Visible Light Communication System Using Silicon Photocell ...

Research ArticleVisible Light Communication System Using Silicon Photocell forEnergy Gathering and Data Receiving

Xiongbin Chen,1,2 Chengyu Min,1 and Junqing Guo1

1State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China2School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing, China

Correspondence should be addressed to Xiongbin Chen; [email protected]

Received 24 September 2016; Revised 9 December 2016; Accepted 20 December 2016; Published 11 January 2017

Academic Editor: Liang Wu

Copyright © 2017 Xiongbin Chen et al.This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Silicon photocell acts as the detector and energy convertor in the VLC system. The system model was set up and simulated inMatlab/Simulink environment. A 10Hz square wave was modulated on LED and restored in voltage mode at the receiver. Anenergy gathering and signal detecting system was demonstrated at the baud rate of 19200, and the DC signal is about 2.77V andAC signal is around 410mV.

1. Introduction

Solar cell has drawn great interest over the past 30 years,and there is a tendency to use it more widely and practically.Visible light communication is also very amazing [1] as a newkind of wireless communication technology with less energyconsumption, higher response speed, and more privacy.

Energy gathering and signal detecting system is a newidea. Energy harvesters are widely used in sensor networks.But energy gathering can be hardly seen in the VLC. Wenoticed that the silicon-based solar panels could receive VLCdata and gather energy at the same time.

Research works in this area can be found in [2]; theresearchers from Korea used a solar cell as a simultaneousreceiver of solar power and visible light communication(VLC) signals. Some research on the efficiency and frequencyresponse of solar cell had been launched.

In our works, solar cell was studied totally under visiblelight. We set up models similar to the real lighting conditionsand run simulations in Matlab/Simulink. Simulation resultsindicate that it is possible to gather energy and receive datathrough the same solar panels.

We implement the systemusing commercial components.Our experiments based on the prototype show that the solarpanels can gather energy for low power circuit and detect theVLC signal at the same time.

2. Model Analysis

In this section, we analyzed the model of LED and solar celland then formulated their relationshipwith some approxima-tions.

2.1. Model of LED Light Source. The LED conforms toLambert emission rule. When the transmitted optical poweris 𝑃𝑡, the received power 𝑃𝑟 (w/m2) is expressed as [3]

𝑃𝑟= {{{

𝑃𝑡𝑚 + 12𝜋𝑑2 cos𝑚 (𝜙) 𝑇𝑠 (𝜓) ⋅ 𝑔 (𝜓) cos (𝜓) 0 ≤ 𝜓 ≤ 𝜓𝑐

0 𝜓 > 𝜓𝑐,(1)

where 𝑑 is the distance between LED and PD, 𝜙 is theirradiance angel, 𝜓 is the incidence angel of PD, 𝑇𝑠(𝜓) is theoptical filter gain, 𝑔(𝜓) is the optical concentrator gain, 𝜓𝑐 isthe field of view of PD, and𝑚 is Lambert emission order.

The SNR for VLC and the illuminance value on PD aregiven as follows:

SNR = 𝛾2𝑃𝑟signal2𝜎2shot + 𝜎thermal

2 + 𝛾2𝑝𝑟ISI2𝐸hor = 𝐼 (0) cos𝑚 (Φ) cos (𝜓)

𝑑2 .(2)

HindawiInternational Journal of OpticsVolume 2017, Article ID 6207123, 5 pageshttps://doi.org/10.1155/2017/6207123

https://doi.org/10.1155/2017/6207123

2 International Journal of Optics

Iph D

Id

C

IRs

V R

Rth

+

−

Figure 1: The equivalent circuit diagram of a typical solar cell.

