Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> C. IoT WIRELESS TECHNOLOGIES' TRADE-OFF <s> Low power wide area (LPWA) networks are attracting a lot of attention primarily because of their ability to offer affordable connectivity to the low-power devices distributed over very large geographical areas. In realizing the vision of the Internet of Things, LPWA technologies complement and sometimes supersede the conventional cellular and short range wireless technologies in performance for various emerging smart city and machine-to-machine applications. This review paper presents the design goals and the techniques, which different LPWA technologies exploit to offer wide-area coverage to low-power devices at the expense of low data rates. We survey several emerging LPWA technologies and the standardization activities carried out by different standards development organizations (e.g., IEEE, IETF, 3GPP, ETSI) as well as the industrial consortia built around individual LPWA technologies (e.g., LoRa Alliance, Weightless-SIG, and Dash7 alliance). We further note that LPWA technologies adopt similar approaches, thus sharing similar limitations and challenges. This paper expands on these research challenges and identifies potential directions to address them. While the proprietary LPWA technologies are already hitting the market with large nationwide roll-outs, this paper encourages an active engagement of the research community in solving problems that will shape the connectivity of tens of billions of devices in the next decade. <s> BIB001 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> C. IoT WIRELESS TECHNOLOGIES' TRADE-OFF <s> A new breed of wireless technologies has emerged under the generic name of low-power, wide-area (LPWA), with a number of common characteristics that make these technologies uniquely suitable for Internet of Things (IoT) applications. These common characteristics include a power-optimized radio network, a simplified network topology, frame sizes in the order of tens of bytes transmitted a few times per day at ultralow speeds, and a mostly upstream transmission pattern that allows the devices to spend most of their time in low-energy deep-sleep mode. These characteristics enable a range of several kilometers and long battery lifetimes, possibly ten years of operation on a single coin-cell. It also enables simple and scalable deploymen <s> BIB002 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> C. IoT WIRELESS TECHNOLOGIES' TRADE-OFF <s> Low-power wide area networking technology offers long-range communication, which enables new types of services. Several solutions exist; LoRaWAN is arguably the most adopted. It promises ubiquitous connectivity in outdoor IoT applications, while keeping network structures and management simple. This technology has received a lot of attention in recent months from network operators and solution providers. However, the technology has limitations that need to be clearly understood to avoid inflated expectations and disillusionment. This article provides an impartial and fair overview of the capabilities and limitations of LoRaWAN. We discuss those in the context of use cases, and list open research and development questions. <s> BIB003
Traditionally, there has been an interest in wireless technologies that can support high data rates and that offer reliable and low latency communications. This trend has best addressed the requirements of human users who are normally greedy for optimum services. The response for this need was the emergence of wireless technologies represented by shortrange systems and long-range cellular systems. High demand for communications between wireless devices has led to technologies that suit the applications running on these devices without needing human intervention. The outcome was a number of technologies that best fit these applications regardless of their differences in features and performances. The first generation of M2M wireless systems managed to realize the IoT. However, there are drawbacks that limit the achievement of this goal. Some of these missing features of short-range M2M wireless systems are long communication distance, low-power consumption and high scalability. There are conflicting goals that make the adoption of these systems impossible. For example, ZigBee is able to range less than 100 m for a maximum of 255 nodes. To cover more area and support more devices, one solution is to have gateways or relays connected in mesh topology, which results in a more complex environment, requiring more sophisticated protocols. Furthermore, such architecture requires complex MAC protocols to reduce packet loss and the number of retransmissions. Moreover, these low data rate systems consume more energy per transmission bit. To overcome the limitations of short-range M2M wireless systems, a new generation of technology has been proposed that minimizes these differences and trade-offs toward better supporting the requirements of IoT applications. There are many trade-offs in short-and long-range M2M wireless systems. For example, LPWA is characterized by low power consumption but wide network range and simple deployment management for an enormous number of low cost devices BIB001 ,  , BIB002 . Different techniques are used by LPWA to achieve these conflicting features. These techniques include the use of a reliable Sub-GHz band, efficient modulation, simple star topology, duty-cycling, simple MAC protocols, and simplified end-devices. Table 2 explains the full list of these techniques. There has been an understanding that licensed wireless technologies are better than unlicensed. This conception may need to be reconsidered with new technologies that operate in unlicensed spectrum and can provide close or similar performance  . As is the case with most technologies, wireless systems have provided cost-performance frameworks, i.e., better performance for higher cost. In the past 30 years, substantial improvements to unlicensed technologies have resulted in considerably higher speeds through better modulation, compression, interference mitigation and efficient spectrum techniques  . Although unlicensed technologies have advantages such as operation in any geographical area free of charge, unlicensed band frequencies (e.g., 5.4 GHz and 5.8 GHz in US) are not exclusively available for use. Licensed systems do not require complex mechanisms to handle spectrum crowding and interference. Systems with such link performance operate at higher SINR and throughput compared with unlicensed systems. On the other hand, systems operate in license exempt frequency bands have a higher collision probability and thus lower SINR and throughput due to highly likely subjection to interference caused by other communications over the same frequency. Before the emergence of LPWA, the industry showed vigilance towards the unlicensed part of the spectrum regardless of the substantial improvements introduced to these systems. The concerns were based on the fact that licensed systems are well prepared against interference and have higher reliability and performance  . The technical differences between licensed and unlicensed technologies have been diminished by LPWA  . They are divided over licensed and unlicensed spectrum technologies. LoRa, SigFox, RPMA and Telensa are unlicensed spectrum based, while Nb-IoT is licensed spectrum-based technology. There have been efforts by a group of mobile operators to standardize the LPWA technologies on licensed spectrum  . Transmission of unlicensed wireless systems does not necessarily mean that devices can stay active for a long time. SigFox, Telensa and LoRa, for example, use the unlicensed spectrum but are restricted to a 1% duty cycle in EU 868 ISM sub-band restricting device activity and channel utilization. It was found that each end device can have a maximum transmission time of 36 s/h BIB003 . As opposed to licensed systems, unlicensed wireless solutions are generally featured by high power consumption. For instance, RPMA operates in unlicensed global band 2.4 GHz ISM and consumes high power due to the use of high spectrum band BIB003 . The data rates of LPWA technologies are relatively low, resulting in more energy incorporated in each transmitted bit. This denotes that, being low power, LPWA is not necessarily a low-energy system. In terms of reliability, most unlicensed LPWA technologies (LoRaWAN, SigFox and Telensa) can provide reliable and robust communication with their low power transmission over the Sub-GHz band, which is not severely affected by attenuation and congestion BIB001 . NB-IoT is a licensed technology that performs at 158.5 kbps; RPMA performs at 624 kbps and is unlicensed. Thus, with respect to LPWA, licensed systems do not necessarily have higher data rates than unlicensed systems.