Radio frequency ranging for wireless sensor network localization.
University of Southampton, School of Electronics and Computer Science,
Wireless sensor networks (WSNs) have a diverse range of industrial, scientific and medical applications where the sensor nodes are of low cost, standard with respect to hardware architecture, processing abilities and communicate using low-power narrow-band radios. Position information of the sensing nodes within those applications is often a
requirement in order to make use of the data recorded by the sensors themselves. On deployment, sensing nodes normally have no prior knowledge of their position and thus a localization mechanism is often a requirement. The process of localizing a 'blind' device consists of ranging estimates or angle measurements to a set of references with a prior knowledge of their position relative to a co-ordinate system and the position computation of the blind device in relation to the fixed references. This research focuses on the process of ranging to enable two-dimensional localization of sensing nodes within WSNs. Alternative ranging methods for the specified application field have not demonstrated their ability to meet the resolution and accuracy (resolution 0.3 m with accuracy better than ± 1.0 m line-of-sight) required. A novel radio frequency (RF) time-of-flight (TOF) ranging system is presented in this work to mitigate those problems. The system has been prototyped using a TI CC2431 development platform with ranging and data packet transfer performed on a single channel in the 2.4 GHz ISM frequency band. The frequency difference between the two transceivers involved with ranging is used to obtain sub-clock TOF phase offset measurement in order to achieve high resolution TOF measurements. Performance results have been obtained for the line-of-sight (LOS), non-line-of-sight (NLOS) and indoor conditions. Accuracy is typically better than 7.0m RMS for the LOS condition over 250.0m and 15.8m RMS for the NLOS condition over 120.0m using a sample average of one-hundred two-way ranging transactions. Indoors accuracy is measured to 1.7m RMS using a 1000 sample average over 8.0m. Corresponding results are also presented for the algorithms suitability for localizing sensor nodes in two-dimensions. Ranging performance is bound by the signal-to-noise ratio (SNR), signal bandwidth, synchronization and frequency difference between devices. This ranging algorithm demonstrates a novel method where resolution and accuracy are improved time dependent in comparison to frequency dependent methods using narrow-band RF.
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