Low Power Wide Area Networks: A Survey <s> Low Power Wide Area <s> As one of the fastest growing technologies, machine-to-machine (M2M) communications are expected to provide ubiquitous connectivity. M2M devices can be used for a wide range of emerging applications that have various communications requirements. While M2M communications have been developed for many years, major challenges still remain with their efficient implementation from the perspective of low energy consumption and wide coverage. To address these challenges, low power wide area (LPWA) technology is investigated as one of the potential candidate solutions. In this article, we first introduce some typical LPWA M2M application scenarios. Given their requirements, we highlight key techniques and standards that are explicitly designed for LPWA M2M communications. Finally, we present an LPWA prototype system to evaluate its performance and demonstrate its potential in bridging a technological gap for future Internet-of-Things (IoT) applications. <s> BIB001 </s> Low Power Wide Area Networks: A Survey <s> Low Power Wide Area <s> In addition to long battery life and low cost, coverage is one of the most critical performance metrics for the low power wide area networks (LPWAN). In this work we study the coverage of the recently developed LoRa LPWAN technology via real-life measurements. The experiments were conducted in the city of Oulu, Finland, using the commercially available equipment. The measurements were executed for cases when a node located on ground (attached on the roof rack of a car) or on water (attached to the radio mast of a boat) reporting their data to a base station. For a node operating in the 868 MHz ISM band using 14 dBm transmit power and the maximum spreading factor, we have observed the maximum communication range of over 15 km on ground and close to 30 km on water. Besides the actual measurements, in the paper we also present a channel attenuation model derived from the measurement data. The model can be used to estimate the path loss in 868 MHz ISM band in an area similar to Oulu, Finland. <s> BIB002
requirements of IoT applications. LPWA technologies offer unique sets of features including wide-area connectivity for low power and low data rate devices, not yet provided by legacy wireless technologies. Their market is expected to be huge: approximately one fourth of overall 30 billion IoT devices are to be connected to the Internet using LPWA networks using either proprietary or cellular LPWA technologies. Figure 1 highlights variety of applications across several business sectors that can exploit LPWA technologies to connect their end devices. These business sectors include but not limited to smart city, personal IoT applications, smart grid, smart metering, logistics, industrial monitoring, agriculture, etc. LPWA networks are unique because they make different tradeoffs than the traditional technologies prevalent in IoT landscape such as short-range wireless networks e.g., ZigBee, Bluetooth, Z-Wave, wireless local area networks (WLANs) e.g., Wi-Fi, and cellular networks e.g. Global System for Mobile Communications (GSM), Long-Term Evolution (LTE) etc. Figure 2 highlights these differences. The legacy noncellular wireless technologies are not ideal to connect low power devices distributed over large geographical areas. The range of these technologies is limited to a few hundred meters at best. The devices, therefore, cannot be arbitrarily deployed or moved anywhere, a requirement for many applications for smart city, logistics and personal health BIB001 . The range of these technologies is extended using a dense deployment of devices and gateways connected using multihop mesh networking. Large deployments are thus prohibitively expensive. WLANs, on the other hand, are characterized by shorter coverage areas and higher power consumption for machine-type communication (MTC). A wide area coverage is provided by cellular networks, a reason of a wide adoption of second generation (2G) and third generation (3G) technologies for M2M communication. However, an impending decommissioning of these technologies  , as announced by some mobile network operators (MNOs), will broaden the technology gap in connecting low-power devices. In general, current cellular technologies do not achieve energy efficiency high enough to offer ten years of battery lifetime. The complexity and cost of cellular devices is high due to their ability to deal with complex waveforms, optimized for voice, high speed data services, and text. For low-power MTC, there is a clear need to strip complexity to reduce cost. Efforts in this direction are underway for cellular networks by the Third Generation Partnership Project and are covered as part of the discussion in Section IV. With a phenomenal range of a few to tens of kilometers BIB002 and battery life of ten years and beyond, LPWA technologies are promising for the Internet of low-power, low-cost, and lowthroughput things. A very long range of LPWA technologies enables devices to spread and move over large geographical areas. IoT and M2M devices connected by LPWA technologies can be turned on anywhere and anytime to sense and interact with their environment instantly. It is worth-mentioning that LPWA technologies achieve long range and low power operation at the expense of low data rate (typically in orders of tens of kilobits per seconds) and higher latency (typically in orders of seconds or minutes). These technologies do not cater to high throughput applications requiring ultra-low latency such as wireless industrial control. Nevertheless, a plethora of applications for smart cities, smart metering, home automation, wearable electronics, logistics, environmental monitoring etc. exchange small amount of data and that also infrequently. Therefore, appeal of LPWA technologies, although limited by its low data rate, is still broad. This is the reason why LPWA technologies generated so much interest after the proprietary technologies such as SIGFOX [6] and LORa  hit the market. At this moment, there are several competing LPWA technologies, each employing various techniques to achieve long range, low power operation, and high scalability. Section II presents these design goals and describes how a combination of different novel techniques actually achieves them. Section III then discusses several early proprietary LPWA technologies and their technical features, highlighting the need for standardization to flourish IoT ecosystem. To this effect, several well-known standard developing organizations (SDOs) such as European Telecommunications Standard Institute (ETSI)  , Third Generation Partnership Project (3GPP)  , Institute of Electrical and Electronics Engineers (IEEE)  , and Internet Engineering Task Force (IETF)  are working towards the open standards for LPWA technologies. Further, multiple industrial alliances are built around individual LPWA technologies to promote new standards. LORa™ Alliance [12] , WEIGHTLESS-SIG [13] and DASH7 Alliance  are a few examples of such special interest groups (SIGs). Section IV covers the standardization efforts led by all these SDOs and SIGs. On a technical side, LPWA providers need to push innovative solutions to overcome the challenge of connecting massive number of IoT and M2M devices. It is indeed not an easy task especially when the heterogeneous LPWA technologies share limited radio resources to render scalable and secure connectivity to low-power and inexpensive end devices. Multiple tradeoffs made by the LPWA technologies bring several challenges, which are discussed in Section V along with possible research directions to address them. Section VI then highlights business considerations for LPWA technologies before finally concluding this paper.