# An Application of a LPWAN for Upgrading Proximal Soil Sensing Systems
## 3 Hardware Design of the AHS
### Communication Module Based on LoRa  
LPWANs have attracted extensive attention, mainly because they can provide affordable connections for low-power devices distributed in very large geographical areas. When realizing the vision of the IoT, LPWAN technologies complement and sometimes even replace conventional wired communication and cellular and short-range wireless technologies, in terms of their performance for various emerging smart city and machine-to-machine applications [21]. Sigfox, LoRa, and NB-IoT are the three leading LPWAN technologies that compete for large-scale IoT deployment, and they have different characteristics that affect the performance of IoT solutions; device connectivity, information delay, and even device battery life [22]. Some of their key characteristics are shown in Table 1.  
LoRa has the characteristics of long-distance and low power consumption, which can prolong the battery life. It uses the unlicensed Sub-1 GHz ISM bands and does not need to pay additional licensing fees. In addition, LoRa can adapt the data rate and allow private networks, while Sigfox and NB-IoT cannot [23]. LoRa, as a representative LPWAN technology, has emerged as an attractive communication platform for the IoT [24,25]. Therefore, in this paper, the mature commercial LoRa module, which was designed based on SemTech sx1278 (Camarillo, CA, USA), was used as the communication module of the AHS, with an adjustable transmission power and a maximum transmission power of 20 dBm; it supported remote wake-up in sleep mode and adopted advanced channel coding technology. Its receiving sensitivity could reach \(-142\) dBm, enabling it to realize long-distance communication under ultralow power consumption. The LoRa gateway was designed based on a sx1301 transceiver controller. The gateway has a higher receiving sensitivity than other technologies, its sight distance coverage radius can reach 5 km, it includes eight receiver channels and one transmission channel (among which 8 receiver channels can receive data simultaneously), and it supports up to 10,000 LoRa terminals, which are convenient for building a massive connection network. It can also support LTE (4G/3G/2G), connect to servers without wiring, and adapt to the multiple access modes of PAAS platforms, such as MQTT, TCP, and Modbus [26].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Sigfox** & **LoRa** & **NB-IoT** \\ \hline Frequency & \begin{tabular}{c} Unlicensed sub-1 \\ GHz ISM bands \\ 10 km (urban), \\ 40 km (rural) \\ \end{tabular} & \
\begin{tabular}{c} Unlicensed sub-1 \\ GHz ISM bands \\ 5 km (urban), \\ 20 km (rural) \\ \end{tabular} &
\begin{tabular}{c} Licensed LTE \\ frequency bands \\ 1 km (urban), \\ 10 km (rural) \\ \end{tabular} \\ Bandwidth & 100 Hz & 250 kHz and 125 kHz & 200 kHz \\ Maximum data rate & 100 bps & 50 kbps & 200 kbps \\ Interference immunity & Very high & Very high & Low \\ Adaptive data rate & No & Yes & No \\ Allow private & No & Yes & No \\ network & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: The key characteristics of LPWAN technologies: Sigfox, LoRa, and NB-IoT.  
Figure 3: Minimum ultralow-power MCU system.