Wearable wireless sensor devices are key components in the emerging technology of personalized healthcare monitoring. Medical data collected by these devices must be secured, especially on the wireless link to the gateway equipment. However, it is difficult to manage the required cryptographic keys, as users may lack the awareness or requisite skills for this task. Alternatively, recent work has shown that two communicating devices can generate secret keys derived directly from symmetrical properties of the wireless channel between them. This channel is also strongly dependent on positioning and movement and cannot be inferred in detail by an eavesdropper. Existing schemes, however, yield keys with mismatching bits at the two ends, requiring reconciliation mechanisms with high implementation and energy costs that are unsuitable for resource-poor body-worn devices.In this work we propose a secret-key generation mechanism which uses signal strength fluctuations caused by incidental motion of body-worn devices to construct shared keys with near-perfect agreement, thereby avoiding reconciliation costs. Our contributions are: (1) we analyse channel measurement asymmetries caused by non-simultaneous probing of the channel by the link end-points, (2) we propose a practical filtering scheme to minimize these asymmetries, dramatically improving signal correlation between the two ends without reducing entropy, and (3) we develop a method to restrict key generation to periods of channel fluctuation, ensuring near-perfect key agreement. To the best of our knowledge, this work is the first to demonstrate the feasibility of generating high quality secret keys with zero reconciliation cost in body-worn networks for healthcare monitoring.
Radio connectivity in wireless sensor networks is highly intermittent due to unpredictable and time-varying noise and interference patterns in the environment. Because link qualities are not predictable prior to deployment, current deterministic solutions to unreliable links, such as increasing network density or transmission power, require overprovisioning of network resources and do not always improve reliability.We propose a new dual-radio network architecture to improve communication reliability in wireless sensor networks. Specifically, we show that radio transceivers operating at well-separated frequencies and spatially separated antennas offer robust communication, high link diversity, and better interference mitigation. We derive the optimal parameters for the dual-transceiver setup from frequency and space diversity in theory. We observe that frequency diversity holds the most benefits as long as the antennas are sufficiently separated to prevent coupling. Our experiments on an indoor/outdoor testbed confirm the theoretical predictions and show that radio diversity can significantly improve end-to-end delivery rates and network stability at only a small increase in energy cost over a single radio. Simulation experiments further validate the improvements in multiple topology configurations, but also reveal that the benefits of radio diversity are coupled to the number of available routing paths to the destination.
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