Experimental characterisation of the off-body wireless channel at 2.4 GHz for dairy cows in barns and pastures

Wireless Sensor Networks (WSNs) provide promising applications in healthcare monitoring of dairy cows. After sensors measure the data in or on the cow’s body (temperature, position, leg movement), this information needs to be transmitted to the farm manager, enabling the evaluation of the health state of the cow. In this work, the off-body wireless channel between a node placed on the cow’s body and an access point positioned in the surroundings of the cows is characterised at 2.4 GHz. This characterisation is of critical importance in the design of reliable WSNs operating in the industrial, scientific and medical (ISM) band (e.g., Wi-Fi, ZigBee, and Bluetooth). Two propagation environments were investigated: indoor (inside three barns) and outdoor (pasture). Large-scale fading, cow body shadowing, and temporal fading measurements were determined using ZigBee motes and spectrum analysis measurement. The path loss was well fitted by a one-slope log-normal model, the cow body shadowing values increased when the height of the transmitter and/or the receiver decreased, with a maximum value of 7 dB, and the temporal fading due to the cow movement was well described by a Rician distribution in the considered environments. As an application, a network planning tool was used to optimise the number of access points, their locations, and their power inside the investigated barns based on the obtained off-body wireless channel characteristics. Power consumption analysis of the on-cow node was performed to estimate its battery lifetime, which is a key factor for successful WSN deployment.