20

The power at different wavelength is measured by a calibrated photodiode power sensors. Devices are excited by a uniformly distributed intensity spot (3080 x 1167 μm2) and the responsivity is calculated by below formula:

퐼푝ℎ표푡표 퐴 R = ( ) 푃𝑖푙푙푢푚𝑖푛푎푡푒푑 푊 where Iphoto is the photocurrent defined as current under illumination minus current under dark, and Pilluminated is the incident power on the PD defined as the total power of light spot multiplied by the device area to spot area ratio.

2.3 Light source and RC time constant measurement

Below is the system diagram for response time measurement:

The laser diode which wavelength is 450 nm is modulated by Thorlabs ITC4001 laser diode temperature controller. Square waves with different frequencies are applied in modulation. The PD is directly connected to the Agilent DSO5034 oscilloscope with an input impedance of 50 .

To confirm the spot size and power intensity distribution, the optical profiles is measured by beam profiling camera Ophir Spiricon silicon CCD cameras SP620U. 21

Figure 9. The profile shows the size of laser spot and power intensity distribution which is Gaussian beam that we can calculate the power illuminated on devices.

−푡/휏 RC time constant is calculated by the charging time function: 푉(푡) = 푉0(1 − 푒 ), where time constant τ called rise time 휏푟. The decay time is calculated by discharging time:

−푡/휏 푉(푡) = 푉0(푒 ), where time constant τ called decay time 휏푑. The cut-off frequency can be calculated as 1/휏푟.

2.4 Light source and RC time constant measurement

Below is the system diagram for frequency spectrum measurement:

The light source is a 404 nm pulse laser modulated in different frequency. The RF modulation signal is controlled by the Tektronix 70001A arbitrary waveform generator

(AWG). The receiver is connected to a spectrum analyzer (SA) to analysis the power amplitude received. The frequency response of bias tee is 250 MHz to 4 GHz. To avoid RC delay, the LED is used gold wire bonding bonded on a designed mount, as shown in figure.10. 22

Figure 10. The LED is used gold wire bonding bonded on a designed mount.

2.4 OFDM

The light source is 450 nm pulse laser which full width at half maximum (FWHM) of pulse is 75 ps. The RF modulation signal is based on a 4-QAM OFDM signal generated off-line by a homemade MATLAB program in the transmitter, and uploaded into a

Tektronix 70001A arbitrary waveform generator (AWG).

The LED is used gold wire bonding bond on the mount which is able to fix SMA connector on the LED. The frequency spectrum of SMA connectors start from DC to 16

GHz. The received analog waveform is captured by a Tektronix DSA 71604C digital sampling oscilloscopes (DSO) with a sampling rate of 100 GS/s, and decoded offline by a

MATLAB program in the receiver unit. 23

3. Chapter Three: Result & Discussion

In the experiment, the size of InGaN/GaN MQWs LED chip is 780 x 330 휇푚2. Fig.11

(a) shows the LED structure and Fig.11 (b) shows the optical view of LED.

Figure 11. (a)The structure of InGaN/GaN MQW LED. (b)The optical view of LED.

3.1 EQE

To detect the basic performance of LED, fig.12 (a) shows the current-voltage characteristics of LEDs. Fig.12 (b) shows the LED emission spectrum and the emission peak wavelength is 448 nm. Fig.12 (c) shows the EQE of LED-based PD. The EQE of LED at 405 nm is nearly 10 % and at 450 nm is lower than 0.05%. However, fig. 12 (a) shows the LED still has photocurrent when it is illuminated 450 nm light. The LEDs can detect the 450 nm light. In the future, this LED can be used in LED to LED communication.

Figure 12. The photonics-electronics performance of LED-based PD. (a) The current- voltage characteristics show the dark current and the photocurrent illuminated 360 nm 24 and 450 nm light. (b)The peak wavelength of LED emission spectrum is 448 nm. (c) External quantum efﬁciency of LED as PD at different wavelength.

3.2 Response Time

To check this LED photoresponse, the 405 nm laser diode is modulated by the OOK illumination switching.

Figure 13. The time response measurement is illuminated 405 nm laser diode which maximum power is 900 mW. (a) The laser diode is modulated 20 Hz OOK signal. (b) The enlarged view shows the fitting curve of rise time and decay time. (c) The frequency spectrum shows the cut-off frequency of whole system is 200 Hz.

Fig.13 (a) shows the photovoltage response at frequency of 20 Hz. Fig.13 (b) is the fitting curve of rise time and decay time of PD. The cut-off frequency can be evaluated by rise time which is 14 kHz. To detect the frequency spectrum, the laser diode is modulated in different frequency. Fig.13 (c) shows the half amplitude of power, that is cut-off frequency (-3dB), is located in 200 Hz. However, this measurement is used the probe station and banana connector which will caused a lot of RC time delay. To measure exact frequency response and avoid the RC delay in the measurement system, the LED is used gold wire bonding bond on a designed mount which fixed the SMA connector.

3.3 Frequency Spectrum

The frequency spectrum shows the photoresponse when LED is illuminated

405nm pulse laser at different frequency. Fig.14 (a) shows a flat of narrow bandwidth is located between 2 GHz to 2.3 GHz. Fig.14 (b) shows the temporal characteristics of LED.

The FWHM of laser pulse is 75 ps and the repetition rate is 2 GHz. The FWHM of photoresponse is 275 ps. The rise time is 225 ps and decay time is 150 ps which is very fast.

Figure 14. The temporal characteristics of LED-based PD. (a) The frequency spectrum sweep from 250 MHz to 4 GHz. (b) PD temporal characteristics response and the repetition rate is 2 GHz.

