WIPT / WPT: Wireless Information and Power Transfer & Harvesting
1. RF Signals as Energy
In WIPT/WPT systems, the transmitter sends radio-frequency (RF) electromagnetic waves instead of light. RF signals carry energy through oscillating electric and magnetic fields.
The power received by an antenna depends on:
- Transmit power
- Distance
- Frequency
- Antenna gains
More transmitted RF power means more harvestable energy at the receiver.
2. RF Energy Harvesting (Rectenna)
The receiver uses a rectifying antenna (rectenna) to convert RF signals into electrical energy.
- RF waves are captured by an antenna.
- The AC RF signal is fed to a rectifier.
- The rectifier converts AC to DC.
- The DC power charges a battery or capacitor.
This is the core principle of wireless power harvesting.
3. Information and Power Separation
The received RF signal contains both energy and information. Two common receiver architectures are used.
(a) Power Splitting (PS)
The received RF power is divided into two parts using a power splitter:
P_EH = β P_total
P_ID = (1 − β) P_total
- PEH: Power for energy harvesting
- PID: Power for information decoding
- β: Power splitting factor
(b) Time Switching (TS)
The receiver alternates operation in time:
- One time slot for energy harvesting
- One time slot for information decoding
4. Analogy
Imagine a radio signal as wind hitting a windmill:
- One part of the wind spins a generator (energy harvesting).
- The other part vibrates a sensor that listens to patterns (information decoding).
The wind can be split either by power (power splitting) or by time (time switching).
5. Practical Examples
Wireless Phone Chargers
- Use near-field electromagnetic coupling
- Power is transferred wirelessly from charging pad to phone
- No data decoding – pure WPT
RFID Tags
- Reader transmits RF signal
- Tag harvests energy from RF
- Uses harvested energy to send information
- Classic example of WIPT
IoT Sensors
- Harvest RF energy from base stations or Wi-Fi
- Operate without batteries
- Transmit sensed data using harvested energy
Wireless EV Charging
- Uses high-power electromagnetic fields
- Transfers energy from ground pad to vehicle coil
- Mainly power transfer; limited data exchange
6. Comparison with Wired Charging
| Feature | Wired Charging | Wireless Power Transfer |
|---|---|---|
| Physical contact | Required | Not required |
| Convenience | Low | High |
| Efficiency | High | Lower |
| Safety | Risk of sparks | Safer sealed systems |
7. Summary
RF signals carry both information and energy. In WIPT/WPT systems, the receiver uses an antenna and rectifier to convert part or all of the received RF power into DC electricity. Power splitting or time switching controls how much energy is used for harvesting versus information decoding.