# Dominant Channel Occupancy for Wi-Fi Backscatter Uplink in Industrial Internet of Things
## 1 Introduction
Moreover, successive transmissions of multiple CTS-to-self packets are not available due to CSMA/CA operations, thus it cannot be a suitable solution to the interference problem. Several studies have tried to differentiate Wi-Fi device channel access through MAC parameter configurations. [PERSON] et al. [18] proposed a PoWiFi system that tunes the contention window (i.e., CW\({}_{\text{min}}\) and CW\({}_{\text{max}}\)) to provide high-priority channel access. However, it only considers one power packet transmission; thus, it suffers from long channel access delays caused by CSMA/CA operations when a series of packets are transmitted to a sensor device. Furthermore, it only tunes the CW\({}_{\text{min}}\) and CW\({}_{\text{max}}\), and such consideration cannot provide dominant priority channel access to Wi-Fi backscatter devices (i.e., Wi-Fi readers and Wi-Fi backscatter tags) while also supporting their burst transmissions. In [19], [PERSON] et al. proposed a dynamic MAC parameter configuration approach based on the IEEE 802.11e standard to maximize network throughput. It uses different arbitration inter-frame space (AIFS) instead of the distributed inter-frame space (DIFS) and different CW\({}_{\text{min}}\) depending on the network congestion status; however, it cannot support Wi-Fi backscatter devices' burst transmissions. In this paper, we propose a dominant channel occupancy (DCO) mechanism for the Wi-Fi backscatter uplink that provides high-priority channel access and reliable burst transmission to Wi-Fi backscatter devices for guaranteeing the timeliness and reliability of the IIoT. The DCO enables the Wi-Fi backscatter tag to deliver its physical sensor data (i.e., tag information) to the Wi-Fi reader without interference from neighboring legacy Wi-Fi devices. In DCO, for high-priority channel access of Wi-Fi backscatter uplink, we consider the following three types of MAC configurations: (1) CSMA/CA starting with short inter-frame space (SIFS); (2) freezing of the backoff period; and (3) reduced CW\({}_{\text{min}}\). On the other hand, the CSMA/CA operation of legacy Wi-Fi: (1) uses a DIFS, not a SIFS; (2) increases the backoff period twice when collision occurs; and (3) uses a fixed CW\({}_{\text{min}}\). Furthermore, we employ SIFS between consecutive packets instead of CSMA/CA to guarantee reliable burst transmission, which means the transmission of a series of same-size packets. The experimental simulations are conducted to verify the effectiveness of the DCO and determine the proper values of the MAC parameters. Specifically, we investigate the channel occupancy of Wi-Fi backscatter devices by comparing the throughput between Wi-Fi backscatter and legacy Wi-Fi devices. The simulations are performed under IEEE 802.11n PHY/MAC environments [20], and both separate and composite cases are observed to check the impacts of each configuration. The simulation results show that the DCO can provide high-priority channel access and dominant channel occupancy to the Wi-Fi backscatter devices in the shared wireless channel. In particular, the reduced CW\({}_{\text{min}}\) has the most significant effect on throughput performance. When two or more MAC configurations are used simultaneously, the Wi-Fi backscatter devices achieve much higher throughput than the separate cases. The results also show that the Wi-Fi backscatter devices can dominantly occupy the shared wireless channel for burst transmission without any interruption by using the SIFS between consecutive packets. The contributions of our work can be summarized as follows. * We propose a new MAC layer solution for the Wi-Fi backscatter uplink, which is essential in most IIoT applications. As mentioned above, the existing Wi-Fi backscatter approach mainly focuses on downlink communication, but most IIoT monitoring applications make little use of their downlink and only require a very small bandwidth (e.g., a few kilobits per second) for command transmissions if utilized.