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Performance Analysis of Ranging with IEEE 802.15.4 Compliant WSN Devices
H. K. Maheshwari and A. H. Kemp

The precision and accuracy of range measurement is an important issue for real time localization in wireless sensor networks (WSNs). This paper reports on round-trip time-of-flight (RT-ToF) and received signal strength (RSS) for point to point range estimation using 2. 4GHz IEEE 802. 15. 4 compliant transceivers. Two main objectives are examined, firstly, the performance limits for ToF and RSS based range measurements are compared with the fundamental Cramer-Rao Lower Bound (CRLB). Secondly the range where the error for RSS ranging is expected to be greater then the error for ToF ranging is considered. We term this the ‘cross-over’ range (CR) of RSS and ToF ranging. Knowledge of this cross-over range can be used to reduce power consumption by disabling the ToF engine and using the RSS within the cross-over range. Performance results and channel parameters have been obtained in outdoor and indoor for the line of- sight (LOS) and non-line-of-sight (NLOS) environments. Both outdoor and indoor experimental results and analysis are presented. For indoor and outdoor unobstructed (LOS) environment over short range, it is demonstrated that RSS is a good candidate for range estimation at ranges less than 7m. Uncertainty in RSS based range estimation increases with distance and beyond 7m severely limits RSS performance. Further analysis over long range demonstrates that ToF is a good candidate for range estimation at greater than 7m.

Keywords: Wireless Sensor Networks, Ranging. Time-of-Flight, Received signal strength, CRLB, Localization

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