# Pattern-Based Decoding for Wi-Fi Backscatter Communication of Passive Sensors
## 2 Related Work  
There have been many research works on backscatter communications with various types of ambient signals. [PERSON] et al. [8] introduced ambient backscatter communication with TV signals that are available nationwide, thus not needing any infrastructure deployment. In Reference [9], two antennas are used by a backscatter receiver to reduce the direct-link interference of TV signals. In Reference [10], the authors used a full-duplex technique to enable immediate feedback by a tag on the same frequency channel. [PERSON] et al. [14] designed backscatter communication for FM radio signals. EkhoNet [15] implemented backscatter communication on UMass Moo and WISP platforms. In Reference [16], the authors proposed a universal backscatter reader that can receive backscattered signals for all available ambient broadcast radio sources. LoRea [17] is a backscatter architecture that combines techniques such as narrow-band backscatter transmissions and self-interference mitigation to achieve long communication range. Braidio [18] dynamically switches between active and passive communication modes according to the available energy. Some research works proposed tag designs for high-order QAM modulation [12; 13]. For reliable decoding of high-order modulated signals against noisy Wi-Fi carrier signals, the approach of the proposed algorithm can still be used, thus enabling better detection of amplitude changes as well as phase changes. Recent work studied Wi-Fi backscatter communication. In Reference [1], a tag backscatters transmissions from a Wi-Fi router, and off-the-shelf Wi-Fi devices decode data bits using per-frame CSI/RSSI values. BackFi [2] improves the data rate of Wi-Fi backscatter communication with a tag's phase-changing modulation and a full-duplex radio at the Wi-Fi router, which is a signal source and a receiver at the same time, to decode backscattered signals against self-interference, but at the expense of an increase of complexity. Passive Wi-Fi [3] generates 802.11b-compatible signals to transmit data bits by backscattering other Wi-Fi signals so that any Wi-Fi device can decode the signals. The work was extended in Reference [4] to generate 802.11b signals from the signals of Bluetooth devices via backscattering. [PERSON] and [PERSON] proposed a transceiver design for backscatter communication to cancel out direct-link interference over an OFDM carrier in Reference [5], and considered multiantenna receivers in Reference [6]. HitchHike [7] performs codeword translation of a received Wi-Fi signal at a tag and generates a signal decodable by a standard 802.11b receiver. FreeRider [19] extends HitchHike for communication with multiple commodity radios such as 802.11g/n Wi-Fi, ZigBee and Bluetooth with the support of multiple tags. Our algorithm requires more information (I/Q samples) from a Wi-Fi receiver chip, but does not rely on a specific version of 802.11. Frequency shift adopted by HitchHike [7] and FS-Backscatter [11] greatly enhances the communication range of Wi-Fi backscatter communication by avoiding the strong interference of a Wi-Fi carrier signal at the receiver side. Our algorithm can also be combined with frequency shift to improve its communication range. There have also been theoretical research works. [PERSON] et al. [20] considered two antennas at a backscatter transmitter and an energy detector that uses the difference in energy levels of the received signals to detect bits at a backscatter receiver. [PERSON] et al. [21] introduced a successive interference cancellation (SIC)-based detector that can achieve near-ML detection performance. [PERSON] et al. [22] formulated a transmission model, designed a data-detection algorithm, and derived a detection threshold in a closed form. In Reference [23], BER performance was derived when 8-PSK modulation and a noncoherent detector were adopted.