Selected Publications
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Position: Wearable Polymorphic Light Sensors.
Ambuj Varshney
ACM WearSys 2019 (co-located with ACM MobiSys) 2019, Seoul, South Korea
Light as a medium for sensing and communication enables new scenarios, such as controlling devices with gestures, or communi- cation for Internet of Things (IoT) devices. However, a limitation of existing systems is that they often sense only a narrow part of the light spectrum. We argue that the ability to sense a broad light spectrum significantly enhances ability of such systems expand- ing possible application scenarios. We demonstrate our work in progress to develop the concept of polymorphic light sensing (PLS). PLS sensor morphs itself according to applications requirements, to track desired parts of the light spectrum (colours, infrared, and ultraviolet light). We couple the PLS sensor with ultra-low power backscatter mechanism, and demonstrate this enables us to sense and communicate the broad spectrum, while operating battery-free.
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TunnelScatter: Low Power Communication for Sensor Tags using Tunnel Diodes
Ambuj Varshney, Andreas Soleiman, Thiemo Voigt
ACM MOBICOM 2019, Mexico
Slides
Due to extremely low power consumption, backscatter has become the transmission mechanism of choice for battery-free devices that operate on harvested energy. However, a limitation of recent backscatter systems is that the communication range scales with the strength of the ambient carrier signal~(ACS). This means that to achieve a long-range, a backscatter tag needs to reflect a strong ACS, which in practice means that it needs to be close to an ACS emitter. We present TunnelScatter, a mechanism that overcomes this limitation. TunnelScatter uses a tunnel diode-based radio frequency oscillator to enable transmissions when there is no ACS, and the same oscillator as a reflection amplifier to support backscatter transmissions when the ACS is weak. Our results show that even without an ACS, TunnelScatter is able to transmit through several walls covering a distance of 18m while consuming a peak biasing power of 57 micro watts. Based on TunnelScatter, we design battery-free sensor tags, called TunnelTags, that can sense physical phenomena and transmit them using the TunnelScatter mechanism.
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Connecting Battery-free IoT Tags Using LED Bulbs
Domenico Giustiniano , Ambuj Varshney, Thiemo Voigt
ACM HotNets 2018, Seattle, USA
We introduce BackVLC, a system to connect battery-free IoT tags using LED bulbs. We make use of bulbs beyond illumination. We send data to the tags with visible light communication (VLC), and retrofit the bulbs with simple circuitry to enable the uplink channel current VLC systems lack, using Radio Frequency (RF) backscatter communication from the tags. Tags process and send data, harvesting energy from light and radio. We present our system design and implementation, evaluate it in preliminary simulation studies and experiments, and discuss the research challenges to develop a complete network architecture. BackVLC is the first work that combines
VLC with RF backscatter.
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Visible Light Communication for Wearable Computing
Ambuj Varshney, Luca Mottola, Thiemo Voigt
Preprint
Visible Light Communication (VLC) is emerging as a means to network computing devices that ameliorates many hurdles of radio-frequency (RF) communications, for example, the limited available spectrum. Enabling VLC in wearable computing, however, is challenging because mobility induces unpredictable drastic changes in light conditions, for example, due to reflective surfaces and obstacles casting shadows. We experimentally demonstrate that such changes are so extreme that no single design of a VLC receiver can provide efficient performance across the board. The diversity found in current wearable devices complicates matters. Based on these observations, we present three different designs of VLC receivers that i) are individually orders of magnitude more efficient than the state-of-the-art in a subset of the possible conditions, and i) can be combined in a single unit that dynamically switches to the best performing receiver based on the light conditions. Our evaluation indicates that dynamic switching incurs minimal overhead, that we can obtain throughput in the order of MBit/s, and at energy costs lower than many RF devices.
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Battery-free 802.15.4 Receiver
Carlos Perez Penichet, Claro Noda, Ambuj Varshney, Thiemo Voigt
ACM/IEEE IPSN 2018, Porto, Portugal
In this paper, We present the architecture of an 802.15.4 receiver that, for the first time, operates at a few hundred microwatts, enabling new battery free applications. To reach the required micro-power consumption, the architecture diverges from that of commodity receivers in two important ways. First, it offloads the power-hungry local oscillator to an external device, much like backscatter transmitters do. Second, we avoid the energy cost of demodulating a phase-modulated signal by treating 802.15.4 as a frequency-modulated one, which allows us to receive with a simple passive detector and an energy efficient thresholding circuit. We describe a prototype that can receive 802.15.4 frames with a power consumption of 361 μW.
