# Low-Power Rectennas in Microwave Wireless Power Transmission
## 2 Low-Power Rectifiers
### Diode Performance Optimization
#### 2.1.1 Input Signal Manipulation  
Schottky diodes require a minimum amount of energy to turn on, and the input voltage must exceed a certain threshold to activate the diode and generate a relatively large dc output voltage [14]. Therefore, increasing the input power and voltage delivered to the diode is an effective method to improve efficiency. As shown in Figure 2, a coupled transmission line (CTL) structure was employed to increase the voltage across the diode, thereby enhancing rectifier efficiency. At 2.4 GHz, when the input powers were \(-5\) dBm, 0 dBm, and 5.5 dBm, the rectification efficiencies reached 60.4%, 67.7%, and 75.3%, respectively [15]. Compared to conventional designs, this structure is more effective in reducing losses caused by the built-in potential \(V_{bi}\) of the PN junction. Similarly, introducing a high-impedance transmission line before the diode can also increase the input voltage. This approach not only achieves impedance matching between the diode and the source but also boosts the peak diode voltage \(V_{D}\) to four times the input voltage \(V_{in}\). At 5.8 GHz with an input power of \(-30\) dBm, an efficiency of 3.5% was achieved [16].  
Another method is waveform manipulation. Under the same average input power, the nonlinear combination of multiple sinusoidal signals can better excite the diode, significantly increasing output dc power compared to single-tone excitation [17]. The efficiency improvement from multi-tone signals depends on the diode parameters, with the most sig  
Figure 2: Schematic of CTL-based rectifier [15].  
nificant benefit observed for diodes with high junction capacitance \(C_{j}\)[18]. Several studies have reported that using multi-tone signals increases the rectification efficiency [19; 20; 21; 22; 23]. In [24], the multi-tone signal optimization problem was formulated as a linear constrained quadratic program, and a globally optimal solution was proposed based on mixed-integer linear programming and branch-and-bound methods. The diode model was simplified to a second-order polynomial curve-fitting model, providing an excellent solution for multi-tone power allocation strategies. With the rapid development of the Internet of Things (IoT) in recent years, simultaneous wireless information and power transfer (SWIPT) technology have attracted huge attention [25]. Various multi-tone modulation techniques such as BASK [26; 27], FSK [28], BFSK [29], and PSK [30; 31] have been introduced, enabling simultaneous transmission of energy and information while improving energy conversion efficiency.  
Adding other environmental energy sources is the most direct way to increase input power. Green energies such as solar energy [32; 33; 34; 35], thermal energy [36; 37; 38; 39], and kinetic energy [40; 41; 42; 43] can be combined with RF energy to achieve higher dc output power. In [36], a cooperative voltage doubler rectifying topology was proposed, seamlessly integrating RF and photovoltaic energy, as shown in Figure 3. The single-frequency rectifier operating at 2.4 GHz exhibited an ultra-wide input power range from \(-20\) dBm to 5 dBm under a light intensity of 232 Lux, achieving an efficiency of 15% at \(-20\) dBm. The wide-band rectifier employed a post-matching technique, using additional capacitors and transmission lines to compensate for the imaginary part of the diode impedance, achieving efficiencies greater than 30% over the 0.8-2.4 GHz range.