## 2 LPWAN IoT Technologies
### SigFox  
SigFox is M2M technology designed by the French start-up SigFox company. It uses UNB technology to accomplish transmission ranges of many km in a license-exempt band within the power constraints [19]. In Europe, for example, the band used is between 868 and 868.2 MHz, whereas in other countries, the band used is between 902 and 928 MHz, based on local regulations. The autonomy, simplicity, cost-effectiveness, short messages, bidirectionality, and complementarity of the SigFox protocol are its primary goals. The flat architecture of the SigFox network is illustrated in Figure 4. It consists of three main components, namely (1) SigFox equipment (devices and base stations), (2) SigFox support or cloud systems (e.g., supervision, backend services, and storage), and (3) Web interface and API (front-end services and data) [20].  
SigFox supports SigFox network operators (SNOs) and works with them as a partner to build their network structure and support it as a service. SNOs deploy proprietary equipment in the form of gateways equipped with cognitive software-defined radios, which are connected to the servers through an IP-based network infrastructure. When compared to LoRa (CSS), the SigFox physical layer is relayed using Gaussian frequency shift keying (GFSK) and differential binary phase shift keying (DBPSK) instead of CSS. It has 100 Hz of bandwidth to transmit data in channels and achieves lower noise levels, higher receiver sensitivity, ultra-low consumed power, and a cheap antenna design [21]. SigFox uses 192 kHz as the total of the spectrum, with a maximum throughput of 100 bps. Due to the lack of synchronization between the device and the network, frequency hopping and replicating the message twice are used in data transmission for a total of three random frequencies to obtain a high QoS.  
Figure 3: Operating modes of NB-IoT and Cat-M1.  
The transmitted power of SigFox is 14 dBm, the equivalent to 25 mW, and typically, the end device's receiver sensitivity is \(-124\) dBm. According to the company, the maximum path loss that can be tolerated by SigFox is 160 dB, which is better than being pathless in conventional cellular networks [22]. SigFox further submits that up to 1 million terminals may be handled at each base station, with coverage in rural regions of 30-50 km and in urban areas of 3-10 km [23]. The daily uplink messages are limited to 140 messages, with a maximum payload length of 12 bytes with up to 100-bps data rates for compatibility with the regional regulations concerning license-free spectrum usage. Nevertheless, the daily downlink (DL) message numbers are limited to four messages with a maximum payload length of eight bytes, therefore having no acknowledgement for all uplink messages [24]. The end device sends uplink messages to the base station and then waits for a short duration to listen to any downlink from the base station.  
Thus, SigFox is the right choice for sensor data acquisition but not for control actuators. SigFox uses both the variety in time and frequency as well as redundancies to improve the reliability of the uplink communication and overcome the lack of adequate support for uplink message acknowledgements [25]. The end devices can send a message many times across different frequency channels and choose a random frequency channel to broadcast their messages autonomously. Moreover, the base stations can scan all of the channels in order to decipher the messages. Therefore, in the EU region, the band from 868.180 to 868.220 MHz is split into 400 100-Hz channels, of which 40 are reserved and unutilized channels [26].