# Simultaneous Lightwave and Power Transfer for Internet of Things Devices
## 3 System Design and Experimental Methodology
### Information Decoding Capability  
To test the information decoding capability of the designed system, it was placed at various distances from the transmitter in a line of sight (LoS) configuration, as depicted in  
Figure 4: Illustration of the system design. IR: infrared; PSOC: programmable system on chip; DAC: digital-to-analog converter; TIA: transimpedance amplifier; OA: operational amplifier.  
Figure 5: A 470 mF supercapacitor charging using sunlight and various light sources placed 30 cm from the solar panel.  
Figure 6. The transmitted signals were modulated using a quadrature amplitude modulation (QAM) OFDM modulation scheme with adaptive bit and power loading. The signals were sent using an arbitrary waveform generator (AWG, Keysight 33600A) with a sampling frequency, \(f_{\text{AWC}}\), set to 1 Msample/s. The bias tee combines the modulation signal with the DC bias from a power supply (Agilent E3649A) to drive the 430-nm laser diode. The receiving circuit was connected to an oscilloscope (Tektronix MSO 4104) using a low power probe to collect the received signal at a sampling rate of 2.5 Msample/s and then process it offline using MATLAB(r).  
A test OFDM signal with a 16-QAM modulation was initially generated following the block diagram in Figure 6. A pseudo-random binary sequence (PRBS) was generated and modulated using QAM. A serial to parallel conversion was then performed. Hermitian symmetry was applied before the inverse fast Fourier Transform (IFFT) to output real values. The size of the IFFT is N\({}_{FFT}\) and was set to 1024. After that, a cyclic prefix whose length was set to 10 was added to limit intersymbol interference and simplify the equalization process on the receiver side. The parallel sequence was then converted into a serial sequence which was then sent by the AWG to the laser diode through the bias tee.  
After the signal was received, it was resampled and synchronized. The serial sequence was then converted to parallel, and the cyclic prefix was removed. FFT was performed, and the symbols were extracted by removing the Hermitian symmetry symbols. Single-tap channel equalization was used after channel estimation was performed based on the training symbols. The symbols were then demodulated and converted to a serial stream of bits, which was then compared to the transmitted signal to determine the bit error rate (BER). All reported data rates were achieved with a BER below the 7%-overhead forward error correction (FEC) BER limit (\(3.8\times 10^{-3}\)). The number of allocated bits per subcarrier was obtained from the channel capacity, as can be seen from Figure 7a, and the received QAM constellations are illustrated in Figure 7b. The power loading factor and the estimated SNR for each subcarrier are shown in Figure 7c. The BER for each sub-carrier is shown in Figure 7d.  
Figure 6: Data transfer experimental setup with the OFDM block diagram. The distance between the 430-nm laser source was set to 2, 10, and 20 m.  
The system performances at different distances are given in Table 2. The gross data rate, \(GDR\), is calculated as:  
\[GDR=\frac{f_{\text{AWG}}}{N_{\text{FFT}}+\text{CP}}\sum_{i=1}^{N_{\text{SC}}} \log_{2}(M_{i}), \tag{4}\]  
where CP is the cyclic prefix length and \(N_{\text{SC}}=485\) is the number of the subcarriers, \(M_{i}=2^{b}\) is the QAM order of the \(i\)th subcarrier, and \(b\) is the number of bits allocated to that subcarrier. The net data rate is calculated by accounting for the training symbols used for channel estimation and synchronization and the 7% FEC overhead. The highest data rate of 1.3 Mbps is obtained at a distance of 2 m from the transmitter. A net data rate of 0.79 Mbps is reached at 20 m.