I. Introduction
The idea of ubiquitous embedded wireless sensor networks (WSNs) for environmental, industrial, and structural monitoring has been envisioned for over a decade. Progress has been made with advances in technology, but the realization of small autonomous nodes with long lifetimes remains an open challenge. Ubiquitous sensor networks consist of thousands of distributed nodes, which makes battery replacement both difficult and costly. This imposes very stringent constraints on the power budget of the accompanying hardware. Therefore, a key technological issue in making wireless sensor nodes widespread is the realization of node electronics with extremely low power consumption.