## 3 Communication System Design
### Half-Duplex Communication Protocol  
In practice, the variations of input voltage, coupling inductance and load resistance are inevitable, leading to changes in primary and secondary current. Moreover, the data carrier and power carrier of forward and backward communication are shared, and there is an interaction between power and  
Figure 11: Bode plot of the designed demodulator.  
Figure 10: Circuit of demodulator.  
information. Therefore, it is necessary to design a half-duplex protocol to enhance communication performance. In the protocol, the length of data frame should be designed as small as possible to achieve high data rate [35]. However, if it is too small, error bits may occur during communication. According to the priority of information exchange, there are two types of protocols for practical application, i.e., command protocol and data protocol. The definition of these two protocols are listed in Tables 2 and 3, respectively. Both of them start with bit \"1\" and end with bit \"1\". To ensure the validity of the transmitted bits, a parity bit is inserted before the stop bit to detect the erroneous bit. Then, four bits representing address matching are added to improve the interoperability for different WPT systems. The total length of command protocol is only eleven, which will facilitate emergency information transmission, such as handshake between devices, device failure, protection information, etc. For the data protocol listed in Table 3, four bits are required to define the type of data, such as voltage, current, power, etc. In order to increase the data rate, the data representing the values of different type information has only ten bits, which is large enough for typical high-resolution voltage and current sensor.  
Figure 12a shows the detailed flowchart of half-duplex communication, where \"Tx\" and \"Rx\" represent the sending and receiving data modes, respectively. These two modes are determined by the initial current in primary or secondary side. If current fluctuation is detected, \"Rx\" mode is selected. The transmitted data is unpacked with protocol above then the beginning state occurs until the received bits are completely processed. If there is a command or data protocol to be sent and the initial current is stable, the \"Tx\" mode is selected. After all bits are packed with protocol and completely sent, the beginning state occurs and waits for the next communication process. The working sequence is shown in Figure 12b, where the \"x\" and \"Rx\" modes work in turn on both side, and these two modes can be regarded a cycling communication process.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Start Bit** & **Address** & **Command** & **Parity Bit** & **Stop Bit** \\ \hline
1 bit & 4 bit & 4 bit & 1 bit & 1 bit \\ \hline \hline \end{tabular}
\end{table}
Table 2: Definition of command protocol.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Start Bit** & **Address** & **Data Type** & **Data** & **Parity Bit** & **Stop Bit** \\ \hline
1 bit & 4 bit & 4 bit & 10 bit & 1 bit \\ \hline \hline \end{tabular}
\end{table}
Table 3: Definition of data protocol.