Mitigation and Exploitation of Multipath Fading in Wireless Sensor Networks

The increasing interest in wireless sensor networks can be promptly understood simply by thinking about what they essentially are: a large number of small sensing self-powered nodes which gather information or detect special events and commu- nicate in a wireless fashion, with the end goal of handing their processed data to a base station. Sensing, processing and communication are three key elements whose combination in one tiny device gives rise to a vast number of applications [5, 46]. Sensor networks provide endless opportunities, but at the same time pose formi- dable challenges, such as the fact that energy is a scarce and usually non-renewable resource. The minimization of energy expenditure in the sensing nodes is a natural direction for wireless sensor network research. Reducing the computational load and minimizing radio power consumption are necessary steps dictated by common sense; on the radio side, the knowledge of the properties of the wireless medium can help minimize packet loss. Wireless analytical models are useful, but the oversimplifica- tions introduced by many of them can lead to wrong considerations and conclusions. For this reason, it is essential to analyze the wireless medium with real hardware. Sensor network design should be performed by considering the interactions between the physical layer and the upper layers: physical layer phenomena should be taken into account in the development of MAC and routing schemes. In the present work, we consider the impact of multipath fading on sensor networks. Chapter 2 focuses on the analysis of the properties of multipath fading, and the verification of the most common models through software simulations and tests based on hardware implementations. In Chapter 3, we use the properties of multipath fading to pro- pose a novel, lightweight protocol for the acquisition of data from the nodes in a wireless sensor network based on the exploitation of mobility and the concept of opportunistic transmissions. Our protocol aims at the avoidance of packet loss by the nodes and the minimization of their energy consumption. While Chapter 3 revolves around the mitigation of the eå¨ects of multipath fading, chapter 4 describes a novel idea for its constructive exploitation. We show how the footprint of multipath fading on the received signal strength can be used for purposes of motion detection. The present work is completed by an appendix where extra details specific to our hardware implementations are provided to the interested reader. In the rest of the present chapter, we provide an overview of the research efforts in the field of wireless sensor networks. We start by listing some interesting applications and research projects currently underway. We then indicate the main peculiarities of sensor networks to give a feeling and basic understanding of what makes them unique and to justify the enormous interest in the field that is being displayed by the community. Finally, we present the most interesting and commonly used hardware platforms that enable wireless sensor network research.