[Paper Review] Simplifying Backscatter Deployment: Full-Duplex LoRa Backscatter

Mohamad Katanbaf, Anthony Weinand+1 more|arXiv (Cornell University)|Nov 8, 2020

Full-Duplex Wireless Communications61 references17 citations

TL;DR

This paper presents the first low-cost, long-range, full-duplex LoRa backscatter reader using commodity LoRa chipsets and a microcontroller with passive components to achieve 78 dB self-interference cancellation. By employing a two-stage tunable impedance network and a hybrid coupler, the system enables co-located transmission and reception, supporting communication up to 300 ft line-of-sight and through obstacles in a 4,000 ft² office, with successful integration into a smartphone-sized form factor and a drone for precision agriculture.

ABSTRACT

Due to the practical challenges in the deployment of existing half-duplex systems, the promise of ubiquitous backscatter connectivity has eluded us. To address this, we design the first long-range full-duplex LoRa backscatter reader. We leverage existing LoRa chipsets as transceivers and use a microcontroller in combination with inexpensive passive elements including a hybrid coupler, inductors, tunable capacitors, and resistors to achieve 78 dB of self-interference cancellation and build a low-cost, long-range, and small-form-factor Full-Duplex LoRa Backscatter reader. We evaluate our system in various deployments and show that we can successfully communicate with a backscatter tag at distances of up to 300 ft in line of sight, and through obstacles, such as walls and cubicles, in a 4,000 ft$^2$ office area. We reconfigure our reader to conform to the size and power requirements of a smartphone, and demonstrate communication with a contact-lens-form-factor prototype device. Finally, we attach our reader to a drone and demonstrate backscatter sensing for precision agriculture with an instantaneous coverage of 7,850 ft$^2$.

Motivation & Objective

• Address the deployment challenges of half-duplex backscatter systems that require physically separated transmitter and receiver units.
• Overcome the limitations of existing full-duplex RFID and backscatter systems, which are large, complex, expensive, and have limited range.
• Enable practical, low-cost, long-range backscatter communication by leveraging commodity LoRa chipsets and passive components.
• Achieve deep self-interference cancellation (78 dB) using only passive elements and a microcontroller, avoiding expensive active components or SDRs.
• Demonstrate real-world applicability through integration into smartphone-sized readers, contact-lens-form-factor devices, and drone-based sensing.

Proposed method

• Use a single-antenna, hybrid-coupler-based architecture to interface the transmitter and receiver with the same RF port.
• Implement a novel two-stage tunable impedance network to dynamically track and cancel self-interference at the receiver by adjusting to antenna impedance variations.
• Leverage the microcontroller to adaptively tune the impedance network in real time, minimizing interference without requiring IQ samples.
• Achieve 78 dB of self-interference cancellation using only passive components (inductors, capacitors, resistors, diodes), avoiding bulky or expensive active RF components.
• Utilize the inherent baseband filtering of commodity LoRa receivers to suppress the residual interference at the offset frequency.
• Integrate the system with a LoRa transceiver and microcontroller to form a compact, low-power, and cost-effective full-duplex reader.

Experimental results

Research questions

• RQ1 Can a low-cost, full-duplex backscatter system be built using only commodity LoRa chipsets and passive components?
• RQ2 Is it feasible to achieve 78 dB of self-interference cancellation using a two-stage tunable impedance network without active cancellation circuits?
• RQ3 Can such a system support long-range communication (up to 300 ft) in real-world environments with obstacles like walls and cubicles?
• RQ4 Can the reader be miniaturized to fit in a smartphone or contact-lens-form-factor device while maintaining performance?
• RQ5 Can the system be deployed on a drone to enable wide-area backscatter sensing in precision agriculture?

Key findings

• The proposed system achieves 78 dB of self-interference cancellation using only passive components and a microcontroller, enabling full-duplex operation on a single LoRa chipset.
• The system successfully communicates with a backscatter tag at distances up to 300 ft in line-of-sight and through walls and cubicles in a 4,000 ft² office environment.
• The reader was reconfigured into a smartphone-sized form factor, demonstrating feasibility for integration into mobile and wearable devices.
• A contact-lens-form-factor prototype device was successfully communicated with using the reader, validating potential for medical and wearable applications.
• When mounted on a drone, the system enabled backscatter sensing over an instantaneous coverage area of 7,850 ft², demonstrating utility in precision agriculture.
• The system outperforms existing half-duplex backscatter systems in deployment simplicity and existing full-duplex systems in cost, size, and range, while avoiding the need for SDRs or expensive active components.