# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 6 Efficient Power and Data Transfer in BackCom
### Scheduling and Resource Allocation
#### 6.3.1 Energy and Data Scheduling  
Energy scheduling is essential for better power harvesting and efficient data transfer. A scheme for backscatter-assisted wireless power communication networks (BAWPCNs) was proposed by [PERSON] et al. [95]. Similarly, a BAWPCN-based model considers energy scheduling using TDMA. This energy scheduling scheme has the trade-off between successful transmission probability and the energy harvesting rate [51]. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**Subsections** & **References Key Points** \\ \hline
[12] & \(\bullet\) The energy used by BackCom devices was in micro-watts. \\
[12] & \(\bullet\) There was no need for a dedicated power source, and ambient signals were utilized to energize. \\
[37] & \(\bullet\) A WiFi source was used as a power source. \\
[32] & \(\bullet\) A hardware prototype achieved complete power by energy harvesting. \\
[34] & \(\bullet\) HitchHike was used; power was harvested from ambient WiFi signals. \\
[35] & \(\bullet\) Passive WiFi chipsets that work on BackCom were introduced. \\
[54] & \(\bullet\) An optimal harvesting ratio was obtained in non-dedicated sources using HTTP. \\
[74] & \(\bullet\) A simple model was used for energy harvesting by tag. \\
[79] & \(\bullet\) An overview of RF-based wireless energy transfer technologies was provided. \\
**Sources of Power in BackCom** & [121] & \(\bullet\) An overview of wirelessly powered communications networks was provided. \\
[122] & \(\bullet\) Bluetooth power consumption from 20 to 60 mW was used. \\
[123] & \(\bullet\) Power consumption of WiFi was greater than 200 mW. \\
[125] & \(\bullet\) Cellphone BTS and TV broadcasts were targeted as signal sources. \\
[126] & \(\bullet\) The lowest attainable input RF power levels were achieved. \\
[127] & \(\bullet\) The transfer of energy was of focus. \\
[128] & \(\bullet\) 5G, mm-waves, and MIMO was used for wireless power transfer. \\
[130] & \(\bullet\) PoWiFi was used, utilizing existing WiFi chipsets for power delivery. \\
[134] & \(\bullet\) The harvest-then-transmit (HTT) protocol was proposed. \\
[135] & \(\bullet\) A mixture of backscatter modulation and wireless power transfer using diodes. \\ \hline
**Energy Management** & [136] & \(\bullet\) Optimal policies for harvested energy management was proposed. \\ \hline
**Efficient Power Transfer** & [127] & \(\bullet\) RF-based energy transfer was used via energy beam-forming from multiple antennas. \\
[137] & \(\bullet\) Energy was harvested to power up WSNs using two different frequencies. \\ \hline
**Technology Conversion** & [36] & \(\bullet\) Bluetooth was converted to WiFi and ZigBee signals, and a single side band backscatter design was suggested. \\
[51] & \(\bullet\) Low cost smart phones were used to work as a ready-made infrastructure for IoT. \\
[97] & \(\bullet\) Backscatter tags were enabled to transmit data via a Bluetooth chipset. \\ \hline
**Frequency Shifting Technique** & [138] & \(\bullet\) Enables the backscatter tag to shift the carrier signals to nearby band. \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Summary of BackCom efficient power and data transfer. BackCom enables each tag to communicate simultaneously through the reflection of incident signals. This simultaneous reflection by each tag causes data collisions [59; 82]. In order to avoid data collisions by tags, priorities and procedures for data transmissions are needed. [PERSON] et al. considered data scheduling for backscatter sensor network to prevent collisions [139]. Moreover, the authors highlighted the data sending technique through a gateway. This technique changes the impedance of antenna while backscattering received RF signals. This work reduced the complexity and power consumption and addressed the data collection problem in addition to data scheduling.