Advanced RF Techniques For Wireless Sensor Networks: The Software-Defined Radio Approach
Traditional wireless sensor node designs follow a common architectural paradigm that connects a low-power integrated radio transceiver chip to a microcontroller. This approach facilitated research on communication protocols that focused on the media access control layer and above, but the closed architecture of radio chips and the limited performance of microcontrollers prevented experimentation with novel communication protocols that require substantial physical layer signal processing. Software-defined radios address these limitations through direct access to the baseband radio signals and an abundance of reconfigurable computing resources, but the power consumption of existing such platforms renders them inapplicable for low-power wireless sensor networking. This dissertation addresses this disparity by presenting a low-power wireless sensor platform with software-defined radio capabilities. The modular platform is built on a system-on-a-programmable chip to provide sufficient reconfigurable computational resources for realizing complete physical layers, and uses flash technology to reduce power consumption and support duty cycling. The direct access the platform provides to the baseband radio signals enables novel protocols and applications, which is evaluated in two ways. First, this is demonstrated by designing the physical layer of a spread-spectrum communication protocol. The protocol is optimized for data-gathering network traffic and leverages spectrum spreading both to enable an asynchronous multiple-access scheme and to increase the maximum hop-distance between the sensor nodes and the basestation. The performance of the communication protocol is evaluated through real-world experiments using the proposed wireless platform. Second, a multi-carrier phase measurement method is developed for radio frequency node localization. Compared to existing interferometric approaches, this method offers more than four orders of magnitude measurement speedup and requires no deliberately introduced carrier frequency offset. The operation of the multi-carrier approach is validated using the new platform in various experiments. The analysis of the collected phase measurement data led to a novel approach for phase measurement-based distance estimation. This model is utilized to derive two maximum-likelihood distance estimators and their corresponding theoretical bounds in order to analyze and interpret the experimental results.