## 6 Experiment  
### Experiment Set-Up  
In this section, we describe the experiment used to evaluate the communication performance of a VLP system using a QADA. The block diagram of the experimental set-up is shown in Figure 14a and the components used in the experiment are listed in Table 2. The data to be transmitted is modulated using ACO-OFDM, which is generated using a software-defined radio (SDR) module (NI-USRP 2950R). Next, the continuous electrical signal at the output of the SDR system is amplified using an electrical amplifier (Mini-circuits ZHL-32A-S). A DC current source is used to generate a 200 mA current, which is then added to the amplified signal using a DC-bias tee circuit (Mini-circuits ZFBT-4R2 GW). The DC current is chosen to be enough to make the LED (Luxeon LXML-PWC2) operate within its linear region. The signal is then used to directly modulate the intensity of the LED. A photo of the experimental set-up is shown in Figure 14b.  
Figure 14: Experimental set-up (**a**) block diagram shows the experimental set-up; (**b**) photo of the experiment; (**c**) photo of the QADA receiver.  
The QADA converts the optical signal into four electrical signals, each of which represents one of the four quadrants. The QADA is made of a square aperture of size \(25\,\mathrm{mm}^{2}\) placed in front of a square quadrant PD (Hamamatsu S5980) with a size of \(25\,\mathrm{mm}^{2}\) at distance \(d=2.5\,\mathrm{mm}\) from the aperture and at a distance of \(40\,\mathrm{cm}\) from the LED. See Figure 15. The QADA front-end is mounted on a custom-built PCB that contains four trans-impedance amplifiers (TIA). The QADA receiver is shown in Figure 14c. The amplified signals are then captured using a data acquisition system (GAGE CSE8389). Finally, the signals are further processed and combined to recover the transmitted data using MATLAB.  
The transmitted signal is generated using an ACO-OFDM signal with 16-QAM modulation. 16-QAM modulation is used because when 4-QAM was used the resultant BER was always zero, so it is not suitable to demonstrate the changes of the BER. An FFT length of 256 and a cyclic prefix (CP) length of 32 are considered. The number of subcarriers used is \(N/4\ -1\); the first subcarrier is not used because it experiences a relatively high attenuation. The total length of the transmitted signal is 2000 OFDM symbols, which represents 512 Kb of data. A baseband daughter board (NI LFTX) is used in the SDR module because the signal required to modulate the LED is a baseband signal. It operates from DC up to 30 MHz. The SDR module replaces the MATLAB signal generation and the arbitrary waveform generator (AWG), which are typically used in similar experiments [40]. Using the SDR for signal generation allows much longer data sequences to be transmitted, and allows most of the processes to be directly implemented and controlled by software. The module used has very high accuracy and thus the hardware induced impairments are small. The USRP 2950R is connected to a personal computer (PC) which runs National Instrument (NI) LabVIEW FPGA 2016. The PC is connected with the module through a PCIeX4 interface card which loads the compiled files from the LabVIEW environment onto the FPGA motherboard. The sampling rate used is 1 MS/s, resulting in an overall data rate of the transmitted signal is approximately 984.4 Kb/s.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **Model** \\ \hline Quadrant Photodiode & Hamamatsu S5980 \\ Amplifier & Mini-Circuits ZHL-32A-S \\ Bias-Tee & Mini-Circuits ZFBT-4R2 GW \\ Software-Defined Radio & National Instruments USRP 2950R \\ Data acquisition system & Gage CSE8389 \\ LED & Luxeon LXML-PWC2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Experimental Components.  
Figure 15: Experiment set-up diagram and the overlap between the light spot and the quadrant PD as the transmitter moves along the \(x\)-axis.