## 4 System Architecture and Design Procedures  
### RF Energy Harvesting System  
A typical RF energy harvesting (RF-EH) system, as shown in Figure 2, consists of crucial components for capturing, converting, and managing ambient RF energy. The system includes an antenna strategically placed to capture ambient RF energy, directing the received power to a rectifier circuit [7]. The rectifier plays a vital role by converting RF power into DC power, which is then stored or managed in a system using capacitors or batteries for future use [47]. Impedance matching connects the antenna and the rectifier and is vital to reduce signal loss and ensure efficient energy transfer, especially with low-input RF signals. A high-quality matching network and low-loss rectifier are critical for optimising power transfer and system performance [48]. This paper presents the design and simulation of an RF-EH system using Keysight Advanced Design System (ADS) 2023 update 1.0 software, developed by Keysight Technologies, headquartered in Santa Rosa, CA, USA. A variable RF power source (_P1\({}_{T}\)one_) with input power ranging from \(-25\) to 20 dBm was used as the antenna input to evaluate system performance across different power levels. The simulation was conducted at 2.4 GHz, targeting indoor IoT applications, commonly using WIFI-4 and WIFI-6. The rectifier generates an output voltage (\(V_{dc}\)) and current (\(I_{o}\)) at a fixed load, as shown in Figure 6, with \(V_{dc}\) exhibiting minimal ripple, stabilised by a 470 pF capacitor. A RF-EH system at low power levels, often as low as \(-25\) dBm (3.2 \(\upmu\)W), poses significant challenges due to the high threshold voltages of conventional diodes [47; 49]. For a typical 50 Ohm antenna, the resulting peak AC voltage may be as low as 5.7 mV, necessitating diodes with low forward voltage drop (\(V_{f}\)) and fast switching speeds to ensure efficient rectification [18; 47]. Schottky diodes, such as the SMS7630 (Skyworks, \(V_{f}\) = 150-250 mV and HSMS-2860 (Broadcom, \(V_{f}\) = 150 mV), are well-suited for these applications due to their low conduction threshold and high-frequency operation. Forward voltage drops and reverse leakage current impact overall rectification efficiency and result in power dissipation in Schottky diodes. Impedance matching networks are employed to optimise power transfer and reduce reflection losses. Multi-stage rectifiers, including  
Figure 6: Delon quadruple rectifier used in the RF energy harvester. voltage multipliers (such as the Delon quadruple rectifier), help compensate for low RF voltage levels, improving DC output without significantly increasing power dissipation. Advanced semiconductor materials, such as graphene-based and tunnel diodes, further enhance energy conversion efficiency with even lower voltage drops and faster response times. Effective thermal management techniques, such as heat sinks and PCB thermal via, prevent overheating and maintain performance stability [12]. This study selected the HSMS-2860 Schottky diode due to its wide operating frequency range (up to 24 GHz), low turn-on voltage, and market availability. By integrating optimised rectification techniques, low-\(V_{f}\) Schottky diodes, and advanced power management strategies, RF energy harvesting systems can achieve higher efficiency, making them viable for low-power IoT and wireless sensor applications [49]. The RF Schottky diode HSMS-2860 was used in this research due to its low forward voltage drop and high switching speed, which make it ideal for rectification. A unique Delon quadruple rectifier (Figure 6), known for its high RF-DC conversion efficiency, was implemented in the RF-EH system to boost voltage conversion and significantly enhance system performance. The rectifier was designed and simulated using ADS software, and its efficiency was calculated using (5). A diode was added in series with the load resistance to improve sensitivity, allowing the RF-EH system to operate effectively down to \(-25\) dBm.