WiTAG: Seamless WiFi Backscater Communication Ali Abedi Cheriton School of Computer Science University of Waterloo ali.abedi@uwaterloo.ca Farzan Dehbashi Cheriton School of Computer Science University of Waterloo farzan.dehbashi@uwaterloo.ca Mohammad Hossein Mazaheri Cheriton School of Computer Science University of Waterloo mh2mazah@uwaterloo.ca Omid Abari Computer Science Department UCLA omid@cs.ucla.edu Tim Brecht Cheriton School of Computer Science University of Waterloo brecht@cs.uwaterloo.ca ABSTRACT WiFi backscatter communication has the potential to enable battery- free sensors which can transmit data using a WiFi network. In order for WiFi backscatter systems to be practical they should be compat- ible with existing WiFi networks without any hardware or software modiications. Moreover, they should work with networks that use encryption. In this paper, we present WiTAG which achieves these requirements, making the implementation and deployment of WiFi backscatter communication more practical. In contrast with existing systems which utilize the physical layer for backscatter communication, we take a diferent approach by leveraging fea- tures of the MAC layer to communicate. WiTAG is designed to send data by selectively interfering with subframes (MPDUs) in an aggregated frame (A-MPDU). This enables standard compliant com- munication using modern, open or encrypted 802.11n and 802.11ac networks without requiring hardware or software modiications to any devices. We implement WiTAG using of-the-shelf components and evaluate its performance in line-of-sight and non-line-of-sight scenarios. We show that WiTAG achieves a throughput of up to 4 Kbps without impacting other devices in the network. CCS CONCEPTS • Networks → Network architectures; Wireless access points, base stations and infrastructure; • Hardware → Wireless devices; Wireless integrated network sensors. KEYWORDS Battery-free communication; WiFi Backscatter; Internet of Things (IoT); 802.11 Networks; Sensors ACM Reference Format: Ali Abedi, Farzan Dehbashi, Mohammad Hossein Mazaheri, Omid Abari, and Tim Brecht. 2020. WiTAG: Seamless WiFi Backscatter Communication. In Annual conference of the ACM Special Interest Group on Data Communica- tion on the applications, technologies, architectures, and protocols for computer Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proit or commercial advantage and that copies bear this notice and the full citation on the irst page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior speciic permission and/or a fee. Request permissions from permissions@acm.org. SIGCOMM ’20, August 10ś14, 2020, Virtual Event, NY, USA © 2020 Copyright held by the owner/author(s). Publication rights licensed to ACM. ACM ISBN 978-1-4503-7955-7/20/08. . . $15.00 https://doi.org/10.1145/3387514.3405866 communication (SIGCOMM ’20), August 10ś14, 2020, Virtual Event, NY, USA. ACM, New York, NY, USA, 13 pages. https://doi.org/10.1145/3387514.3405866 1 INTRODUCTION Backscatter systems are very attractive as a means of communica- tion for wireless sensors in applications ranging from implantable body sensors to farming [21, 31, 32, 38]. Because they do not re- quire batteries they can have a lower cost, smaller form factor, and require less maintenance. Traditional backscatter systems (such as RFIDs) require a specialized reader to read the tag values. The high cost and large form factor of these readers have made them diicult to deploy and have limited the adoption of RFID tags in many appli- cations. To overcome these challenges, researchers have designed WiFi backscatter systems. The vision is to design backscatter tags that can be read using WiFi devices, thus reducing the complexity and cost of deploying backscatter systems by using existing WiFi infrastructures instead of specialized readers. Wi-Fi backscatter [14] is the irst WiFi backscatter system that en- ables communication with commodity WiFi devices. Unfortunately, due to self-interference between WiFi transmission and backscatter signals the range of this system is very limited [38]. BackFi [4] and Passive WiFi [15] try to increase the range of communica- tion, however they require specialized hardware which hinders the widespread deployment of these systems. Ideally WiFi backscatter systems need to satisfy the following key requirements: • Compatible with existing WiFi access points: They should be compatible with already deployed commodity access points (APs), including 802.11n and 802.11ac standards, without re- quiring hardware or software changes. • Work with encrypted WiFi networks: Most WiFi net- works are secured using encryption. Therefore, WiFi backscat- ter systems should work with WiFi networks that use WPA or WEP encryption. • Battery-free: Similar to traditional backscatter tags (RFIDs), WiFi backscatter tags need to be extremely low-power so that they can harvest their energy from the environment and operate without requiring a battery. If the above requirements are satisied, we can envision hav- ing battery-free wireless sensors which do not require specialized readers and can be deployed in environments with existing WiFi infrastructure. However, to the best of our knowledge, no current backscatter system satisies all of these requirements. Recently, a group of systems namely, FS-Backscatter [38], HitchHike[36],