# Dominant Channel Occupancy for Wi-Fi Backscatter Uplink in Industrial Internet of Things
## 4 Performance Evaluation
### Simulation Results
In composite cases, the Wi-Fi backscatter devices can dominantly  
Figure 6: Variation of throughput for separate cases: (**a**) reduced CW\({}_{\min}\) = 15; (**b**) reduced CW\({}_{\min}\) = 7; (**c**) reduced CW\({}_{\min}\) = 3; and (**d**) reduced CW\({}_{\min}\) = 1.  
occupy the channel. In Figure 7b, the Wi-Fi backscatter devices occupy 95% of the channel on average, since they use a small CW\({}_{\text{min}}\) and fixed backoff period. In particular, as shown in Figure 7c,d, when CW\({}_{\text{min}}\) is set to 1 and the duration of DIFS is set to 16 \(\upmu\)s (i.e., SIFS) and when all configurations are simultaneously employed, the Wi-Fi backscatter devices dominantly occupy the channel. Figure 7: _Cont._  
Figures 8 and 9 show the variation of the number of consecutive packet transmissions and throughput for the burst transmission, respectively. For the burst transmission, we set the Wi-Fi helper to transmit 100 packets consecutively. Figure 8 exhibits the variation of the number of burst transmissions when the number of devices increases. The Wi-Fi helper transmits 100 packets without any interruption. The reason for this is that they wait only for SIFS between consecutive packets. The neighboring legacy Wi-Fi devices use the CSMA/CA mechanism, which causes DIFS and random backoff delay; thus, they do not have a chance to transmit packets during the burst transmission. The number of consecutive packet transmissions of the legacy Wi-Fi devices decreases slightly when the number of devices increases, since the number of devices that have to compete for channel access increases. Figure 7: Variation of throughput for composite cases: (**a**) CSMA/CA starting with SIFS + freezing of backoff period; (**b**) freezing of backoff period + reduced CW\({}_{\text{min}}\) = 1; (**c**) CSMA/CA starting with SIFS + reduced CW\({}_{\text{min}}\) = 1; and (**d**) all of the MAC configurations. Figure 9 shows the variation of throughput for burst transmission. In Figure 9a, the Wi-Fi backscatter devices employ burst transmission without MAC configurations for initial channel access. In this case, the throughput of the Wi-Fi backscatter devices decreases as the number of devices increases. The reason for this is that both types of devices have the same chance of channel access due to the use of CSMA/CA. Figure 9b shows the variation of throughput of the Wi-Fi backscatter devices when they use all of the MAC configurations. The simulation results demonstrate that the Wi-Fi backscatter devices can dominantly occupy the wireless channel. In this case, the Wi-Fi backscatter devices always access the channel earlier than the others, since they use the CSMA/CA starting with SIFS, freezing of the backoff period, and reduced CW\({}_{\min}\) (i.e., CW\({}_{\min}\) = 1). In addition, Wi-Fi helper transmits the packets successively without any interruption because they only wait for SIFS between consecutive packets. Figure 8: Variation of the number of consecutive packet transmissions. Figure 9: \(Cont\). Figure 10 shows the throughput variation and the number of consecutive packet transmissions for multiple pairs of Wi-Fi backscatter devices. In the simulation, 20 pairs of devices are deployed in total; the number of Wi-Fi backscatter device pairs started from one and rose to 10 in single-step increments, and the rest is set to the pairs of legacy Wi-Fi devices. All MAC configurations are employed for the pairs of Wi-Fi backscatter devices, and \(\text{CW}_{\text{min}}\) is set to three. In addition, the number of consecutive packets for burst transmission is set to 100. In Figure 10a, the Wi-Fi backscatter devices generally exhibit higher throughput than those of the legacy Wi-Fi due to the use of DCO. However, it sharply decreases as the number of Wi-Fi backscatter device pairs increases, because each Wi-Fi helper competes with all others for channel access using CSMA/CA. Figure 10b shows the number of consecutive packet transmissions when deploying multiple pairs of Wi-Fi backscatter devices.