# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 4 BackCom: Signal Processing and Detection
### Signal Detection in BackCom
#### 4.2.2 Non-Coherent Detection
Similarly, the detection of BiS was proposed to improve the BER performance in comparison to earlier BiS radio receivers [66]. The non-coherent channel coding was incorporated within a BiS radio framework [73]. The receivers in this framework have less computational complexity as compared to maximum likelihood (ML) coherent receivers, and the corresponding BER results were obtained and compared to their coherent counterparts. Figure 17: Non-Coherent Detection for BackCom. Subsequently, a transmission model was formulated while considering multiple antennas in the reader [91]. They also proposed a different method of signal detection in the absence of CSI. **Discussion:** Table 4 summarizes Section 4, which covers coding, interference, decoding, and signal detection schemes in BackCom. Moreover, various techniques for channel coding are discussed. The primary purpose of these coding schemes is to enhance the reliability, throughput, and range of backscattered signals. The key points are discussed below:  
1. The standard coding scheme in BackCom employs FM0 and the Miller code, which has a maximum rate of one half [70]. In other words, each information bit adds one additional bit for redundancy, before transmitting data to the wireless channel. This technique provides redundancy to the data, at the cost of a significant bandwidth. Hence, these types of codes are beneficial in large bandwidth scenarios. 2. An improved-rate 6/8 channel block codes for Amb-BackCom are suggested to address the need for high data rates in limited bandwidth scenarios [71]. 3. The renowned cyclic error correcting codes, with short length, are preferred to improve throughput when the carrier emitter is far from the reader [66]. 4. Another way to improve the communication range in BackCom is to enable concurrent transmissions instead of serial transmissions. This type of scenario is suitable for tags and readers with multiple antennas. Moreover, Microcode and BST focus on simultaneous transmissions of data for BackCom [32,68], whereas concurrent transmissions require multiple antennas, which consequently require more space. 5. The BackCom data rate can be enhanced by utilizing the phase information of backscattered signals: negative phase, positive phase, or no backscattering [72]. This technique is preferred when there is no available phase information. 6. Coding and modulation can be used together, for acquiring a high data rate and an extended range, at the cost of higher complexity [74]. 7. The detection techniques were analyzed to observe variations with particular receivers. A distinct detector is suggested if received signals are obtained from multiple antenna tag receivers [91]. 8. Coherent detection is preferable in a bistatic setup due to the availability of phase information by the near carrier emitter [66]. 9. Non-coherent detectors are preferred for scenarios such as Amb-BackCom, as the phase information of backscattered signals is hard to identify [84]. 10. For optimal detection, an ML detector is chosen and can be utilized as the reference to establish other optimal detectors [65]. Let us now consider communication modes, modulation schemes, and multiple access techniques in the upcoming section to accommodate more tags.