RF Propagation Path-loss Patterns for ISM Band In-field WSN Agricultural Applications

Abstract

The objective of this study was to evaluate and compare in-field radio frequency signal propagation and data transmission performance at 915MHz and 2.4GHz band wireless sensor network (WSN) links. Two kinds of commercial wireless sensor motes using these bands as transmitters were wirelessly connected to their corresponding receivers and a hand-held spectrum analyzer. Indexed packets transmitted from each transmitter were captured by a spectrum analyzer to measure path-loss and synchronously received by a receiver using equal mote model to calculate packet delivery rate (PDR). Experiments were implemented in a research field where wheat was planted. Impact factors considered were: plant canopy height, transmitter height, receiver height, transmitter-to-receiver distance (T-R distance), and carrier frequency. Results indicated that RF signal at higher frequency band was more subject to plant canopy height. Univariate ANOVA results indicated in-field RF signal path-loss was more subject to system configurations and plant height as well as their interactions for using higher carrier frequency. Pearsons correlation results indicated that path-loss and PDR were more correlated with each other in higher plant heights for both models of motes. Estimated marginal means plots indicated that the best performance in general, if taking all plant canopy heights into consideration, might happen when the transmitter was installed at a height of 2 m and the receiver was mounted at the height of 3 m for using both kinds of motes. Two distances of 80 m and 70 m were defined for communication at 915MHz and 2470MHz carrier frequencies, respectively, as a stable communication distance for wheat field WSN applications with a minimum transmitter/receiver antenna height of 1.0 m under a maximum plant height of 0.8 m.