# LPWAN-Based Vehicular Monitoring Platform with a Generic IP Network Interface
## 3 Technological Background
### LoRaWAN  
LoRaWAN is one of the most prominent LPWAN-based technologies. Concretely, it is based on LoRa, which is a modulation based on the Chirp Spread Spectrum (CSS) modulation technique and developed by Semtech [26]. LoRa follows the following precepts: (i) Reduced transmission bit-rates, (ii) use of low-frequency bands (sub-GHz bands), and (iii) limited end-device communication capabilities, in order to enable long transmission distances (of more than 15 km) with reduced power consumption, that permits battery lifetimes of years [3]. Thereby, LoRa uses reduced data rates of just a few kilobits per second (kbps), which permits the development of receiving modules with high sensitivity. This fact permits the receiving equipment to decode the received signal at very low power-levels. This transmission robustness is further improved by the CSS-based modulation, as stated above [26]. In addition, LoRa uses unlicensed low-frequency bands, that is, Industrial, Medical, and Scientific (ISM) bands (such as 868 MHz and 911 MHz in Europe and America, respectively). The use of sub-GHz bands improves penetration and transmission distances, in comparison with other typical, higher-frequency, bands, such as 2.4 GHz. Aiming at saving energy in the end-devices, the communications in LoRaWAN systems are limited in both the number of transmissions per node per day, and the length of each transmission [27]. This is also related to the use of unlicensed frequency bands, so the number of transmissions and their duration need to be controlled, in order to avoid overuse by certain applications or nodes.  
Regarding the architecture of LoRaWAN networks, as shown in Figure 2, there is another layer on top of the PHY level (LoRa) that gives the name to the whole system, LoRaWAN. Actually, it defines the MAC-layer mechanisms and other architectural aspects, such as network security and its organization. Regarding the latter, LoRaWAN makes use of a star or star-of-stars topology (Figure 3). This architecture permits simple and direct communication between end-devices and the gateway, hence avoiding other configurations with higher complexity (such as mesh networks). Observe in Figure 3 that end-devices directly transmit to the gateway in one single hop (i.e., without the help of the rest of the network).  
Figure 3: LoRaWAN network architecture.  
Figure 2: LoRaWAN stack.  
Considering mobility issues, end-devices transmit LoRaWAN messages in broadcast mode regardless of their position, movement, or speed. This means that any LoRaWAN gateway that hears the message will forward it to the correspondent network server (network service in Figure 3); this way, changes or hand-overs among gateways are completely seamless for end-devices.  
Finally, an interesting feature of LoRaWAN is the possibility of adapting its PHY parameters, namely LoRa parameters, depending on the transmission conditions. Therefore, LoRa has three different configuration parameters: Spreading Factor (SF), Coding Rate (CR), and bandwidth (BW). The SF adjusts the spreading level of the transmitted signal, in comparison with the original one. By enlarging transmission spreading, the link robustness is increased at the expense of reducing the data-rate. The transmission robustness may be additionally increased by including redundant information in each frame. The CR defines the number of redundant bits, in comparison with the useful data, and may be tuned using different pre-defined values in the European 868 MHz ISM band. Finally, LoRa transmissions usually employ a BW of 125 kHz, but other configurations are supported as well (such as 250 kHz or 500 kHz).