# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 5 BackCom: Wireless Communications
### BackCom Communication Modes
#### 5.1.1 Full-Duplex BackCom  
Full-duplex communication mode is widely used in BackCom and Wireless Powered Communication Networks (WPCNs). The WPCN with a full-duplex hybrid (energy and information) access point is considered to enable energy harvesting capabilities for wireless users [92]. Moreover, dual antennas are used in H-AP for operation in full-duplex mode. While the first antenna broadcasts wireless energy to users in the downlink, the other antenna receives data using time division multiple access in the uplink. [PERSON] et al. presented the first demonstration of a monostatic full-duplex BackCom system that considers the carrier source co-located with a commercial Bluetooth chipset [97]. This system allows for the transmission of information from specially designed Bluetooth low energy (BLE) backscatter tags to a commercial Bluetooth chipset, without any software modifications.  
A full-duplex scheme for Amb-BackCom devices is considered to relay the information [93]. The relay node can send its information, while receiving signals from the source node. [PERSON] et al. designed a full-duplex radio that uses balun cancellation, a signal inversion, and an adaptive cancellation technique [107]. The circuits of balun cancellation can tune their cancellation circuits in variable environments. A full-duplex BackCom network to address the interference problem was suggested in [77]. This network considers a multi-access design for time hopping spread spectrum. It allows a coexisting reader-tag link to set up a one-way wireless energy transfer and dual-way wireless information exchange.  
A full-duplex BackCom system for wireless communication systems, namely \"ReflectFX,\" was proposed by [PERSON] et al. [108]. ReflectFX prevents self-interference in half and full-duplex modes.  
\begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**References** & **Subsections** \\ \hline
[77, 78, 92, 93, 94, 95] & **Communication Modes** \\
[95] & \(\bullet\) Full-duplex \\ \hline
[50] & **Modulation Techniques** \\
[66, 96] & \(\bullet\) Amplitude Modulation \\
[97] & \(\bullet\) Frequency Modulation \\
[90] & \(\bullet\) Gaussian Frequency Shift Keying \\
[98] & \(\bullet\) Differential Modulation \\ \hline
[98] & **Quadrature Amplitude Modulation** \\ \hline \hline \end{tabular}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
[68, 99, 100, 101] & **Multiple Access** \\
[102, 103] & **TDMA** \\
[104] & **OFDMA** \\
[101, 105, 106] & **OFDMA** \\ \hline \hline \end{tabular}
\end{table}
Table 5: BackCom: wireless communications.  
Moreover, it allows backscatter nodes to reuse interfering received carrier signals for the transmission of data. Likewise, IBFD wireless communications utilize a full-duplex mode on the same frequency band for wireless terminals to transmit and receive information [78]. Previously, network designers were unable to attain a full-duplex feedback channel on battery-less devices. However, the usefulness of the full-duplex feedback channel is demonstrated in [94]. This channel can facilitate a network layer and link layer mechanisms to improve BackCom performance. Optimal policies for time allocation in full-duplex, backscattered assisted wirelessly powered communications networks (BAWPCNs) were achieved to maximize the throughput of the BackCom system [95].