# A Comprehensive Study of the Use of LoRa in the Development of Smart Cities
## 4 Results of the Analysis of the Selected Works
### Analysis of the Device Layer
#### 4.3.1 Sensors
Through the evaluation of the amount of light that is backscattered by the particles, it is possible to determine the air quality [62]. Some aspects that are important when using sensors are (i) timestamp and (ii) sensor identification. The timestamp between a node and the gateway must be synchronized; this allows validation of the information received by the nodes as new or old [25]. For sensor identification, a Local ID could be used. This is useful for knowing which device is sending the information. Figure 14: Sensors used in smart city applications. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Subcategory** & **Sensing Parameters** & **Type of Sensors** & **Distance GW-Sensor** & **Ref** \\ \hline Agriculture & \begin{tabular}{c} Humidity, Temperature, \\ Luminosity, Solar radiation, Soil, \\ Conductivity, Ph \\ \end{tabular} & \begin{tabular}{c} Ultrasonic \\ Temperature \\ Humidity \\ Soil \\ \end{tabular} & 30 cm–15 km & [63; 64; 65; 22; 66; 63; 64; 22; 65; 66] \\ \hline Healthcare & Health signs & Biosensors & 3 m & [61] \\ \hline Energy & \begin{tabular}{c} Light intensity \\ Motion \\ Voltage \\ Temperature \\ Humidity \\ \end{tabular} & Temperature & 15 km & [18; 37] \\ \hline Traffic & \begin{tabular}{c} Motion \\ Occupancy \\ \end{tabular} & \begin{tabular}{c} Magnetic \\ Ultrasonic \\ \end{tabular} & 500 m–1 km & [41; 42] \\ \hline Environment & \
\begin{tabular}{c} CO\({}_{2}\), NO\({}_{2}\), O\({}_{3}\) Concentration \\ Weather \\ \end{tabular} &
\begin{tabular}{c} Gas \\ Temperature \\ \end{tabular} & 200 cm–5 km & [27; 67; 68] \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Types of sensors used in different subcategories of smart city applications. The power consumption of sensors varies depending on their type. Although the consumptions are low (voltage ranges between 2.2 to 5 V and average is less than 1 mA), it is necessary to consider energy consumption in the design of IoT solutions, since the end nodes depend on low-powered lithium batteries. Table 10 shows the most common voltage and amperage ranges used by sensors. On the basis of the SLR carried out in this work, we could identify several IoT-LoRa projects using DTH XX series sensors due to their easiness of configuration, low cost and good accuracy. Additionally, the availability of libraries for this type of sensors in different programming languages allows time and effort reduction for the development and implementation of LoRa-based IoT solutions. The sensor DTH11 and DTH 22 are digital sensors; DTH22 allows temperature measurements in the range from \(-40\) to \(80\,\mathrm{\SIUnitSymbolCelsius}\) with accuracy of \(\pm 0.5\,\mathrm{\SIUnitSymbolCelsius}\) and humidity measurements in the range from 0% to 100% RH (relative humidity) with an accuracy of 2% RH. DTH11 works in a temperature measuring range from 0 to \(50\,\mathrm{\SIUnitSymbolCelsius}\) with an accuracy of \(\pm 2.0\,\mathrm{\SIUnitSymbolCelsius}\) and in a humidity range from 20% to 90% with an accuracy of 4% RH. The sensors DTH11 and DTH22 can transmit bits to the nodes of LoRa ecosystems. Table 11 shows the bits sent in different group of bytes [64; 65; 66]. A complete transmission of information from DTH 11 and DTH 22 consists of 40 bits (5 bytes) in a period of time of 4 ms. Two bytes correspond to the temperature measurement, two bytes to the humidity measurement, and 1 byte to errors checksum. DTH11 and DTH 22 are not high-precision sensors, but their accuracy is acceptable. DTH11 can be used in tests or home projects, while DTH22 operates with medium accuracy and can be used in monitoring implementations at the city level. Table 12 shows a comparison of these two types of sensors based on their level of accuracy. Some points to consider related to the connection between sensors and nodes are that values lower than 3.3 V can present problems, the power of the nodes must consider the limited capacities of the batteries, and the distance between sensors and nodes must be taken into account.