Toward Reliable Wireless Sensor Networks Energy-Aware Distributed interference Management for Unlicensed Bands

Sammanfattning: Wireless sensor networks have been proposed as a cost effective and easy to deploy alternative to traditional wired systems in a multitude of application scenarios ranging from industrial automation to healthcare monitoring. They are expected to enable an unparalleled number of new services that will bring countless benefits to society. However, low power communications of sensor nodes operating in unlicensed bands face several challenges and are easily corrupted by transmissions of other collocated wireless networks. This problem has recently raised reliability concerns which have been tremendously enhanced by the proliferation of wireless devices we have been witnessing during the last years in the few available regions of the spectrum.This dissertation studies how to achieve reliable communications by proposing different ways for the energy aware management of the radio interference problem. The use of wireless sensing technologies has been envisaged in a broad variety of settings: for this reason it is not possible to identify a unifying solution for the problem of interference, but rather it is necessary to tailor the design of communication schemes accounting for the specific communication paradigm adopted by sensors, the traffic pattern generated by the expected application, as well as for the channel conditions experienced by nodes.When packet transmissions are addressed to a single receiver, cognitive access schemes can be utilized and sensors can opportunistically select for their transmissions when to access a certain channel or which channel to access so as to avoid interference. We provide an energy aware design for communication schemes implementing these ideas and evaluate their energy performance by means of experiments using real hardware. Our results indicate that the first approach should be considered only for sporadic packet transmissions over channels presenting limited interfering activities; channel adaptation should instead be preferred for large bulks of data or when the risk of operating in heavily interfered frequency bands is high. We further propose and evaluate a novel adaptive frequency hopping algorithm: this algorithm has been shown to be very efficient in mitigating the negative effects of interference allowing at the same time to avoid the use of the energy costly spectrum sensing algorithms required by cognitive access schemes. However, none of these three approaches may be suitable for scenarios where packet transmissions are addressed to multiple receivers. To deal with the packet losses that nodes may experience over noisy or interfered channels we envisage the use of fountain codes and show how it is possible to engineer such a coding solution so as to reduce the complexity and overhead introduced by the encoding and decoding procedures. The resulting codes provide an efficient way for disseminating data over multi-hop wireless sensor networks. Results obtained in this dissertation can be of great utility for designers of sensor applications who can use them in order to select the most energy efficient way to achieve reliable interference-aware communications.