To get more details of frequency spectrum, the LED-based PD is connected to DSO which sampling rate is 100 GS/s and swept frequency from 250 MHz to 5 GHz in different voltage. Fig.15 shows the frequency spectrum of LED-based PD measured by the DSO, revealing a flat region from 2 GHz to 2.3 GHz which can be used to load the OFDM modulation. The bias voltage applied in device affects capacitance of PD which also affects 26 the frequency spectrum, so the LED-based PD is applied in different reverse bias to find the suitable reverse bias.

Figure 15. The frequency spectrum of LED-based PD is applied in different bias.

3.3 OFDM

Below will investigated suitable condition for the OFDM modulation.

3.4.1 Bias voltage dependence

Figure 16. The OFDM testing depends on different bias voltage. (a) The SNR value with six sub-carriers pass the FEC limit. (c) BER and SNR value pass the FEC limit. 27

To find the suitable reverse bias applied in LED-based PD, the frequency spectrum is shown in fig.15. Then, laser diode is modulated the 4-QAM with 6 subcarriers to detect the SNR. Fig. 16(a) shows the LED-based PD can pass the FEC limit that it can detect the signal modulated in OFDM, only the last sub-carrier at 2.46 GHz cannot pass the FEC limit.

Fig. 16(b) shows the BER and SNR of all different reverse bias pass the FEC certification.

Table .1 shows the BER has smallest value (0.0025) and the SNR has highest value (8.9704 dB) at -2V that the best reverse bias applied in LED-based PD.

Bias Voltage (V) BER SNR (dB)

-1 0.0028 8.8532

-2 0.0025 8.9704

-3 0.0027 8.9012

Table 1. The BER and SNR value is measured in different bias voltage. All the reverse bias pass the FEC certification.

3.4.2 Start frequency

The LED-based PD is applied in a reverse bias at -2 V and investigated the start frequency. The laser diode is modulated in 4-QAM and 6 sub-carriers mode. Due to the frequency division of each sub-carrier is 23.44 MHz, the corresponding bandwidth is 140

GHz. The start frequency test start from 2.15 GHz to 2.53 GHz, because there is a flat in the frequency spectrum measured before. Fig. 17 shows the BER and SNR value of start frequency only at 2.34 GHz pass the FEC limit. Table.2 shows BER is 0.0018 and SNR is

9.2664 at start frequency is 2.34 GHz. 28

Figure 17. The BER and SNR plot shows start frequency only 2.34 GHz pass the FEC limit.

Start Frequency (GHz) BER SNR (dB)

2.53 0.0052 8.1641

2.43 0.0038 8.3179

2.34 0.0025 8.9704

2.25 0.0018 9.2664

2.15 0.0053 8.154

Table 2. The value of BER and SNR with different start frequency.

3.4.3 Bandwidth

To investigate the suitable bandwidth of LED-based PD, the start frequency is set as 2.34 GHz and the applied -2 V bias voltage. The frequency division of each sub-carrier is 23.44 MHz, so the bandwidth increase with the different numbers of sub-carrier. The laser diode is modulated with 140, 210, and 280 MHz corresponding to 6, 9, and 12 sub- 29 carriers respectively. Fig.18 (a) shows the SNR testing depend on different bandwidth.

Obliviously, there is a large fade at 2.35 GHz only the last sub-carrier of 140 MHz neat the

FEC limit. Fig.18 (b) shows the BER and SNR test of different bandwidth. Only 210 MHz pass the BER and SNR test. Furthermore, the subcarrier in full bandwidth should be considered. The bandwidth is 210 MHz which consist of the most sub-carriers pass the

FEC limit and wide bandwidth.

Figure 18. The start frequency of bandwidth testing is 2.34 GHz. (a) Bandwidth test with different subcarriers numbers. (b) The BER and SNR which OFDM bandwidth is 420 Mbit/s are pass the FEC limit.

Due to using 4-QAM modulation, the bits data rate of 210 MHz is corresponding to 420 bit/s. Table.3 shows the different bandwidth corresponding to the sub-carriers number and the data rate respectively.

Bandwidth (MHz) Subcarrier Number Data Rate (Mbit/s)

140 6 280

210 9 420

280 12 560

Table 3. The bandwidth correspond to different sub-carrier and data rate. 30

3.4.4 Summary

Depend on the bias test, the suitable reverse bias applied in LED-based PD is -2 V.

The OFDM modulation conditions is modulated in 9 sub-carriers corresponding to bandwidth 210 MHz with start frequency 2.25 GHz. The OFDM modulated in with 4-QAM corresponds to 420 Mbit/s.

4. Chapter Four: Conclusion

In conclusion, this dissertation investigates the photodetection characteristics of a

InGaN LED-based PD. By operating the LED in reverse bias of -2 V, we can used it as a wavelength selective PD in the blue region (405-nm). Moreover, the LED-based PD resulted in rise time of 71 µs, indicating high speed temporal characteristics. In addition, the output waveform responds immediately to picosecond-impulse and the frequency response revealed high performance at 2.25 GHz region. Furthermore, by implementing a 4-QAM OFDM codification scheme, the bandwidth efficiency of the LED-based PD can be significantly improved. Even there are some fades in the bandwidth, the signals can be corrected by FEC, leading to an enhanced operational bandwidth of 210 MHz and a transmission data rate of 420 Mbit/s in VLC. Since LEDs are currently installed in multiple applications such as mobile phone networks, automatic robotics, wearable devices, indoor lights, and traffic signals, by using LED-based PDs for VLC, the integration cost can be reduced notoriously. Therefore, LED-based PD have a great potential for the next generation of VLC application.

BIBLIOGRAPHY

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