Leggiero: Analog WiFi Backscatter with Payload Transparency

2023-06-18

ACM SIGMOBILE International Conference on Mobile Systems, Applications, and Services

AI Summary

Battery-free backscatter sensing in AIoT faces two key bottlenecks: high microcontroller unit (MCU) power consumption and poor coexistence with WiFi. This paper proposes the first payload-transparent analog WiFi backscatter system: it eliminates the MCU entirely and directly modulates sensor signals onto the RF phase in the analog domain using a varactor diode, precisely embedding the modulation into the channel state information (CSI) phase of WiFi’s long training field (LTF). The approach enables end-to-end analog phase modulation and zero-intrusive embedding of raw analog sensor data into commodity WiFi CSI—without any hardware modification to existing WiFi devices. Experimental results show a mere 30 μW power consumption at 400 Hz sampling—4.8× lower than the state-of-the-art. Furthermore, the system supports uplink multimodal sensing while preserving the primary WiFi link’s throughput without degradation.

Technology Category

• Application Domains: Internet of Things, Sensor Networks & Smart Cities
• Machine Learning: Hardware-aware ML
• Humans and AI: Brain-Sensing and Analysis

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Security and Privacy: Large-scale security measurements
• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation

Abstract

Backscatter is an enabling technology for battery-free sensing in today's Artificial Intelligence of Things (AIOT). Building a backscatter-based sensing system, however, is a daunting task, due to two obstacles: the unaffordable power consumption of the microprocessor and the coexistence with the ambient carrier's traffic. In order to address the above issues, in this paper, we present Leggiero, the first-of-its-kind analog WiFi backscatter with payload transparency. Leveraging a specially designed circuit with a varactor diode, this design avoids using a microprocessor to interface between the radio and the sensor, and directly converts the analog sensor signal into the phase of RF (radio frequency) signal. By carefully designing the reference circuit on the tag and precisely locating the extra long training field (LTF) section of a WiFi packet, Leggiero embeds the analog phase value into the channel state information (CSI). A commodity WiFi receiver without hardware modification can simultaneously decode the WiFi and the sensor data. We implement Leggiero design and evaluate its performance under varied settings. The results show that the power consumption of the Leggiero tag (excluding the power of the peripheral sensor module) is 30μW at a sampling rate of 400Hz, which is 4.8× and 4× lower than the state-of-the-art WiFi backscatter schemes. The uplink throughput of Leggiero is suficient to support a variety of sensing applications, while keeping the WiFi carrier's throughput performance unaffected.

Problem

• Reducing power consumption in WiFi backscatter sensing systems
• Enabling coexistence with ambient WiFi carrier traffic
• Achieving payload transparency in analog WiFi backscatter

Innovation

• Analog WiFi backscatter with payload transparency
• Varactor diode circuit eliminates microprocessor
• Embeds sensor data in WiFi CSI