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LoRea: A Backscatter architecture that achieves a long communication range
Ambuj Varshney, Oliver Harms, Carlos Penichet, Christian Rohner , Frederik Hermans, Thiemo Voigt
ACM SENSYS 2017, Delft, Netherlands
There is the long-standing assumption that radio communication in the range of hundreds of meters needs to consume mWs of power at the transmitting device. In this paper, we demonstrate that this is not necessarily the case for some devices equipped with backscatter radios. We present LOREA an architecture consisting of a tag, a reader and multiple carrier generators that overcomes the power, cost and range limitations of existing systems such as Computational Radio Frequency Identification (CRFID). An off-the-shelf implementation of LOREA costs 70 USD, a drastic reduction in price considering commercial RFID readers cost 2000 USD. LOREA's range scales with the carrier strength, and proximity to the carrier source and achieves a maximum range of 3.4 km when the tag is located at 1 m distance from a 28 dBm carrier source while consuming 70 micro watt at the tag. When the tag is equidistant from the carrier source and the receiver, we can communicate upto 75 m, a significant improvement over existing RFID readers.
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Battery-free Visible Light Sensing
Ambuj Varshney, Andreas Soleiman, Luca Mottola, Thiemo Voigt
ACM VLCS (Co-located with ACM MobiCom) 2017, Utah, USA
Best Paper Award
ACM Student Research Competition Winner
We present the design of the first Visible Light Sensing (VLS) system that consumes only tens of μWs of power to sense and communicate. Unlike most existing VLS systems, we require no modification to the existing light infrastructure since we use unmodulated light as a sensing medium. We achieve this by designing a novel mechanism that uses solar cells to achieve a sub- micro watt power consumption for sensing. Further, we devise an ultra-low power transmission mechanism that backscatters sensor readings and avoids the processing and computational overhead of existing sensor systems. Our initial results show the ability to detect and transmit hand gestures or presence of people up to distances of 330 m, at a peak power of 20 micro watts. Further, we demonstrate that our system can operate in diverse light conditions (100 lx to 80 klx) where existing VLS designs fail due to saturation of the transimpedance amplifier (TIA).
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Towards wide-area backscatter networks
Ambuj Varshney, Carlos Perez Penichet, Christian Rohner, Thiemo Voigt
ACM HotWireless (Co-located with ACM MobiCom) 2017, Utah, USA
Backscatter communication reflecting or absorbing ambient wireless signals enables transmissions at several orders of magnitude lower energy cost when compared to conventional low-power radios. The past few years have seen signficiant progress with systems demonstrating the ability to synthesise transmissions that are compatible with WiFi, ZigBee or BLE at micro watts of power consumption. However, these systems achieve a maximum communication range of tens of meters which severely limits the possible applications. On the other hand, our recent system LoRea demonstrates that backscatter communication can achieve a significantly longer range reaching upto a few kms when the tag is co-located with the carrier source. In our vision, such a large range could be a key enabler to develop wide-area networks of battery-free sensors. In this paper, we build on our system LoRea and identify issues of improving the reliability of weak backscatter links, increasing the range and supporting the operation of multiple tags as the key challenge to our vision, and present our preliminary efforts to address them.
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modBulb: A Modular Light Bulb for Visible Light Communication
Kasun Hewage, Ambuj Varshney , Abdalah Hilamia, Thiemo Voigt Thiemo Voigt
ACM VLCS (Co-located with ACM MobiCom) 2016, New York, USA
Due to several interesting properties such as large bandwidth and immunity against radio interference, Visible Light Com- munication (VLC) has caught the attention of the research community. Current efforts are, however, hampered by the lack of open source platforms. We present modBulb, an open, modular light bulb. modBulb is a VLC transmitter that can be customized to the application’s requirements. modBulb enables modulation and other processing through an MCU for flexibility and ease of programming or an FPGA for applications that require higher efficiency. Furthermore, modBulb supports several driving circuits to balance the trade-off between energy efficiency and switching noise. Last but not least, the light source itself can be selected according to the application’s requirements. We present experiments that demonstrate modBulb’s salient properties.
