## 3 Development of RF WPT-Enabled IoT Application
### Feasibility Analysis and Design Dimensions Definition  
To analyze whether the target application is feasible, we start with a brief analysis of the application's energy utility. The state-of-the-art accelerometers (e.g., LIS2 DE12 TR from STMicroelectronics (Geneva, Switzerland) or BMA253 from Bosch Sensortec (Kusterdingen, Germany) consume about 30-300 \(\upmu\)W in average when active, depending on the sampling rate. The temperature sensors (e.g., SI7053-A20-IM from Silicon Labs (Austin, TX, USA)--below 10 \(\upmu\)W in average for one sample per second (sps) measurement. The consumption of a system-on-chip composing a BLE transceiver and a microcontroller (e.g., CC2650 from Texas Instruments (Dallas, TX, USA)) is on the order of 60 mW while transmitting with the microcontroller active (with the on-air time of below 2 ms) and below 30 \(\upmu\)W in idle. Therefore, the expected mean power consumption is in the range of 200-500 \(\upmu\)W. Note that, to simplify the analysis of the applicability of energy harvesting and WPT for other applications and scenarios, we provide Table 2, capturing the consumption of the state-of-the-art commercial sensors. Next, we consider the amount of energy, which one can deliver to the node over WPT. For this, we start by analyzing the frequency regulations. Since the system will be deployed in Europe, we refer to the documents [60; 61]. Note that, to the best of our knowledge, none of the regulatory agencies to this day have issued specific regulations for RF-based WPT systems. For this reason, we consider the regulations for the non-specific short-range devices operating in ISM bands, which are summarized in Table 3.  
Next, we assume that both WPT transceiver and receiver antennas are omnidirectional (e.g., dipoles with 2.15 dBi gain) and that the efficiency of RF-DC conversion is 50% (refer to Table 1). In addition, we use the free-space propagation model to account for the losses in the channel. Consider that the RF energy is sent in 2.4 GHz frequency band with the maximum Effective Isotropic Radiated Power (EIRP) of 10 mW. At one meter distance, the propagation loss is in the order of 37 dB and the total harvested power is in the order of 100 \(\upmu\)W, which is below the level needed by our targeted application. Next, we consider the h1.6 band featuring 500 mW effective radiatcycle limitation. At a distance of about 2.8 m, the harvested power exceeds 0.5 mW, thus enabling supplying the target device. Finally, for band g3, we can estimate that the target level of 500 \(\upmu\)W is met at a distance of about 2.5 m. The calculated maximum levels of power transferable via WPT for the different ISM bands accounting for the duty cycle restrictions are summarized in Figure 2.  
As one can see from Figure 2, among all the ISM bands available, the h1.6 band enables transferring the maximum amount of energy. In addition, the presented results reveal that the amount of energy available exceeds the amount required for powering our target application. Therefore, we can conclude that the targeted application is feasible. Note that the previous analysis implies the use of omnidirectional antennas and the match of polarization between the WPT transfer and receive antennas. Meanwhile, for the targeted use case, this is also possible to employ directional antennas, thus reducing the power losses in the radio channel.