Visible Light Communication System Using Silicon Photocell for Energy Gathering and Data Receiving

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


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 new kind of wireless communication technology with less energy consumption, higher response speed, and more privacy.
Energy gathering and signal detecting system is a new idea.Energy harvesters are widely used in sensor networks.But energy gathering can be hardly seen in the VLC.We noticed that the silicon-based solar panels could receive VLC data and gather energy at the same time.
Research works in this area can be found in [2]; the researchers from Korea used a solar cell as a simultaneous receiver of solar power and visible light communication (VLC) signals.Some research on the efficiency and frequency response of solar cell had been launched.
In our works, solar cell was studied totally under visible light.We set up models similar to the real lighting conditions and run simulations in Matlab/Simulink.Simulation results indicate that it is possible to gather energy and receive data through the same solar panels.We implement the system using commercial components.Our experiments based on the prototype show that the solar panels can gather energy for low power circuit and detect the VLC signal at the same time.

Model Analysis
In this section, we analyzed the model of LED and solar cell and then formulated their relationship with some approximations.

Model of LED Light Source.
The LED conforms to Lambert emission rule.When the transmitted optical power is   , the received power   (w/m 2 ) is expressed as [3] where  is the distance between LED and PD,  is the irradiance angel,  is the incidence angel of PD, () is the optical filter gain, () is the optical concentrator gain,   is the field of view of PD, and  is Lambert emission order.The SNR for VLC and the illuminance value on PD are given as follows: (2)  It can be formulated as where  1 is the number of solar cells in parallel,  2 is the series number,   is the light current,  0 is the diode saturation current,  is the output voltage of solar cell,  is the output current, and  is a constant which is typically in the rang 1 to 3. As  th ≫   , if set then (3) can be written as For solar cell, the light current is positively proportional with received illuminance power: is the illuminance power of solar cell.The standard sun light illuminance power at normal room temperature is 1000 w/mm 2 . SC is the short circuit current.We can set  = 0, so the solar cell works in the open state; then (5) can be expressed as (7) 2.3.Model of the System.For our system, solar cell is used as the PD.The two models can be connected by making   = .
In this way, (3) can be expressed as follows: Combine ( 5), (6), and (7) together: ℎ is the load resistance of solar cell.In conclusion,  1 ,  2 , and  3 are constants related to  1 ,  2 ,  SC , and  OC , so the relationship between  of solar cell and the LED power can be formulated as (11): (11)

Results and Discussions
We set up the two models in Matlab/Simulink and combined them for simulation.Solar cell model was simulated separately first.The model is based on the equations of ( 5), (6), and (7).Assuming that it works in the stable room temperature at 298 K, we chose solar cell AM-5308 for our experimental study.Parameters are set in Table 1.
The LED illumination model and Si photocell array model were combined to simulate the practical system.Figure 2 shows that  OC for 4 × 4 and  SC for 2 × 8 are half of values for 4 × 8 arrays individually. OC of 2 × 8 and 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 curves through the simulations under different illumination from 300 Lx to 1000 Lx.These numbers represent the daily scene illumination value, including living room, library, hospital  illumination value is above 500 Lx.It is stable for supplying power.The simulation value and experiment matched perfectly in Figure 6.
Then, a square signal as in Figure 7 is modulated on LED as the transmitting data.The period of the square signal is 0.1 s.The duty cycle is 50%.
The output power of the solar cell depends on the load resistance.The maximum output power about 1.2 × 10 −3 W can be achieved, when load resistance is 4 kΩ, under illumination at 300 lx.The output power of solar cell with different load resistance is shown in Figure 8; the -axis unit is 10 KΩ.
The output voltage of solar cell rises to 2.5 V after several pulses.The waveform of output voltage of solar cell under continuous pulse modulation is shown in Figure 9; the -axis unit is 1 S.
An energy gathering and signal detecting system was demonstrated as Figure 10.To fit the working condition of solar cell, we used a 15 W LED which could simulate the different indoor lighting conditions.The distance between the 2 × 8 photocell array and the 15 w LED is 1.8 m.The illumination value on photocell was 690 lx, when the LED was not modulated.The illumination value on photocell was 637.5 lx, when the LED was modulated.The baud rate of computer's output was 19200.The output data was the repetition of "A5" in HEX form and the polarity was reversed by RS485 converter chip.The yellow line in Figure 11 represents a DC coupled output signal of the silicon photocell which is about 2.77 V.The green line in Figure 11 represents the AC coupled output signal of the silicon photocell, filtered by a 0.1 F coupling capacitor.And the AC signal is around 410 mV.The baud rate and AC amplitude could be higher after one stage amplifier circuit [5].

Conclusion
In our works, we set up a model of solar cell VLC system which was simulated in Matlab/Simulink.We had verified the correction of the model and gave reasonable design to optimize the system.
The energy gathering and signals detecting system was demonstrated.The data rate of it is 19200 bps.The DC voltage of photocell was about 2.77 V which is enough for low voltage power supply circuits.The AC voltage of photocell was about 410 mV and could be optimized by one stage amplifier circuit.It was proved that solar cell can act as energy converting and detecting device simultaneously in VLC system.
The channel influences [6], response of solar cell to frequency, room lighting conditions, and other factors were ignored in our model.Further studies can take these factors into consideration.At the same time, we will optimize the design for the actual application.

Figure 1 : 2 . 2 .
Figure 1: The equivalent circuit diagram of a typical solar cell.

Figure 10 :
Figure 10: Energy gathering and signal detecting demo system.

Table 1 :
Parameters for solar cell.
I (A)