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This paper gives a tutorial overview of ultrawide-band communications systems for sensor networks. In particular, we describe the IEEE 802.15.4a standard, which is currently being developed. While most nodes (reduced-function devices) in sensor networks usually have to consume little energy, and are constrained with respect to the complexity of the processing they can perform, some nodes (full-function devices) do not show these restrictions. We describe a hybrid modulation, coding, and multiple access scheme that is particularly suited for heterogeneous networks that contain both FFDs and RFDs. The scheme is a generalization of the well-known time-hopping impulse radio (TH-IR). It employs systematic coding, joint pulse-position modulation and phase shift keying, as well as a combination of polarity scrambling and time-hopping. We also describe two-way ranging algorithms that serve as the basis for geolocation in 802.15.4a, and we discuss the methods for how the ranging information can be kept secret from snoopers.