2.2. Model of Solar Cell. The equivalent circuit diagram of atypical solar cell is as shown in Figure 1 [4].

It can be formulated as

𝐼 = 𝑁1𝐼𝑃 − 𝐼0 {exp[𝑞 (𝑉 + 𝐼𝑅𝑠)𝑁2𝐴𝐾𝑇 ] − 1} − 𝐶𝑑𝑉𝑑𝑡− 𝑉 + 𝐼𝑅𝑠𝑅th

,(3)

where 𝑁1 is the number of solar cells in parallel, 𝑁2 is theseries number, 𝐼𝑃 is the light current, 𝐼0 is the diode saturationcurrent, 𝑉 is the output voltage of solar cell, 𝐼 is the outputcurrent, and 𝐴 is a constant which is typically in the rang 1 to3. As 𝑅th ≫ 𝑅𝑠, if set

𝐾0 = 𝐴𝐾𝑇𝑞 (4)

then (3) can be written as

𝐼 = 𝑁1𝐼𝑃 − 𝐼0 {exp[(𝑉 + 𝐼𝑅𝑆)𝑁2𝐾0 ] − 1} − 𝐶𝑑𝑉𝑑𝑡 . (5)

For solar cell, the light current is positively proportional withreceived illuminance power:

𝐼𝑃 = 𝑆1000𝐼SC. (6)

𝑆 is the illuminance power of solar cell. The standard sunlight illuminance power at normal room temperature is1000w/mm2. 𝐼SC is the short circuit current.

We can set 𝐼 = 0, so the solar cell works in the open state;then (5) can be expressed as

𝑁1𝐼𝑃 = 𝐼0 [exp( 𝑈OC𝑘0𝑁2) − 1]

𝐼0 = 𝑁1𝐼𝑃exp (𝑈OC/𝑘0𝑁2) − 1 .

(7)

2.3. Model of the System. For our system, solar cell is used asthe PD. The two models can be connected by making 𝑃𝑟 = 𝑆.In this way, (3) can be expressed as follows:

𝐼 = 𝑁1 𝑃𝑟1000𝐼SC −𝑁1𝑃𝑟𝐼SC

exp (𝑈OC/𝑁2𝑘0) − 1⋅ [exp(𝑈 + 𝑈/𝑅ℎ ⋅ 𝑅𝑆𝑁2𝑘0 ) − 1] .

(8)

Combine (5), (6), and (7) together:

𝐼 = 𝑁1𝑃𝑟𝐼SC1000 {1 − 11000 [exp (𝑈OC/𝑁2𝑘0) − 1]

⋅ [exp(𝑈 + (𝑈/𝑅𝑛) 𝑅𝑆𝑁2𝑘0 ) − 1]} .(9)

𝑅ℎ is the load resistance of solar cell. In conclusion, 𝑘1, 𝑘2,and 𝑘3 are constants related to 𝑁1, 𝑁2, 𝐼SC, and 𝑈OC, so therelationship between 𝑈 of solar cell and the LED power canbe formulated as (11):

𝐼 = 𝑈𝑅ℎ (10)

𝑈 = {{{𝑃𝑡𝑚 + 12𝜋𝑑2 cos𝑚 (𝜙) 𝑇𝑠 (𝜓) ⋅ 𝑔 (𝜓) cos (𝜓)

1𝑘1 [1 −

1𝑘2𝑅ℎ exp(

𝑈𝑘3)] 0 ≤ 𝜓 ≤ 𝜓𝑐

0 𝜓 > 𝜓𝑐.(11)

3. Results and Discussions

We set up the two models in Matlab/Simulink and combinedthem for simulation.

Solar cell model was simulated separately first.Themodelis based on the equations of (5), (6), and (7). Assuming that itworks in the stable room temperature at 298K, we chose solarcell AM-5308 for our experimental study. Parameters are setin Table 1.

The LED illumination model and Si photocell arraymodel were combined to simulate the practical system.Figure 2 shows that 𝑈OC for 4 × 4 and 𝐼SC for 2 × 8 arehalf of values for 4 × 8 arrays individually. 𝑈OC of 2 × 8and 4 × 8 are 3∼3.5 V, which possibly charge lithium battery.In Figure 3, we got the 2 × 8 arrays solar cell’s I-V curvesthrough the simulations under different illumination from300 Lx to 1000 Lx. These numbers represent the daily sceneillumination value, including living room, library, hospital

International Journal of Optics 3

Table 1: Parameters for solar cell.

