# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 3 BackCom: An Effective Solution  
The high expectations of emerging 5G technology to connect massive IoT devices to the Internet [14; 26] and to enhance the bandwidth of cellular network [19; 22; 23; 29; 30] made users expect a solution to the limited battery life problem in cellular networks, since the major constraint with wireless communications is the limited battery life. The problem of limited battery life can be effectively addressed if there are battery-free communications. The popular technique in recent ultra-low power communications is BackCom, which is based on RFID technology and receives power from nearby sources [12; 31; 32; 33; 34; 35; 36; 37]. BackCom enabled the first battery-free cell phone with all passive components [8]. The battery-free cell phone is an ultra-low power device that consumes very little power and does not require a battery. Moreover, it does not possess any digital signal processing component in the handset. The analog voice signals are modulated directly on the carrier waves by utilizing BackCom and amplitude modulation. Subsequently, the nearest base station is responsible for receiving analog signals from the cellphone for all digital processing of the signals. The special microphone in the cellphone works as a capacitor that generates sufficient power to energize the earphones, in conjunction with other energy harvesting components. In summary, BackCom devices do not require any battery but harvest power from incident RF signals. Hence, BackCom is the desired solution to combat the energy crisis in emerging wireless networks. BackCom can also lead us to new communications protocols suitable for future battery-less devices.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Subsections** & **References** & **Key Points** \\ \hline \multirow{6}{*}{**Power Enhancement in mm-waves–A Cellular Perspective**} & [18] & \(\bullet\) RF-based energy harvesting from base station was proposed in a cognitive cellular network. \\  & [19] & \(\bullet\) mm-wave frequencies possess the potential to increase the achievable rate and harvested power, compared to the beachfront spectrum. \\  & [20] & \(\bullet\) The mm-wave identification system for frequencies above 70 GHz was introduced. The transponder size of the receiver enables BackCom for low power device communications. \\  & [22] & \(\bullet\) Wireless power transfer for mm-waves cellular networks was discussed. \\  & [23] & \(\bullet\) Energy harvesting performance was analyzed for low power devices powered by mm-waves. \\ \hline \multirow{6}{*}{**Power Management in Disasters–A D2D Perspective**} & [4] & \(\bullet\) Survival in sharing mechanisms uses D2D-based communication to overcome battery power limitations during disasters. \\  & [24] & \(\bullet\) A D2D discovery mechanism takes advantage of the proximity area concept to establish energy-efficient D2D communications. \\  & [25] & \(\bullet\) A D2D-based messaging solution was proposed to overcome the battery power limitation in disaster situations. \\ \hline \multirow{6}{*}{**Enhancing Lifetime of Devices–An IoT Perspective**} & [26] & \(\bullet\) An energy-efficient network architecture based on sensor and relay nodes was discussed to avoid the energy hole problem. \\  & [27] & \(\bullet\) The Quark OS was introduced for low power IoT devices with tiny energy buffers. \\  & [28] & \(\bullet\) An energy-efficient multi-sensing platform WuLoRa (Wake up Long Range) was proposed for an IoT network. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of energy constraints in wireless communications.