**3. Results**

Simulation results and fabrication of our system is presented in this section. Spectrum sensing, circuits and antenna are simulated using MATLAB, Advance Design System (ADS) and Computer Simulation Technology (CST) software respectively. We consider the frequency band 500 MHz to 1 GHz for maximum power RF signal extraction. At the end, our fabricated charger circuit is tested in laboratory environment.

#### **3.1 Spectrum sensing simulations**

We consider *M* random base stations in the aforementioned frequency band. Using OFDM transmitter, the output signal is generated and by exploiting energy detection, our desired RF signal for energy harvesting circuit is obtained. Parameters of simulation are given in **Table 1**. Note that for digital to analog converter, a13 th order Butterworth filter is used (normalized cut off frequency equals to \_\_\_1 <sup>20</sup>).

RC pulse shaping time response is as follows [35].

$$S\_{RC}(t) = \text{sinc}\left(\frac{t}{T\_s}\right) \times \frac{\cos\left(\frac{\pi at}{T\_s}\right)}{1 - \frac{4\alpha^2 t^2}{T\_s^2}}\tag{1}$$

**99**

**Figure 5.**

**Figure 4.**

*RF Energy Harvesting System and Circuits for Charging of Wireless Devices Using Spectrum…*

After finding the frequency in which our desired RF signal exists, spectrum should be fed to a filter with the central frequency of 915 MHz and the bandwidth of 10 MHz. In **Figure 5**, filter characteristics in terms of frequency response is given. Note that this filter has the ability to be tuned to select the maximum frequency each time. Also, this filter should reject the rest of frequency band otherwise we face some challenges such as power loss and circuit design complexity.

*Frequency response of OFDM transmitter in our frequency band with QPSK modulation in receiver end.*

*Frequency response of Butterworth filter with central frequency of \$915\$ MHz and bandwidth of 10 MHz.*

*DOI: http://dx.doi.org/10.5772/intechopen.84526*

where *α* and *Ts* are roll-off factor (takes values from 0 to 1) and symbol repetition rate respectively.

As it can be seen in **Figure 4**, there are four signals available in the spectrum and in order to find the RF signal with maximum power, the area under each signal should be calculated i.e. its power. After applying energy detection we find that in this case, maximum power RF signal happened at 915 MHz. Therefore, the filter is set to select this signal out of the spectrum.


**Table 1.** *Stimulation parameters.* *RF Energy Harvesting System and Circuits for Charging of Wireless Devices Using Spectrum… DOI: http://dx.doi.org/10.5772/intechopen.84526*

After finding the frequency in which our desired RF signal exists, spectrum should be fed to a filter with the central frequency of 915 MHz and the bandwidth of 10 MHz. In **Figure 5**, filter characteristics in terms of frequency response is given. Note that this filter has the ability to be tuned to select the maximum frequency each time. Also, this filter should reject the rest of frequency band otherwise we face some challenges such as power loss and circuit design complexity.

**Figure 4.** *Frequency response of OFDM transmitter in our frequency band with QPSK modulation in receiver end.*

**Figure 5.** *Frequency response of Butterworth filter with central frequency of \$915\$ MHz and bandwidth of 10 MHz.*

*A Guide to Small-Scale Energy Harvesting Techniques*

model.

**3. Results**

equals to \_\_\_1

<sup>20</sup>).

tion rate respectively.

laboratory environment.

**3.1 Spectrum sensing simulations**

For the goal of this chapter, an accurate and effective battery model is proposed based on battery model proposed in [34]. This model provides an easy extraction procedure, gives run time, static and transient responses and also contains all of the electrodynamic characteristics of the batteries. **Figure 3** shows this proposed

Simulation results and fabrication of our system is presented in this section. Spectrum sensing, circuits and antenna are simulated using MATLAB, Advance Design System (ADS) and Computer Simulation Technology (CST) software respectively. We consider the frequency band 500 MHz to 1 GHz for maximum power RF signal extraction. At the end, our fabricated charger circuit is tested in

We consider *M* random base stations in the aforementioned frequency band.

\_\_*t Ts*) <sup>×</sup>

where *α* and *Ts* are roll-off factor (takes values from 0 to 1) and symbol repeti-

As it can be seen in **Figure 4**, there are four signals available in the spectrum and in order to find the RF signal with maximum power, the area under each signal should be calculated i.e. its power. After applying energy detection we find that in this case, maximum power RF signal happened at 915 MHz. Therefore, the filter is

**Parameters Value (OFDM)** Number of sub-carriers 2048 Occupied of sub-carriers 1024 Sampling frequency (MHz) 20 Number of oversampling 4 Pulse shaper Raised cosine Number of random frequency 4 IFFT length 4096 Bandwidth (MHz) 10

cos( \_\_\_ *t Ts* ) \_\_\_\_\_\_\_\_ <sup>1</sup> <sup>−</sup> <sup>4</sup>*α*<sup>2</sup> *<sup>t</sup>* <sup>2</sup> \_\_\_\_ *Ts* 2

(1)

Using OFDM transmitter, the output signal is generated and by exploiting energy detection, our desired RF signal for energy harvesting circuit is obtained. Parameters of simulation are given in **Table 1**. Note that for digital to analog converter, a13 th order Butterworth filter is used (normalized cut off frequency

RC pulse shaping time response is as follows [35].

*SRC*(*t*) <sup>=</sup> *sinc*(

set to select this signal out of the spectrum.

**98**

**Table 1.**

*Stimulation parameters.*

**Figure 6** shows power for filter, spectrum and the output of filter (Antenna input signal). We use OFDM transmitter as mentioned earlier with QPSK modulation.
