3.2 Simulation

coefficient between the channels. Even if the multipath gain of the source node and the non-source node is the same or slightly higher, the ratio of peak power will still have a large gap when the channel correlation coefficient is very small. In the scattering-rich environment, as long as the distance is enough, the correlation of impulse response of multipath channel decreases to a very low level. The spatial focusing effect of TR is adaptive and can reduce electromagnetic radiation pollution.

Spatial focusing effect of time reversal electromagnetic wave: Electric field distribution Z = 0 plane of the source

Through section 2, we can get the idea that TR technique can increase power gain by the temporal focusing effect and reduce the electromagnetic radiation pollution by the spatial focusing effect. In WPT system, the important part is the rectifier antenna. According to [5, 10], the rectifier antenna consists of a diode and a low-pass filter to transform radio frequency (RF) input signal into direct current (DC). The collected DC energy is a function of input power level and RF-DC conversion efficiency. RF-DC conversion efficiency is not only a function of rectifier antenna design but also a function of input waveform, that is, input power and shape. Therefore, we consider to design the frequency point amplitude to improve

Through the above introduction, we consider the system model as Figure 6. Because the two stages of TR, the transmitter and receiver both have two antennas. In the transmitter, the channel estimation module can extract the channel state information (CSI) between two devices. Then the time reversal module can use the CSI to obtain the time reversal signal. According to the rectenna circuit model, a non-convex problem about the received signal and the output voltage can be extracted. Usually, a non-convex problem should be converted into a convex problem [11]. We propose that the sequential convex programming algorithm can solve the problem. More detail can be found in [12] due to the space limitations. Finally, the sequential convex programming module will get the optimal frequency point

energy efficiency based on the diode circuit with TR technique.

3. WPT system model and analysis

node (a) in NO-TR system (b) in TR-WPT system.

Recent Wireless Power Transfer Technologies

amplitude according to rectenna circuit.

3.1 System model

98

Figure 5.

Firstly, we consider a multipath channel response based on a metal cavity like in Figure 7, with the measurement in frequency domain by Vector Network Analyzer (VNA) [13].

In [14], it proposes that the larger the bandwidth, the more the number of multipath like in Figure 8. In the above analysis and Eq. (2), the performance of TR is related to the multipath. In addition, the factor of SCP is frequency point numbers. Therefore, we compare three system models, namely, direct transmission (DT), TR, and TR-SCP, in different bandwidth (20–200 MHz) and frequency point numbers (2–8).

#### 3.3 Result and analysis

In the simulation, we set the carrier frequency as 5.8GHz and the emitting power as 36 dBm. Then we combine the measured channel with Matlab module like in Figure 6 to perform the experiment. As in Figure 9a, the TR-SCP and TR systems perform much better than the DT system because the TR technique can take

Figure 7. The metal cavity.

and rich multipath environment and can adapt to a variety of wireless power

Long-Distance Wireless Power Transfer Based on Time Reversal Technique

Science Foundation under Grant (2018B030311049).

This work was supported by the National Natural Science Foundation of China (NSFC) under Grant (61871193) and by the key project of Guangdong Natural

transmission scenarios.

DOI: http://dx.doi.org/10.5772/intechopen.89078

Acknowledgements

Author details

101

Bin-Jie Hu\*, Zhi-Wu Lin and Peng Liao

provided the original work is properly cited.

\*Address all correspondence to: eebjiehu@scut.edu.cn

Technology, Guangzhou, China

School of Electronic and Information Engineering, South China University of

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

#### Figure 8.

Percentage of captured energy versus the number of significant eigenvalues with a single antenna [14].

Figure 9. Result of the simulation (a) power and bandwidth (b) power and frequency points.

advantage of the multipath channel. What's more, as the bandwidth becomes bigger, their performance is better. As in Figure 9b, because the TR-SCP can make use of CSI based on the diode circuit, its performance will be better than TR system. And when the number of frequency point is from 2 to 8, the system can get more information about CSI. Therefore, in Figure 9b, the TR-SCP system will be better and better than the TR system as the number increases.

#### 4. Conclusion

This chapter firstly introduces the principle of TR technique. Then according to the focusing effect of TR, we propose TR be applied to WPT. Because the conversion efficiency is associated with the waveform, we propose SCP algorithm based on diode circuit to design the amplitude of the emitting signal. Finally, we combine the SCP with the TR to propose a SCP-TR system model. Based on the results of the simulation, our design model can achieve better performance on wireless power transmission. Therefore, we believe that our model is robust and stable in a complex and rich multipath environment and can adapt to a variety of wireless power transmission scenarios.
