# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 4 BackCom: Signal Processing and Detection
### BackCom Signal Processing Aspect
#### 4.1.2 Interference in BackCom  
BackCom tags could be embedded into everyday objects of IoT with next-generation 5G networks, thus raising the connectivity of smart objects with ubiquitous networks based on ultra-low power consumption. The primary challenge associated in accomplishing ultra-low power communication is the high reliability and efficiency of communication between BackCom tags. However, nodes in BackCom systems are not able to sense each other's transmissions, thus suffering from collisions and interference. Figure 13 shows signal levels with and without collisions. In Figure 13a, the transmission is shown by a single node. Signals have two levels of magnitude and could be distinguished as \"0\" and \"1.\" Figure 13b delineates the collision between the signals of two nodes. The received signal has four levels of magnitude corresponding to \"00,\" \"01,\" \"10,\" and \"11.\" To address the collision problem, a communication system named \"Buzz\" was proposed by [PERSON] et al. [59]. Buzz handles all the BackCom tags as one virtual sender. Buzz also treats collisions as code across all the bits, transmitted by nodes. In this way, Buzz allows only a small number of nodes to have collisions at a time. Moreover, the authors made collisions to act like a sparse code that could be decoded by a customized sensing algorithm.  
Figure 12: Coding in BackCom.  
[PERSON] et al. suggest anti-collision algorithms for BackCom based on bit collision detection for the FM0 code and miller sub-carrier sequence [76]. Throughput of the suggested algorithms is greater than the conventional binary tree algorithm.  
The work in [77] addresses the interference issue in BackCom by proposing a full-duplex BackCom network design. To reduce in-band full-duplex (IBFD) self-interference, a remote device was proposed, and this device could communicate over the same frequency band as in IBFD communication [78]. \"Near-far\" is a problem in wireless communications in which a receiver catches high power signals, making it difficult for the receiver to detect low power signals [79]. Similarly, \"doubly near-far\" is a condition observed in energy harvesting networks [54], where the devices closer to the access point harvest more energy in the downlink and require less power to achieve the given SNR. Therefore, this condition allow devices far from the access point to harvest less energy, but the devices require high power to obtain the targeted SNR [80].  
To compensate the path loss in mm-wave BackCom, a power efficient solution was proposed [81]. This solution allows one to equip tags with retro-directive arrays that have a maximum reflection gain direction of incidence. [PERSON] et al. discussed sensor collisions (interference) in the backscattered network and elaborated on the feasibility of anti-collision performance in high-density backscatter sensor networks [82]. Moreover, the results show that the performance of backscatter sensor networks depends on the modulation scheme used at each sensor node. In addition, the interference and noise generated from surrounding nodes and wireless communication channels, respectively, render the backscattered signals weaker at the receiver end. Therefore, the task of the BackCom decoder is to decode the information from the low power backscatter signals.