# Dominant Channel Occupancy for Wi-Fi Backscatter Uplink in Industrial Internet of Things
## 4 Performance Evaluation
### Simulation Results  
Figures 5 and 6 show the variation of average throughput for the Wi-Fi backscatter and legacy Wi-Fi devices when the number of devices is changed. In these simulations, only a single MAC configuration is employed for the Wi-Fi backscatter devices (i.e., separate cases). In Figure 5a,b, the Wi-Fi backscatter devices employ the \"CSMA/CA starting with SIFS\" and the \"freezing of the backoff period\", respectively. Overall, the MAC configuration can improve the average throughput of Wi-Fi backscatter devices. The CSMA/CA starting with SIFS and the freezing of the backoff period improves the throughput by 85% and 29%, respectively. This is because the Wi-Fi backscatter devices \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline PHY/MAC model & IEEE 802.11n & Traffic application & CBR \\ Simulation time & 1000 ms & Date rate & 300 Mbps \\ SIFS & 16 \(\upmu\)s & Payload & 2304 bytes \\ DIFS & 34 \(\upmu\)s & PHY header & 16 bytes \\ Slot time & 9 \(\upmu\)s & MAC header & 30 bytes \\ Maximum retransmission & 6 & CW\({}_{\text{min}}\), CW\({}_{\text{max}}\) & 15, 511 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. Figure 4: Examples of network topology: (a) four devices; (b) eight devices; (c) 14 devices; and (d) 20 devices. have a higher priority of initial channel access than the legacy Wi-Fi devices in both cases. Specifically, the former can reduce the waiting time for channel access, and the latter can reduce the backoff delay when collision occurs. The variation of throughput for \"reduced CW\({}_{\text{min}}\)\" is shown in Figure 6. From the results, we observe that the reduced CW\({}_{\text{min}}\) has the most significant effect on the channel occupancy. In Figure 6a, the throughput performance of the Wi-Fi backscatter and legacy Wi-Fi devices is almost the same, since both devices maintain the same value of CW\({}_{\text{min}}\). In this case, the throughput performance of each device is slightly different, since the device randomly selects the number of  
Figure 5: Variation of throughput for separate cases: (**a**) CSMA/CA starting with SIFS; and (**b**) Freezing of backoff period. backoff slots (i.e., [0, CW\({}_{\min}\)]). When the CW\({}_{\min}\) of the Wi-Fi backscatter devices is reduced to 7 and 3 (i.e., Figure 6b,c), the Wi-Fi backscatter devices obtain higher throughput compared with the case where CW\({}_{\min}\) = 15 because they have fewer backoff slots than the legacy Wi-Fi devices. In Figure 6d, CW\({}_{\min}\) is set to 1. In this case, the Wi-Fi backscatter devices exhibit the highest throughput. The reason for this is that the number of backoff slots, which is the main cause of channel-access delay, selected is only to \"0\" or \"1.\" However, the separate cases cannot guarantee dominant-channel occupancy for the Wi-Fi backscatter devices when the number of devices increases. This is because the increased number of devices leads to the increased number of legacy Wi-Fi devices with a small value of CW\({}_{\min}\). Figure 6: \(Cont.\)  
Figure 7 shows the variation of average throughput for the composite cases where two or more MAC configurations are simultaneously employed for the Wi-Fi backscatter devices. Similar to the separate cases, the channel occupancy of the Wi-Fi backscatter devices is higher than that of the legacy Wi-Fi devices. However, the dedicated devices in composite cases exhibit higher average throughput than separate cases. This is because the advantages of each configuration are compositely applied to the Wi-Fi backscatter devices. Specifically, as shown in Figure 7a, the Wi-Fi backscatter exhibit 128% higher average throughput when the CSMA/CA starting with SIFS and freezing of the backoff period are employed at the same time.