# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 1 Introduction
### Ambient BackCom: A Solution to Limited Battery-Life  
Conventional RFID systems utilize a dedicated radio source and reader. Unlike the traditional RFID system, ambient BackCom (Amb-BackCom) does not need a dedicated RF signal source, such as an oscillator or signal generator with the reader. However, Amb-BackCom explores the advantage of RF signals available in the vicinity (e.g., TV, WiFi, cellular signals, etc.), thus revolutionizing wireless communications [12]. Figure 3 delineates the communication between passive tags by using backscattered TV signals. Currently, researchers are showing great interest in making future communications based on the reflection of ambient RF signals to combat the limited battery life problem in wireless communications [13].  
5G communication has advantages of high-power mm-waves, in addition to directional beam communication with multi-input multi-output (MIMO) antenna arrays [14], enabling devices to send power beams in a certain direction. Thus, we explore BackCom, which can enable battery-free  
Figure 2: Block diagram battery-free cellphone.  
communications in emerging wireless networks. Existing work has shown a detailed overview of the latest developments in ambient and bistatic scatter types of BackCom [13]. Authors have covered various channel models of BackCom and their theoretical analysis along with experimental measurements. They also discussed the antenna designs in detail. Our survey deliberately avoided content details that is extensively covered in existing work [13], such as antenna design, communication channels, visible light BackCom, and energy harvesting networks. However, our approach suggests the BackCom for emerging battery-free heterogeneous wireless networks. Our work organization explores BackCom with a 5G perspective, which may be suitable for future battery-free communication. Furthermore, our approach bridges the gap between previous works [13] and recent developments, by covering the latest research related to BackCom, battery-free communication, and emerging networks.  
This paper aims to survey BackCom as an emerging technology to solve the limited battery life problem in next-generation wireless heterogeneous networks. Figure 4 presents a taxonomy graph of our survey paper. To be precise, the following aspects are covered in this paper.  
* We discuss the energy constraints in wireless communications and suggest BackCom with emerging 5G wireless as an effective solution to this long-lasting problem.
* We present signal processing and signal detection schemes for BackCom.
* We demonstrate different communication techniques for BackCom including communication modes, modulation techniques, and multiple access schemes.
* We highlight efficient power and data transfer schemes for BackCom, in addition to an increase in the communication range and the reliability of data.
* We show various applications of BackCom, its challenges, and future directions for emerging wireless networks.  
The rest of the article is organized as follows. Section 2 focuses on the energy problem in different wireless communications perspectives. The evolution, types, and protocols for BackCom are discussed in Section 3. Section 4 discovers BackCom signal processing aspects, such as coding, interference, decoding, and signal detection schemes. Communication modes, modulation methods, and multiple access techniques for BackCom are explained in Section 5. Section 6 considers different power sources used in BackCom in addition to efficient power and data transfer techniques, such as scheduling, resource allocation, and optimization. The BackCom coverage problem and range extension along with the reliability and security of data are illustrated in Section 7. Different applications of BackCom are shown in Section 8. Section 9 presents the challenges and future directions for BackCom in next-generation 5G wireless networks. Finally, the paper is concluded in Section 10.  
Figure 3: Passive tags communicating using backscatter communication (BackCom).