Ashay Dhamdhere

Transmission Power Control in Body Area Sensor Networks for Healthcare Monitoring

This paper investigates the opportunities and challenges in the use of dynamic radio transmit power control for prolonging the lifetime of body-wearable sensor devices used in continuous health monitoring. We first present extensive empirical evidence that the wireless link quality can change rapidly in body area networks, and a fixed transmit power results in either wasted energy (when the link is good) or low reliability (when the link is bad). We quantify the potential gains of dynamic power control in body-worn devices by benchmarking off-line the energy savings achievable for a given level of reliability.We then propose a class of schemes feasible for practical implementation that adapt transmit power in real-time based on feedback information from the receiver. We profile their performance against the offline benchmark, and provide guidelines on how the parameters can be tuned to achieve the desired trade-off between energy savings and reliability within the chosen operating environment. Finally, we implement and profile our scheme on a MicaZ mote based platform, and also report preliminary results from the ultra-low-power integrated healthcare monitoring platform we are developing at Toumaz Technology.

Experimental study of mobility in the soccer field with application to real-time athlete monitoring

Live monitoring of athletes during sporting events can help maximise performance while preventing injury, and enable new applications such as referee-assist and enhanced television broadcast services. A major challenge is the extraction of athlete physiological data in real-time, since the radio range of body-worn sensor devices is limited, necessitating multi-hop routing mechanisms. However, little is known about the highly dynamic operating conditions on a soccer field under which communication protocols need to operate. In this work we conduct field experiments in which we outfit first-division soccer players with sensor devices and record their inter-connectivity during a real game. Our first contribution profiles the key properties of the dynamic wireless topologies arising in the soccer field, and highlights the consequences for routing mechanisms. We show that the topology is in general sparse, with short encounters and power-law distributed inter-encounters. Importantly, the co-ordinated movement of players in the field gives rise to significant correlations amongst links, an aspect that can potentially be exploited by routing. Our second contribution develops a model for generating synthetic topologies that mirror connectivity in a real soccer game, and can be used for simulation studies of routing mechanisms. Its novelty lies in explicitly modelling the underlying auto-correlation and cross-correlation properties of the links, from which derived measures such as inter-encounter times and neighbourhood distributions follow. Our study is an important first step towards understanding and modelling dynamic topologies associated with sports monitoring, and paves the way for the design of real-time routing algorithms for such environments.

Experiments with wireless sensor networks for real-time athlete monitoring

Real-time physiological monitoring of athletes during sporting events has tremendous potential for maximizing player performance while preventing burn-out and injury, and also enabling exciting new applications such as referee-assist services and enhanced television broadcast. Emerging advanced monitoring devices have the right combination of light weight and unobtrusive size to allow truly non-intrusive monitoring during competition. However their small battery capacities, limited wireless ranges and susceptibility to body effects make real-time data extraction a challenge, particularly in sports with a large playing area. In this work we present the novel application of body area sensor networks to monitoring soccer players in a soccer field. We begin by outlining the challenges in experimental data collection and elaborate on the design choices we have made. Secondly, we show that the inherent characteristics of the operating environment lead to unacceptably high delays for direct transmissions from the players to the base stations. This leads to our third contribution, namely a multi-hop routing protocol that balances between the competing objectives of resource consumption and delay.

Algorithms for Transmission Power Control in Biomedical Wireless Sensor Networks

Wireless sensor networks are increasingly being used for continuous monitoring of patients with chronic health conditions such as diabetes and heart problems. As biomedical sensor nodes become more wearable, their battery sizes diminish, necessitating very careful energy management. This paper proposes feedback-based closed-loop algorithms for dynamically adjusting radio transmit power in body-worn devices, and evaluates their performance in terms of energy savings and reliability as the data periodicity and feedback time-scales vary. Using experimental trace data from body worn devices, we first show that the performance of dynamic power control is adversely affected at long data periods. Next for a given data period we show that modifying the transmit power at too long timescales (around a minute) reduces the efficacy of dynamic power control, while too short a time-scale (few seconds or less) incurs a high feedback signaling overhead. We therefore advocate an intermediate range of time-scales (when permitted by the data periodicity), typically in the few tens of seconds, at which the control algorithms should adapt transmit power in order to achieve maximal energy savings in body-worn sensor devices used for medical monitoring.

An experimental study of wireless connectivity and routing in ad hoc sensor networks for real-time soccer player monitoring

Live physiological monitoring of soccer players during sporting events can help maximise athlete performance while preventing injury, and enable new applications for referee-assist and enhanced television broadcast services. However, the harsh operating conditions in the soccer field pose several challenges: (a) body-mounted wireless sensor devices have limited radio range, (b) playing area is large, necessitating multi-hop transmission, (c) wireless connectivity is dynamic due to extreme mobility, and (d) data forwarding has to operate within tight delay/energy constraints. In this paper, we take a first step towards characterising wireless connectivity in the soccer field by undertaking experimental work with local soccer clubs, and assess the feasibility of real-time athlete monitoring. We make three specific contributions: (1) We develop an empirical profile of radio signal strength in an open soccer field taking into account distance and body orientation of the athlete. (2) Using data from several soccer games we profile key characteristics of wireless connectivity, highlighting aspects such as small power-law inter-encounters and link correlations. (3) We develop practical multi-hop routing algorithms that can be tuned to achieve the right balance between the competing objectives of resource consumption and data extraction delay. We believe our study is the first to characterise the wireless environment for mobile sensor networks in field sports, and paves the way towards realisation of real-time athlete monitoring systems.