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Augmenting IoT Networks with Backscatter-Enabled Passive Sensor Tags
Carlos Perez Penichet, Frederik Hermans, Ambuj Varshney , Thiemo Voigt
ACM HotWireless (Co-located with ACM MobiCom) 2016, New York, USA
The sensing modalities available in an Internet-of-Things (IoT) network are usually fixed before deployment, when the operator selects a suitable IoT platform. Retrofitting
a deployment with additional sensors can be cumbersome, because it requires either modifying the deployed hardware or adding new devices that then have to be maintained. In this paper, we present our vision and work towards passive sensor tags: battery-free devices that allow to augment existing IoT deployments with additional sensing capabilities without the need to modify the existing deployment. Our passive sensor tags use backscatter transmissions to communicate with the deployed network. Crucially, they do this in a way that is compatible with the deployed network’s radio protocol, and without the need for additional infrastructure. We present an FPGA-based prototype of a passive sensor tag that can communicate with unmodified 802.15.4 IoT devices. Our initial experiments with the prototype support the feasibility of our approach. We also lay out the next steps towards fully realizing the vision of passive sensor tags.
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Directional Transmissions and Receptions for High-throughput Bulk Forwarding in Wireless Sensor Networks
Ambuj Varshney, Luca Mottola, Mats Carlsson, Thiemo Voigt
ACM SenSys 2015, Seoul, South Korea
We present DPT: a wireless sensor network protocol for bulk traffic that uniquely leverages electronically switchable directional (ESD) antennas. Bulk traffic is found in several scenarios and supporting protocols based on standard antenna technology abound. ESD antennas may improve performance in these scenarios; for example, by reducing chan- nel contention as the antenna can steer the radiated energy only towards the intended receivers, and by extending the communication range at no additional energy cost. The corresponding protocol support, however, is largely missing. DPT addresses precisely this issue. Our results, obtained in a real testbed using 802.15.4-compliant radios and custom ESD antennas we built, indicate that DPT ap- proaches the maximum throughput supported by the link layer, peaking at 214 kbit/s in the settings we test.
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dRTI: Directional Radio Tomographic Imaging
Bo Wei, Ambuj Varshney, Wen Hu, Neal Patwari, Thiemo Voigt , Chun Tung Chou
ACM/IEEE IPSN 2015, Seattle, USA
Radio tomographic imaging (RTI) enables device free local- isation of people and objects in many challenging environ- ments and situations. Its basic principle is to detect the changes in the statistics of radio signals due to the radio link obstruction by people or objects. However, the local- isation accuracy of RTI suffers from complicated multipath propagation behaviours in radio links. We propose to use inexpensive and energy efficient electronically switched di- rectional (ESD) antennas to improve the quality of radio link behaviour observations, and therefore, the localisation accuracy of RTI. We implement a directional RTI (dRTI) system to understand how directional antennas can be used to improve RTI localisation accuracy. We also study the impact of the choice of antenna directions on the localisa- tion accuracy of dRTI and propose methods to effectively choose informative antenna directions to improve localisa- tion accuracy while reducing overhead. Furthermore, we analyse radio link obstruction performance in both theory and simulation, as well as false positives and false negatives of the obstruction measurements to show the superiority of the directional communication for RTI. We evaluate the per- formance of dRTI in diverse indoor environments and show that dRTI significantly outperforms the existing RTI locali- sation methods based on omni-directional antennas.
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A complete list of my publications can be found at this page .
Top-tier ACM conferences and workshops in mobile computing, computer networking and networked embedded systems, i.e., MOBICOM, SENSYS, IPSN, and HOTNETS, are highly selective venues selecting on an average one out of five submitted papers. Conferences are the primary source to dissementate scientific results in my area of research. These conferences are comparable or exceed many IEEE journals in their selectivity, visibility, and impact.