Parameter ValueAreas 3 × 36mm2

Open circuit voltage 𝑈OC = 0.3VShort circuit current 𝐼SC = 15 uASeries resister 𝑅𝑠 = 0.0052ΩStandard condition 𝐸V = 100 LxParallel number of solar cells 𝑁1 = 2series number of solar cells 𝑁2 = 8Load resistance 𝑅ℎ = 0∼5000ΩCapacity 20 nFTemperature 𝑇 = 298KElectronic charge 𝑞 = 1.62𝑒 − 19𝑐Boltzmann’s constant 𝐾 = 1.38066𝑒 − 23Gain of optical filter 𝑇𝑠(𝜓) = 1Field of view Ψ = 60∘Half angel of LED Φ = 60∘Lambert constant 𝑚 = 1Distance 𝑑 = 1.0mOptical concentrator gain 1Power of LED 15WIlluminance 68 lm/W

I(A

)

1.4 × 10−4

1.2 × 10−4

1.0 × 10−4

8.0 × 10−5

6.0 × 10−5

4.0 × 10−5

2.0 × 10−5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

U (V)

2 ∗ 8

4 ∗ 4

4 ∗ 8

Figure 2: I-V curves for different solar cell arrays (𝐸V = 300 Lx).

operating room, and sports venue. Power properties of differ-ent arrays under different illumination values are also simu-lated in Figures 4 and 5. The output power of 2 × 8 arraysunder 300 Lx and 50KΩ is 1.4 × 10−4W.The single receivingarea of Si photocell chip is 3 × 36mm2. For 2 × 8 arrays, thearea is 1728mm2. So the efficiency of the 2 × 8 array is 8.1%.The spectral response of Si photocell chip made influence onthe received light power as our LED light is mainly made upof blue and yellow.

The output voltage in simulation and experiment isamong 2.7 V to 3.5 V. It increased to saturation state when the

I(A

)

1.4 × 10−4

1.6 × 10−4

1.8 × 10−4

2.0 × 10−4

1.2 × 10−4

1.0 × 10−4

8.0 × 10−5

6.0 × 10−5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

U (V)

1000Lx750Lx

500Lx300Lx

Figure 3: I-V curves of 2 × 8 arrays under different illumination.

2 ∗ 8

4 ∗ 4

4 ∗ 8

0 100k 200k 300k 400k 500kR (Ω)

1.6 × 10−4

3.2 × 10−4

2.8 × 10−4

2.0 × 10−4

2.4 × 10−4

1.2 × 10−4

8.0 × 10−5

4.0 × 10−5

0.0

P(W

)

Figure 4: P-R curves for different solar cell arrays (𝐸V = 300 Lx).

illumination value is above 500 Lx. It is stable for supplyingpower. The simulation value and experiment matched per-fectly in Figure 6.

Then, a square signal as in Figure 7 is modulated on LEDas the transmitting data. The period of the square signal is0.1 s. The duty cycle is 50%.

The output power of the solar cell depends on the loadresistance. The maximum output power about 1.2 × 10−3Wcan be achieved, when load resistance is 4 kΩ, under illumi-nation at 300 lx. The output power of solar cell with differentload resistance is shown in Figure 8; the 𝑥-axis unit is 10 KΩ.

The output voltage of solar cell rises to 2.5 V after severalpulses. The waveform of output voltage of solar cell undercontinuous pulse modulation is shown in Figure 9; the 𝑥-axisunit is 1 S.

An energy gathering and signal detecting system wasdemonstrated as Figure 10. To fit the working condition ofsolar cell, we used a 15W LED which could simulate the

4 International Journal of Optics

0 50.0k 100.0k 150.0k 200.0k 250.0k 300.0kR (Ω)

P(W

)

750Lx500Lx300Lx

4.0 × 10−4

3.5 × 10−4

2.0 × 10−4

3.0 × 10−4

2.5 × 10−4

1.5 × 10−4

1.0 × 10−4

5.0 × 10−5

0.0

Figure 5: P-R curves under different illumination.

