# Dominant Channel Occupancy for Wi-Fi Backscatter Uplink in Industrial Internet of Things
## 3 Design of the DCO
### High-Priority Channel Access  
The DCO guarantees the timeliness of Wi-Fi backscatter uplink communication by providing high-priority channel access to the Wi-Fi backscatter devices. For high-priority channel access, the DCO uses three types of MAC configurations: (1) CSMA/CA starting with SIFS; (2) freezing of the backoff  
Figure 1: Wi-Fi backscatter uplink communication: (**a**) architecture; and (**b**) operation.  
period; and (3) reduced CWmin. Note that the CSMA/CA operation of legacy Wi-Fi devices: (1) uses a DIFS, not a SIFS; (2) increases the backoff period twice when collision occurs; and (3) uses a fixed CWmin. The first MAC configuration reduces the waiting time of the Wi-Fi backscatter devices spent at the beginning of CSMA/CA by replacing the DIFS with SIFS. Specifically, it reduces the waiting time by \"2 \(\times\) slot time\", since DIFS is calculated as \"SIFS + 2 \(\times\) slot time.\" Consequently, Wi-Fi backscatter devices start the next step (i.e., the backoff procedure) earlier than the legacy Wi-Fi devices in the vicinity. The second configuration can reduce the backoff delay when collision occurs. After collision occurs, the DCO makes the Wi-Fi backscatter devices maintain the length of backoff period, given as [0, 2\({}^{\text{BE}}\)-1], where BE is the backoff exponent. The minimum and maximum BEs (i.e., _MacMinBE_ and _MacMaxBE_, respectively) are pre-determined by the system, and 2\({}^{MacMinBE}\)-1 is equal to CWmin [23; 24]. To maintain the length of the backoff period after collision, the DCO uses a fixed value of BE (i.e., _MacMinBE_) regardless of the collision. The freezing of the backoff period increases the probability that Wi-Fi backscatter devices will have fewer backoff slots than other legacy Wi-Fi devices. The last configuration can reduce the backoff delay for initial channel access by reducing the value of CWmin. The reduced CWmin decreases the maximum number of backoff slots for initial channel access. Considering that the number of backoff slots is randomly selected in [0, CWmin], the reduced CWmin makes the Wi-Fi backscatter devices have fewer backoff slots than legacy Wi-Fi devices for initial channel access.  
Figure 2 shows a comparison of initial channel access between (Figure 2a) the DCO and (Figure 2b) legacy Wi-Fi. In the figure, the DCO defers the backoff procedure for SIFS, which is a shorter waiting time than that of legacy Wi-Fi devices. In addition, the DCO has shorter backoff delay than the legacy Wi-Fi because it uses the reduced CWmin. Since the channel-access delay of the DCO is shorter than that of the legacy Wi-Fi, it can provide high-priority channel access to Wi-Fi backscatter devices, which allows the Wi-Fi backscatter devices to transmit a packet earlier than others. The legacy Wi-Fi device defers its own transmission when it detects a busy channel via carrier sensing; thus, the high-priority channel access can guarantee the timeliness of the Wi-Fi backscatter devices regardless of the legacy Wi-Fi devices in the vicinity.  
Figure 2: Comparison of initial channel access: (**a**) DCO; and (**b**) legacy Wi-Fi.