## 3 Results and Discussion  
In the experiment, a commercially available red LD is employed. It is based on AlGaInP with a multi-quantum well (MQW) structure. It has a maximum optical output power of about 130 mW. Figure 3 shows the measured characteristics of the red LD. The threshold currents is about 60 mA. The inset in Figure 3 shows the measured optical spectrum and the peak emission wavelength is 640 nm.  
\begin{table}
\begin{tabular}{c c} \hline \hline  & **System Parameter** \\ \hline FFT Size & 512 \\ Subcarrier & 125 \\ Cyclic Prefix & 32 \\ LD Drive Current & 70 mA \\ AWG Bandwidth & 18 GHz \\ PD Bandwidth & 1.2 GHz \\ RTO Bandwidth & 16 GHz \\ Transmission Distance & 1.5 m \\ \hline \hline \end{tabular}
\end{table}
Table 1: System Parameters.  
Figure 2: Experiment of the proposed PowDM VLC system. AWG: arbitrary waveform generator; RTO: real-time oscilloscope. Inset: a photo of the experimental setup.  
As the two individual data signals are superimposed in the power domain and the constellation diagrams are shaped according to the power allocation, it is important to obtain the optimum power ratio to maximize the system throughput and to minimize the bit error ratio (BER) of both data. To simple the DSP of Tx and Rx, the power ratio is applied to all the subcarriers. Figure 4 shows measured BER against power ratio of Data 2 to Data 1 of the PowDM VLC system. The power ratio has a significant effect on Data 1 since it will highly affect the shape of the constellation diagram of the main signal, introducing high error during the implementation of the SIC. We can also observe that the power ratio only has a small effect on the Data 2. This is because the demodulation of the 16-quadrature amplitude modulation (QAM) like signal is divided into four quadrants, and the constellation points in each quadrant will be mapped to the same Data 2 logic bit. The constellation diagrams of Data 1 and Data 2 are also illustrated in Figure 4. In the experiment, when the power ratios are larger than 5 dB, the BER performances of Data 1 and Data 2 are 2.41 \(\times\) 10\({}^{-3}\) and 3.94 \(\times\) 10\({}^{-3}\). Both data can satisfy the 7% pre-forward-error-correction (pre-FEC) BER of 3.8 \(\times\) 10\({}^{-3}\).  
Figure 5 shows the measured BER curves of the two individual signals and the PowDM signal at different data rate. By considering the FEC limit, Data 1 can achieve a data rate of 1.66 Gbit/s at BER of 2.20 \(\times\) 10\({}^{-3}\) and Data 2 can achieve a data rate of 6.41 at BER of 1.21 \(\times\) 10\({}^{-3}\). Hence, the maximum data rate achieved by the PowDM signal fulfilling the pre-FEC BER limit is 8.07 Gbit/s at the BER of 1.7 \(\times\) 10\({}^{-3}\).  
Finally, the SNRs of the Data 1 and Data 2 in the proposed PowDM signal are also evaluated. Figure 6 shows the measured SNRs of the two individual signals against all the OFDM subcarriers in the proposed PowDM VLC system. The average SNR for Data 1 is 10.96 dB and the average SNR for Data 2 is 14.45 dB. The corresponding achieved data rates for Data 1 and Data 2 are 1.66 Gbit/s and 6.41 Gbit/s, respectively.  
Figure 3: Measured characteristics of the red LD used in the experiment. Inset: measured optical spectrum.  
Figure 4: Measured BER against power ratio of Data 2 to Data 1 of the PowDM VLC system. Insets: constellation diagrams of Data 1 and Data 2.  
Figure 5: Measured BER curves of the two individual signals and the PowDM signal.