# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 7 BackCom: Range Extension and Reliability
### Reliability and Security  
Achieving reliability and security in a network is an important task in BackCom. Passive tags do not have a signal generator component, and always seek signals from nearby sources. These external sources may risk the security and reliability of tag data. The reliability issue in BackCom was addressed by modeling uplink transmissions in backscatter networks [59]. Moreover, this approach considers transmissions by only a single virtual transmitter and deals with collisions as a sparse rateless code among nodes.  
[PERSON] et al. addressed the reliability of backscattered radio systems using multiple antennas [62]. Similarly, security attacks using D2D communications include interference attack. This type of attack is on privacy and can be executed by statistically or logically determining data transmission patterns between devices [182]. Another type of security attack is the distributed denial of service attack, which can jam autonomous D2D communications and stop the exchange of data between devices. These above-mentioned types of attacks can also exist in IoT with backscatter devices and need to be addressed carefully. Furthermore, the carrier signals used to energize passive devices for retrieving data may be exploited by hackers. The overhearing of data without permission is called an eavesdropper attack. This attack is more common in BackCom. Several security measures have been taken to solve this problem [184; 109].  
Figure 38: Hybrid D2D Amb-BackCom.  
Figure 39 shows a secure BackCom assisted by nearby relays. This scenario considers no direct link between reader and tag, and BackCom is only possible with the help of relays [184]. All relays communicate in half duplex mode, having a single antenna and perform amplify-and-forward task. The relay node (forward) is selected from all relays, to power up the tag by transmitted carrier waves. The relay node (backward) is selected nearer to the reader, to receive the backscattered signals from tags. The nearer selection of relay node improves the secrecy rate and avoids attack from eavesdroppers.  
**Discussion:** Table 11 summarizes the range extension of BackCom devices in addition to the reliability and security of data. Key points are discussed below.  
1. To enhance the communication range of BackCom in a BiS scenario, a coherent receiver is suggested [66], the BackCom LoRa scheme performs well for long distance communications [129]. Quantum tunneling-based backscattering [167] and power amplifiers [136] are also suggested for an extension of range.
2. BackCom tags can use multiple antennas and MIMO system designs for the enhancement of communication range. These designs are suitable for large size tags that are used in MicroMo [32], multi-antenna BiS [171] and unitary query schemes [174].
3. RFID MIMO systems for a dyadic backscatter channel also improve the communication range of the dyadic channel [74].
4. A full-duplex relaying scheme for BackCom based on the division of time slots increases the communication range by simultaneous energy reception and data transmission [93].
5. A D2D-based communication scenario improves the coverage in passive devices [182].  
It is observed from the above literature that, besides BackCom range extension, researchers are also trying to address interference attack, denial of service attack, and eavesdropping attacks for the sake of BackCom security. However, there is much room for improvement in range extension in addition to the reliability and security of data.  
Figure 39: Secure BackCom assisted by helping relays.