Energy saving is a problem of growing importance due to the low-energy utilization and increasing environmental awareness. However, the challenge of energy optimization is to assure the accuracy of the energy forecast model [38].
According to the Global Standards Initiative on the IoT, a device labeled as IoT must meet seven criteria related to sensors: Internet connectivity, processors, energy efficiency, cost-effectiveness, quality, reliability, and security. In order to be attractive for use in commercial applications, IoT devices need to factor in low power consumption, long distance communication, and affordability [18].
Low power consumption, low transceiver chip cost, and extensive coverage area are the main characteristics of the low-power extensive area network (LPWAN) technologies. We expect that LPWAN can be part of enabling new human-centric health and wellness monitoring applications [39].
LPWAN, which stands for low-power wide-area network, has great importance in the IoT domain. Most of the IoT projects are based on the following requirements: extended range, low data rate, low energy consumption, and cost-effectiveness.
Today, many technologies are based on the LPWAN concept. Among the best-known technologies of this type are Sigfox, LoRa, third generation partnership project (3GPP), Weightless, Ingenu, WAVIoT, nWave, Telensa, Cyan’s Cynet, Accellus, and SilverSpring’s Starfish. Applications such as infrastructure monitoring and metering scenarios, or smart traffic, are the ones that most use LPWAN technology. However, other fields that test the potential of this technology have recently emerged. These fields include smart health care and well-being monitoring.
LPWAN technology makes it possible to increase the distance necessary for the communication of the sensors and the base stations, up to hundreds of meters or even tens of kilometers. On the other hand, the complexity of the network decreases proportionally to this distance.
The main reasons why LPWAN technology is so used nowadays, especially in health care monitoring and well-being applications [40], are the low cost and the low energy consumption for sensors devices.
In technologies such as Sigfox, LoRa, and NB-IoT, the end devices are not being used outside the operation; most of the time they are put in sleep mode, and this can reduce the energy consumption. Although this technique helps to achieve low energy consumption, for NB-IoT technology, this is counterbalanced by the additional energy consumption caused by the synchronous communication and QoS handling, as well as the demand for current of its OFDM/FDMA access modes. NB-IoT technology ensures that the end devices have a shorter lifetime, as compared to Sigfox and LoRa. However, as a trade-off, NB-IoT devices ensure low latency.
Regarding LoRa devices, they are divided into several classes: class A, class B, or class C. Class A is characterized by the fact that the devices of this class start to transmit the packets at any moment in ALOHA fashion; also, the packets will be transmitted using a channel that is selected randomly. After the transmission, there are two receiving windows opened by the sensor node that will be used by the base station to reach the device in the downlink. The devices of class B are different from those of class A because they have additional periodic receive slots. Class C devices increase energy consumption by continually staying in receive mode unless they are transmitting. The devices of class C are used to manage the low-bidirectional latency, to the detriment of increased energy consumption. In addition, there are handover-related signaling procedures, specific to LoRa devices, which help saving energy to the sensor nodes and transmit the packets successfully.
The devices based on the NB-IoT communication protocol use in turn the LTE protocol and also minimize and make its functionalities better. The LTE protocol is the one which broadcast the valid information for all end devices from a cell, and it is maintained to the minimum size and its minimum occurrence while the broadcasting back-end system utilizes the battery power of each end device and achieves the resources. Accordingly, to all mentioned above, the system can be cost-efficient, and the battery consumption small to obtains up to 10 years of battery lifetime with a transmission rate of 200 bytes per day on average [41].
Overall, it is recommended to use Sigfox and class-A LoRa devices for applications that transmit a small amount of data and are not being influenced by the latency. Regarding the applications that need low latency, there are better options such as the LoRa class-C devices and NB-IoT.
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