Ambuj Varshney

dRTI: directional radio tomographic imaging

Radio tomographic imaging (RTI) enables device free localisation of people and objects in many challenging environments 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 localisation accuracy of RTI suffers from complicated multipath propagation behaviours in radio links. We propose to use inexpensive and energy efficient electronically switched directional (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 localisation accuracy of dRTI and propose methods to effectively choose informative antenna directions to improve localisation 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 performance of dRTI in diverse indoor environments and show that dRTI significantly outperforms the existing RTI localisation methods based on omni-directional antennas.

LoRea: A Backscatter Architecture that Achieves a Long Communication Range

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). LOREA achieves this by: First, generating narrow-band backscatter transmissions that improve receiver sensitivity. Second, mitigating self-interference without the complex designs employed on RFID readers by keeping carrier signal and backscattered signal apart in frequency. Finally, decoupling carrier generation from the reader and using devices such as WiFi routers and sensor nodes as a source of the carrier signal. An off-the-shelf implementation of LOREA costs 70 USD, a drastic reduction in price considering commercial RFID readers cost 2000 USD. LOREAs 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 μW 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.

Augmenting IoT networks with backscatter-enabled passive sensor tags

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 networks 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.

Battery-free Visible Light Sensing

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-μW 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 330m at a peak power of μWs. 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).

Using Directional Transmissions and Receptions to Reduce Contention in Wireless Sensor Networks

Electronically Switched Directional (ESD) antennas allow software-based control of the direction of maximum antenna gain. ESD antennas are feasible for wireless sensor network. Existing studies with these antennas focus only on controllable directional transmissions. These studies demonstrate reduced contention and increased range of communication with no energy penalty. Unlike existing literature, in this chapter we experimentally explore controllable antenna directionality at both sender and receiver. One key outcome of our experiments is that directional transmissions and receptions together considerably reduce channel contention. As a result, we can significantly reduce intra-path interference.

Directional transmissions and receptions for high throughput burst forwarding

Many sensor network applications generate large amounts of sensed data. These often need to be delivered reliably to the sink node for further processing. In such applications, high communication throughput allows for more data to be sensed. Intra-path interference is a problem in reliable forwarding of data and affects the end-to-end throughput. We show that using antennas that allow directional transmissions and receptions significantly reduces intra-path interference and enables high throughput forwarding of packet bursts over multiple hops using only one wireless channel.

Towards Wide-area Backscatter Networks

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 μWs 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 up to 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.

Poster Abstract: LocaLight - A Battery-Free Passive Localization System Using Visible Light

Most existing indoor localization systems are battery-powered and use the changes in Radio Frequency (RF) signals to localize objects. In this paper, we present LocaLight: a battery-free indoor localization system that localizes objects using visible light by tracking the shadow they cast. By sensing a drop in the intensity of ambient light caused by the presence of a shadow, LocaLight localizes the object. Since the position of the shadow can be predicted, it is possible to localize the object in a sensitive area by carefully positioning the light sensors and the overhead lights. Our initial results suggest that LocaLight achieves an accuracy comparable to many of the state-of-the art solutions that use RF.

Passive sensor tags: demo

The sensing capabilities of an Internet-of-Things (IoT) network are usually fixed at deployment. Adding new sensing modalities is a cumbersome process because it requires altering the deployed hardware. We introduce passive sensor tags that allow to easily and seamlessly add new sensors to existing IoT deployments without requiring hardware modifications or additional energy sources. Passive sensor tags employ backscatter communication to generate transmissions that can be decoded by the radio transceivers present in todays IoT devices. Furthermore, unlike recent works, our approach does not require dedicated infrastructure to generate the unmodulated carrier used for backscatter communication. The demo showcases our prototype of a passive sensor tag collecting sensor data and delivering it to unmodified commodity IoT devices using passive 802.15.4 transmissions.

modBulb: a modular light bulb for visible light communication

Due to several interesting properties such as large bandwidth and immunity against radio interference, Visible Light Communication (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 applications 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 applications requirements. We present experiments that demonstrate modBulbs salient properties.