# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 4 BackCom: Signal Processing and Detection
### BackCom Signal Processing Aspect
#### 4.1.1 BackCom Coding  
Similar to wirelessly powered communication, the data in BackCom is encoded with certain coding techniques before the start of transmission over RF channels. The purpose is to support the recovery of (lost) data across noisy channels. Channel coding is also performed for efficient transmission over wireless communication channels. Coding schemes proposed for a conventional wireless system not only consume a high amount of power but involve high computational complexity. Hence, these techniques are not suitable for low power passive devices. To standardize RFID BackCom encoding schemes, EPC Global provided the EPC Gen-2 communication protocol. This standard suggests two encoding schemes for backscatter base-band signals: (i) the Miller code and (ii) the bi-phase space (FM0) code.  
Both of these channel codes have the highest transmission rate of one half [70]. However, to address the need for high data rates in BackCom, the improved rate channel codes employ a 6/8 rate balanced block code for higher throughput [71]. Figure 12a shows the Gen-2 FM0 encoding and its  
\begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline \# & **Types of BackCom** & **Explanation** & **References** \\ \hline \multicolumn{4}{c}{**Dedicated Source and Reader**} \\ \hline
1 & Dedicated BackCom & \(\bullet\) Uses a single dedicated source and reader for each tag. & [35] \\
2 & Ambient BackCom & \(\bullet\) Uses surrounding signals from different sources to power up tags. & [12; 37; 65] \\ \hline \multicolumn{4}{c}{**Arrangement of Source and Reader**} \\ \hline
3 & Bistatic Scatter (BiS) & \(\bullet\) A signal generator is dislocated from the reader, and a separate signal source is placed near the tag to reduce path loss. & [50; 66] \\ \hline
4 & Monostatic BackCom & \(\bullet\) The same antenna is used for transmission and reception, and no separate antennas are needed. & [39] \\ \hline
5 & Hybrid BackCom & \(\bullet\) The tag works in ambient mode, dedicated bistatic mode, or both. & [52] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary for Types of BackCom.  
corresponding code bits, whereas Figure 12b delineates a 6/8 rate balanced block code and its relevant wave forms.  
[PERSON] et al. proposed a \"microcode\"-based mechanism for Amb-BackCom [32]. Microcode supports concurrent transmissions and has a longer transmission range compared to traditional Amb-BackCom. The authors suggest a coding technique by considering three states of tags response: (i) backscattered signals with the same phase; (ii) backscattered signals with an inverted phase; and (iii) no backscattered signals [72]. Likewise, short length cyclic error-correcting codes for the BiS network are employed to improve transmission rates [66]. Subsequently, short block length channel codes, with very low encoding complexity, are considered for BiS radio [73]. These channel codes can adopt bistatic scattering on large-scale, ultra-low price wireless sensor networks (WSNs). Meanwhile, the coupled usage of coding and the modulation of energy shows better results for energy harvesting in BackCom [74]. [PERSON] et al. proposed run-length limited codes for the backscatter system, which were used for a reduction in self-interference and an improvement in data rates [75]. After coding, the backscattered signals were sent to the reader via transmission channels. These channels added noise and interference due to the presence of similar signals. The next section focuses on the interference component of BackCom.