Practical voltageSimulated voltage

1000 20000 500 30002500 35001500

E� (Lx)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

U(V

)

Figure 6: Comparison of simulation and experiments result forvoltage and illumination.

different indoor lighting conditions. The distance betweenthe 2 × 8 photocell array and the 15 w LED is 1.8m. Theillumination value on photocell was 690 lx, when the LEDwas not modulated. The illumination value on photocell was637.5 lx, when the LEDwasmodulated.The baud rate of com-puter’s output was 19200. The output data was the repetitionof “A5” in HEX form and the polarity was reversed by RS485converter chip. The yellow line in Figure 11 represents a DCcoupled output signal of the silicon photocell which is about2.77V. The green line in Figure 11 represents the AC coupledoutput signal of the silicon photocell, filtered by a 0.1𝜇Fcoupling capacitor. And the AC signal is around 410mV. Thebaud rate and AC amplitude could be higher after one stageamplifier circuit [5].

1

0

−1

0 0.2 0.4 0.6 0.8 1

Initial signal

Figure 7: Initial signal.

0 0.5 1 1.5 2 2.5 3

1.2

1

0.8

0.6

0.4

0.2

0

×10−3

P (W)

Figure 8: Power of solar cell.

U (V)3

2.5

2

1.5

1

0.5

00 0.5 1 1.5 2 32.5

Figure 9: Voltage of solar cell.

4. Conclusion

In our works, we set up a model of solar cell VLC systemwhich was simulated in Matlab/Simulink. We had verifiedthe correction of the model and gave reasonable design tooptimize the system.

The energy gathering and signals detecting system wasdemonstrated.The data rate of it is 19200 bps.TheDC voltageof photocell was about 2.77Vwhich is enough for low voltagepower supply circuits. The AC voltage of photocell was about410mV and could be optimized by one stage amplifier circuit.It was proved that solar cell can act as energy converting anddetecting device simultaneously in VLC system.

The channel influences [6], response of solar cell tofrequency, room lighting conditions, and other factors wereignored in our model. Further studies can take these factorsinto consideration. At the same time, we will optimize thedesign for the actual application.

Competing Interests

The authors declare that they have no competing interests.

International Journal of Optics 5

Figure 10: Energy gathering and signal detecting demo system.

Figure 11: Output signals of silicon photocell.

Acknowledgments

This work is supported in part by the National Key BasicResearch Program of China (Grant no. 2013CB329204), inpart by the National High Technology Research and Devel-opment Program of China (Grant no. 2015AA033303), and inpart by Science and Technology Planning Project of Guang-dong Province, China (Grant no. 2014B010120004).

References

[1] A. Jovicic, J. Li, and T. Richardson, “Visible light communica-tion: opportunities, challenges and the path to market,” IEEECommunications Magazine, vol. 51, no. 12, pp. 26–32, 2013.

[2] S.-M.Kim, J.-S.Won, and S.-H.Nahm, “Simultaneous receptionof solar power and visible light communication using a solarcell,”Optical Engineering, vol. 53, no. 4, Article ID 046103, 2014.

[3] T.-H. Do andM. Yoo, “Optimization for link quality and powerconsumption of visible light communication system,” PhotonicNetwork Communications, vol. 27, no. 3, pp. 99–105, 2014.

[4] E. Koutroulis, K. Kalaitzakis, and N. C. Voulgaris, “Develop-ment of amicrocontroller-based, photovoltaicmaximumpowerpoint tracking control system,” IEEE Transactions on PowerElectronics, vol. 16, no. 1, pp. 46–54, 2001.

[5] J. Guo, X. Chen, H. Li, Y. Huang, and H. Chen, “The responseproperties analysis of silicon photovoltaic cell array in visiblelight communication system,”Optoelectronics Laser, vol. 26, no.3, pp. 475–479, 2015.

[6] K. Lee, H. Park, and J. R. Barry, “Indoor channel characteristicsfor visible light communications,” IEEE Communications Let-ters, vol. 15, no. 2, pp. 217–219, 2011.

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