Characterization of WiFi signal range for agricultural WSNs

Abstract

WiFi is starting to be adopted in agricultural settings because of the increasing use of data-centric farming devices and applications. Some of these applications require high data rates that other radio standards cannot offer, for example, realtime yield mapping, drone-based image upload and viewing and video monitoring. It is therefore desirable for farming wireless sensor networks (WSNs) to also utilise available on-farm WiFi networks. However, little information is available on WiFi signal propagation in these environments, as agricultural WSNs have traditionally been based on other radio standards. Therefore, the 2.4GHz WiFi signal propagation characteristics in real outdoor agricultural cropping environments were investigated using infield data loggers. Three distinct farming environments were studied, (i) bare fields, (ii) cotton fields, and (iii) ponded-water rice fields. We studied the effects of (i) weather conditions, (ii) crop growth and (iii) water depth on signal strength across an entire growing season. We also studied the range of reliable data transfer in each environment as a function of height of the logger WiFi antenna above the crop. Crop growth status was found to be much more significant in determining signal strength than weather conditions, with signal strength declining by 8dB over the season in a cotton field, and by 20dB in a rice field. In rice and cotton crops, provided the radios remain 20cm above the crop canopy, ranges in excess of 1km were measured. Significantly greater ranges are predicted if the antenna is more than 40cm above the top of the crop. Regression models were fitted to the measurements to allow predictions and recommendations, with correlation coefficient of determination (R2) values of better than 0.9 in most cases. Radio path loss exponents depended on the environment, and were typically between 3 and 5.