# Demonstrating the Potential of a Low-Cost Soil Moisture Sensor Network
## 3 Results and Discussion
### LoRaWAN Performance  
LoRaWAN technology is well-known for its long-range data acquisition with low power consumption, however LoRaWAN has limited messaging capabilities which may cause transmission delay or even data loss in the network. Therefore, it is useful to evaluate the data communication capability of the sensor network. Various settings in the evaluation can have an effect on the data transmission and the Packet Error Rates (PERs) of the network. A high PER will consequently increase the duration of data transmission. Therefore, by analysing the transmission time between the sampling time (at individual nodes) and the data collection time (i.e., the time at which the gateway receives the data), it is possible to determine the delay. The delay includes uplink (time on air) and the default time offsets for receiving the frame on the gateway side [34]. Since the time stamp associated with the individual nodes collecting a measurement is not available, it was estimated using the sampling frequency at the node and the \(t=0\) (i.e., the time stamp at which the nodes collect the first measurement with the new sampling frequency). Using the newly estimated sampling times and the time difference between two adjacent samples it was possible to calculate the delay (Figure 6). The delay medians for the four sampling frequencies are 31 s, 31 s, 32 s and 32 s, respectively. Thus, there is no observed significant difference in data transmission delay among different sampling rates. However, as shown in Figure 6, there is an increase in the median delay with distance from the gateway, independent of the sampling frequency. For example, in the \"30 min per sample\" subplot, the IQRs of delays for the distances from 40 m to 460 m are 0, 0.017, 0.383 and 0.458 min, respectively, while the medians for the distance 100 m, 300 m and 460 m are 0.133, 0.517 and 0.55 min. This observation is a consequence of the Adaptive Data Rate (ADT) scheme used in LoRaWAN, which aims to minimise energy consumption and maximize throughput by adjusting the data rate for every end node. ADR controls the transmission parameters, namely Bandwidth (BW), Spreading Factor (SF), Transmission Power (TP) and Coding. Rate (CR) [35]. ADR changes the data rate based on simple rules. For example, if the link budget is high, the data rate can be increased by increasing the SF, while if the link budget is low, the data rate can be lowered by decreasing the SF [36]. The sensors tested operate at  
Figure 5: Errors and sources of errors associated with sensor installation. (**a**) Data showing sensor response before and after partial exposure to air—example provided in (**b**); (**c**) data showing sensor response before and after the presence of an air interface between the probe and soil—introduced through repeated shaking; (**d**) data drift associated with tines deflection. an SF between 7-12, and it is known that large SFs allow for a longer communication range while increasing the time on-air and consequently the off-period duration [36]. Accordingly, there is trade-off between SF and transmission range, with lower delays and lower SFs present for shorter ranges [36]. The higher delay for the nodes positioned at 40 m nodes was caused by the positioning in relation to the gateway. The sensor nodes at the other distances were in the antenna field of view (i.e., facing the antenna) while the nodes at 40 m were positioned behind it. This was later confirmed to be the cause, by positioning the nodes in the same field of view. In addition to the four ranges presented here, a fifth range of approximately 740 m was originally used in the experimental design. However, this range was excluded from the analysis as more than 50% of the data packets were lost. According to the manufacturer's specifications, the antenna should provide a range of up to 2 km with no clear line of sight and up to 10 km with a clear line sight, which is not substantiated by these results.