# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 6 Efficient Power and Data Transfer in BackCom
### Power Transfer Management and Technology Interchange
#### 6.2.2 Technology Conversion and Frequency Shifting Technique  
The technology conversion technique enables BackCom to use the frequency spectrum of two technologies, by transforming the frequency band of one wireless technology to another [36]. Figure 32  
Figure 31: Efficient power transfer in BackCom.  
delineates the concept of technology conversion, which converts the Bluetooth frequency to a WiFi spectrum. Similarly, WiFi devices can be used for the creation of Bluetooth- and ZigBee-compatible wireless signals for BackCom [36]. Moreover, this work suggests a single side-band backscatter design, which can cause a frequency shift on one side of the carrier. The suggested technique leads to the communications between contact lens and smartphones and smart-watches. [PERSON] et al. proposed smartphone Bluetooth 4.0 Low Energy (BLE) [51]. They utilized low cost, commonly available smartphones, and other handy devices, to work as a ready-made infrastructure for IoT nodes. These devices are supported by the ultra-low power modulated BackCom scheme (MBS), which is commonly used in ultra high frequency (UHF) RFID systems. Moreover, MBS is now compatible with BLE software and hardware. Similarly, a BLE-compatible full-duplex BackCom system model for mobile devices was proposed to demonstrate monostatic full-duplex communications [97]. This model further considers the carrier source co-located with the Bluetooth chipset. This enables specially designed BLE-based backscatter tags to transmit information via a Bluetooth chipset, without any software modifications.  
The frequency shifting technique allows backscatter tags to shift the carrier frequency to a nearby spectrum [138]. This type of scenario shifts carrier signals to a nearby non-overlapping band of frequency (using Bluetooth and a WiFi band) [97]. This scheme also separates the frequency range of backscattered signals from the primary (signals) band to strengthen decoding.  
**Discussion:** Table 9 highlights different power sources for BackCom in addition to energy management and efficient power transfer techniques. Moreover, technology conversion techniques and frequency shifting techniques are also given:  
1. The energy consumption of BackCom devices is in micro-watts [8; 32]. This small amount of power can be extracted from surrounding (ambient) radio waves [12], such as WiFi signals [37], cellphone signals [125] and TV signals [93].
2. The optimal harvesting ratio and different harvesting modes for BackCom are discussed. Energy management policies for energy harvesting using BackCom were discussed [136].
3. Efficient power transfer schemes use energy beam-forming and multiple antennas for BackCom [127].
4. Technology conversion in BackCom utilizes the frequency spectrum of Bluetooth and converts those signals to WiFi and Zigbee signals [36].
5. The frequency shifting technique in BackCom allows the carrier signals to shift in nearby frequency bands [138].  
Figure 32: Technology conversion in BackCom.