Smarter Radios for Energy Efficiency in Wireless Sensor Networks

The constraints of Wireless Sensor Networks scenarios require the introduction of optimization techniques at different design levels: from the hardware to the software and communication protocol stack. In fact, the design of energy efficient WSNs involves an appropriate Hardware/Software co-design oriented to the concerned application. Given the event driven and multitasking nature of WSNs applications, one could think of adding different processing units that cooperate to manage events and tasks in an optimal way. Then, the complexity of tasks performed by the main processing unit can be reduced and energy efficiency can be achieved. In this PhD thesis we study protocols that leverage the implementation of smart radios. The idea of smart radios is introducing intelligence into the radio chip; in this way, it will be able to take decisions and perform several tasks in an autonomous way and without any intervention of the main CPU. The CPU will be charged of bootstrapping the network and, after a stable state is reached, it can remain inactive most of the time while the radio chip provides a given set of services. The proposed protocol is called Wake on Idle and it provides integrated neighborhood maintenance and low duty-cycle medium access control. These services are provided based on analog transmissions that are time encoded; then, as soon as the network enters the stable state ( i.e. the topology is formed and nodes are associated and synchronized) digital processing of frames is not needed. Since it relies on low level information, Wake on Idle can be easily implemented on hardware and integrated into the radio chip; then, it works as a coprocessor that provides high level services ( i.e. neighborhood maintenance and medium access) to the main processing unit. Through theoretical analysis and a preliminary implementation we demonstrate the feasibility of the protocol and we show several interesting characteristics that help achieving energy efficiency and good performance. Then, we further exploit analog signaling to optimize duty cycle of existing medium access control protocols. We propose a mechanism called Sleep on Idle and it is based on the exchange of analog busy tones. Sleep on Idle can also be integrated into the smart radio to take decisions about whether the CPU has to be woken up. We apply the decision mechanism to the slotted IEEE802.15.4 standard and validate it through simulations and experimentations. The results show an important gain in terms of energy consumption and network reactivity.