# Dominant Channel Occupancy for Wi-Fi Backscatter Uplink in Industrial Internet of Things
## 1 Introduction  
Recently, Wi-Fi backscatter has been considered as one of the emerging communication technologies that may help realize the industrial Internet of things (IIoT), which can improve the connectivity, efficiency, and scalability of smart machines in both enterprises' manufacturing processes and supply chain monitoring and management systems [1; 2; 3; 4; 5]. Wi-Fi backscatter uses radio frequency (RF) signals as a power source and reuses the existing Wi-Fi infrastructure to provide RF-powered devices with Internet connectivity. Therefore, it allows billions of Wi-Fi devices to connect to the Internet without charging or swapping batteries. The Wi-Fi backscatter consists of two types of communication: uplink communication and downlink communication. In uplink communication, the RF-powered device transmits its tag information to the Wi-Fi device, while the Wi-Fi device transmits data to the RF-powered device via downlink communication [6]. In particular, uplink communication is important for IIoT, since most IIoT applications aim to provide monitoring services using a number of physical sensors, which generate a huge amount of uplink-centric traffic [7; 8]. However, the uplink communication of the existing Wi-Fi backscatter is easily interfered with by the transmissions of legacy Wi-Fi devices in the vicinity, since it commonly uses a carrier sense multipleaccess with collision avoidance (CSMA/CA) mechanism, as defined in the Wi-Fi standards [9; 10; 11; 12]. Thus, it is difficult to guarantee timeliness and reliability, which are essential system requirements of the IIoT. The existing Wi-Fi backscatter cannot satisfy the strict delay constraint of the IIoT because of the long channel access delay caused by CSMA/CA operations and suffers from frequent packet losses and data corruptions. This is because the interferences of the neighboring legacy Wi-Fi devices make it difficult to guarantee consecutive transmissions of the Wi-Fi backscatter tag (i.e., RF-powered device). Note that Wi-Fi backscatter tags only transmit one bit of their own tag information according to whether they are reflecting the Wi-Fi signal. [PERSON] et al. [13] developed a special type of shield, called a comb reflection frequency selective surface (CR-FSS) that can be installed on a wall to physically prevent interference between Wi-Fi access points (APs). However, CR-FSS cannot prevent interference between Wi-Fi APs placed in open spaces. [PERSON] et al. [14] presented a frame length and charging time adaptation in computational radio frequency identification (CRFID) backscatter communication, for which CRFID devices would use a dedicated link to communicate with each other. The use of a dedicated link might help prevent interference, but it cannot be adopted in the case of a Wi-Fi backscatter uplink where a number of Wi-Fi backscatter tags share a common channel through which they deliver their tag information. It is strongly necessary to design a new medium access control (MAC) protocol for dedicated use in the Wi-Fi backscatter uplink to address the interference problem in Wi-Fi backscatter uplink communication, since the MAC protocol decides which device is used, and both when and how long it occupies the wireless channel when a number of network devices are trying to transmit packets simultaneously [15; 16]. [PERSON] et al. [6] and [PERSON] et al. [17] developed dedicated MAC protocols for the downlink communication of Wi-Fi backscatter, which commonly use CTS-to-self packets to prevent interference with the neighboring legacy Wi-Fi devices. The CTS-to-self packet is used to set the network allocation vector (NAV) for neighboring legacy Wi-Fi devices to defer their transmissions by up to 32 ms, which is insufficient to deliver uplink-centric traffic from the Wi-Fi backscatter tag in IIoT applications. Specifically, the Wi-Fi reader should use multiple channel measurements to decode each bit transmitted by the Wi-Fi backscatter tag in the Wi-Fi backscatter uplink, due to the noise of the environment.