Characterization of Low-Antenna Ultrawideband Propagation in a Forest Environment

Impulse ultrawideband (UWB) communication promises a number of benefits for use in wireless sensor networks, particularly in forest environments, where it has the potential to provide robust operation, along with the ability to combine communications with precision position location. In this paper, we present measurement results and empirical models for UWB signal propagation in a forest environment. Path-loss measurements were performed using a 620-ps duration UWB pulse with a frequency range of 830-4200 MHz. More than 22 000 measurements were recorded in 165 locations in four diverse forest environments in Virginia and Maryland, USA. Transmitters and receivers were separated by distances that range from 4 m to 50 m. Large-scale path loss was most closely modeled by log-distance propagation, with path-loss exponents ranging from 2.5 to 3.8. Small-scale fading analysis indicated that UWB signals experience Rician fading, with K-factors in the range of 10-16 dB. K-factors were found to depend on forest type, with the medium-density forest providing the greatest number of multipath components and highest overall K-factor. Multipath component analysis demonstrated that a forest is a fairly rich multipath environment; however, in most cases, approximately 90% of the available energy was contained within the strongest 3-15 multipath components. Postprocessing analysis divided the UWB pulse into three frequency subbands and demonstrated that model parameters also had frequency dependence. These measurements and models should aid in the development of future UWB outdoor sensor networks.