Secure key loss recovery for network broadcast in single-hop wireless sensor networks

Symmetric encryption of data at the base-station using time-varying keys has been proposed as an attractive method for securing broadcasts in wireless sensor networks: symmetric decryption keeps computational costs at sensor nodes low, while time-varying group keys protect the network against key compromise at any of the receivers. However, a significant problem is that interference or disconnections may cause a receiver to miss broadcast packets and the dynamic keys contained therein, rendering it unable to participate in subsequent broadcasts. In this paper, we develop a scheme which allows receivers to recover from key loss in a secure, efficient, and scalable manner. Our scheme appends recovery information to each broadcast message to help out-of-sync receivers reattach probabilistically using an older key. We analyze our scheme to quantify the recovery probability as a function of system parameters, and deduce fundamental asymptotic bounds on recovery. We further prototype our scheme on the MicaZ mote platform and show that it is lightweight and efficient. Our solution offers a highly configurable, efficient and scalable method for key recovery in large sensor networks that require secure broadcasts.

Modeling signal strength of body-worn devices

Body-wearable devices for physiological monitoring are fast becoming a reality — by 2014, 420 million wearable wireless devices are expected to be in use, of which 90% will be for sports and fitness applications. We envisage the use of ultra-lightweight wearable devices for monitoring athletes in field sports such as soccer for quantifying, assessing and improving game performance. To this end, in this paper we present an empirical characterization of the radio signal strength of sensor devices mounted on an athletes body. We fit simple analytical models to our empirical data, highlighting how the signal degrades with distance as well as orientation of the body. Our model aids in improved protocol design and locationing services that take into account propagation effects of the human body.

Experiments in Adaptive Power Control for Truly Wearable Biomedical Sensor Devices

Emerging body-wearable devices for continuous health monitoring are severely energy constrained and yet required to offer high communication reliability under fluctuating channel conditions. Such devices require very careful management of their energy resources in order to prolong their lifetime. In our earlier work we had proposed dynamic power control as a means of saving precious energy in off the-shelf sensor devices. In this work we experiment with a real body-wearable device to assess the power savings possible in a realistic setting. We quantify the power consumption against the packet loss and establish the feasibility of dynamic power control for saving energy in a truly-body-wearable setting.

A per-hop security scheme for highly dynamic wireless sensor networks

Certain popular wireless sensor network applications, including disaster recovery, battlefield communication and athlete monitoring, are characterized by extensive node mobility, intermittent contact between nodes and a highly dynamic network topology. Traditional routing protocols and security schemes are designed for essentially static networks and do not perform well in this case. This has given rise to a new multi-hop routing paradigm, that of “mobility-assisted” routing in which nodes make strategic data store-and-forward decisions on a per-hop basis. In this paper we discuss the security challenges relevant to mobility-assisted routing and propose a scheme to secure data communication between nodes in highly mobile sensor networks. Our solution utilizes symmetric-key encryption to ensure data confidentiality and varies encryption key in a verifiable, non-forgeable manner to allow easy authentication. This scheme also provides data freshness, semantic security and per-hop encryption to enable secure data aggregation. To validate our basic assumptions and fine-tune our scheme, we collect and analyze link connectivity statistics from a dynamic sensor network application, athlete monitoring during a first-division university soccer club match. We show that our scheme is well-suited for certain dynamic environments and serves as an effective first step towards securing communications for mobile sensor networks.

A key loss recovery scheme for secure broadcasts in wireless sensor networks

Authenticity and secrecy of broadcast message content is important in wireless sensor networks deployed for battlefield control, emergency response, and natural resource management. Encryption of broadcast data requires the key to vary in time, typically via a key chain, so that a key compromised at a receiver does not compromise broadcast security for the entire network. An unfortunate consequence of time-varying keys is that a receiver that misses (due to packet loss) one or more keys from the chain cannot decrypt subsequent messages, thereby getting excluded from all broadcasts. In this paper we develop a scheme that allows receivers to recover from one or a few lost keys by having the transmitter probabilistically reuse old keys from the chain. Our scheme makes the broadcast system more robust to packet loss, at the expense of increasing vulnerability to compromised old keys. Analysis of our scheme shows how the trade-off can be controlled by tuning parameters, and a prototype implementation on a MicaZ mote testbed demonstrates the feasibility of our scheme in real sensor network platforms.