# Simultaneous Lightwave and Power Transfer for Internet of Things Devices
## 2 Background  
SLIPT-based systems can be installed in locations where changing batteries or providing continuous powering is expensive or not possible. An illustration of a generic network of self-powered devices is shown in Figure 1. In a SLIPT-based system, solar panels can operate at any given moment in one of two modes: photovoltaic or photoconductive mode. The photovoltaic mode is the energy harvesting mode, while the photoconductive corresponds to the information signal decoding mode. There are three possible SLIPT configurations: time switching, power splitting, and space switching. In Table 1, we offer a comparison between the different SLIPT methods. The methods are compared in terms of hardware complexity, whether EH and ID are performed simultaneously on time, whether it is possible to perform maximum power point tracking (MPPT) [16] or not, and cost. We recall that time switching consists of using the same receiver to decode data signals and harvest energy over two different time slots. The power splitting approach aims to split an incoming beam using a power splitter into two quantities, one for ID and a second for EH. The complexity of this approach lies mainly in the design of the power splitting component. One device can harvest energy from multiple light sources and receive information signals from at least one transmitter within the space splitting approach. It is not possible to use an MPPT only for the case of power splitting. \begin{table}
\begin{tabular}{c l l l} \hline \hline
**Approach** & **Time Switching** & **Power Splitting** & **Space Splitting** \\ \hline Hardware Complexity & Low & High & Moderate \\ \hline Real-Time EH and ID & No & Yes & Yes \\ \hline MPPT & Yes & No & Yes \\ \hline Cost & Low & Moderate & High \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between different SLIPT configurations. The solar panel circuit model for energy harvesting is illustrated in Figure 2a. In the DC model, the photocurrent generation phenomenon can be substituted by a current source \(I_{PH}\). The non-linear relationship between the voltage and the current of the SC can be modeled by a diode, \(D\), that is connected in parallel to the photocurrent source. The leakage current can be expressed as an internal resistance in parallel, \(R_{SH}\), and the internal voltage drop by the cell interconnections can be represented as a series resistor \(R_{S}\)[17]. The load, which can be a supercapacitor or a battery, is represented as a pure resistive load \(R_{L}\). The output current \(I_{S}\) is the following:  
\[I_{S}=I_{PH}-I_{D}-I_{SH}, \tag{1}\]  
where \(I_{D}\) and \(I_{SH}\) are the currents through the parallel diode and the shunt resistance, respectively [5]. \(I_{D}\) can be expressed as follows [5]:  
\[I_{D}=I_{0}(e^{\frac{V_{GH}}{mV_{T}}}-1), \tag{2}\]  
Figure 1: Illustration of a generic network architecture of self-powered devices. Figure 2: Solar panel circuit model for (**a**) energy harvesting and (**b**) information decoding. where \(n\) is the diode ideality factor and \(V_{T}\) is the thermal voltage. In high frequency, the parasitic capacitances of the solar cell start to affect its output; due to this, a capacitor \(C\) must be inserted in parallel with the resistor \(R_{SH}\). The diode in AC can be substituted by its small-signal corresponding resistor \(r\) once it is considered that the AC component of the light signal has a minor variation compared to the magnitude of the DC component. Figure 2b shows the AC signal representation of an SC in which \(i_{PH(w)}\) is the photocurrent of the AC signal. Here, we mainly focus on time switching SLIPT. Our switching circuit has two modes when EH mode is active; the circuit can be modeled as a small resistance \(R_{SW}\), in series with an inductor \(L_{SW}\), that represent the wire connection from the switch to the load \(Z_{1}\). When ID is active, the switching circuit is disconnected from the load.