Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> The emerging Internet of Things (IoT) is getting closer to reality every day, with standards and protocols tailored to specific applications. Although some technologies follow a ‘make once, fit all’ approach, others are designed for specific applications such as smart-metering where the devices are often stationary with low jitter and throughput requirements. ETSI is already working towards a new standard for such applications, having specified a Specification Group to study what they call Low Throughput Networks (LTN) and propose a communications standard. In the meantime industrial solutions have entered the market such as, among others, Sigfox's and OnRamp Wireless technologies and LoRaWAN. Although this may lead to fragmentation of the IoT, they are worth examining to gain insights from their innovations and approaches to solving problems that are common in IoT technologies, and more specifically, security, energy management and resource constrains. This paper examines the wireless technologies of LoRaWan, Sigfox and OnRamp Wireless, the emerging leaders in IoT smart metering applications. These have so far not been studied in any peer-reviewed paper. Furthermore we discuss the suitability of Low Throughput Networks for certain applications and how the above technologies would work in such scenarios. <s> BIB001 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> This paper provides an overview of the Internet of Things (IoT) with emphasis on enabling technologies, protocols, and application issues. The IoT is enabled by the latest developments in RFID, smart sensors, communication technologies, and Internet protocols. The basic premise is to have smart sensors collaborate directly without human involvement to deliver a new class of applications. The current revolution in Internet, mobile, and machine-to-machine (M2M) technologies can be seen as the first phase of the IoT. In the coming years, the IoT is expected to bridge diverse technologies to enable new applications by connecting physical objects together in support of intelligent decision making. This paper starts by providing a horizontal overview of the IoT. Then, we give an overview of some technical details that pertain to the IoT enabling technologies, protocols, and applications. Compared to other survey papers in the field, our objective is to provide a more thorough summary of the most relevant protocols and application issues to enable researchers and application developers to get up to speed quickly on how the different protocols fit together to deliver desired functionalities without having to go through RFCs and the standards specifications. We also provide an overview of some of the key IoT challenges presented in the recent literature and provide a summary of related research work. Moreover, we explore the relation between the IoT and other emerging technologies including big data analytics and cloud and fog computing. We also present the need for better horizontal integration among IoT services. Finally, we present detailed service use-cases to illustrate how the different protocols presented in the paper fit together to deliver desired IoT services. <s> BIB002 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> The Internet of Things aims to connect several billions of devices. Terminals are expected to be low cost, low power, and able to achieve successful communication at long range. While current Machine-to-Machine technologies tend to use spreading factors to meet the required specifications, we propose a more sophisticated use of redundant waveforms in a scheme called Turbo-FSK. This scheme involves Frequency-Shift-Keying (FSK) modulation at the transmission, and a turbo-decoder dedicated to the FSK waveforms at the receiver. Highly robust communication is achieved with a mere transmitter, as complexity is deported on the receiver side. Results are compared to common modulations using spreading factors, a significant gain in performance is achieved even with small packet sizes. <s> BIB003 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> This paper proposes an analytical framework for the prediction of decoding error probabilities in heterogeneous wireless environments, where transmissions from various radio nodes with distinct Poisson-arrival rates and packet lengths populate the channel. Random channel access without feedback is assumed, where partial packet collisions can lead to the loss of packets. The analysis is based on the modeling of the collision length distribution between competing nodes. With recent results from information theory in the finite block length regime, we provide bounds on achievable decoding error probabilities for a given interference scenario. The new framework enables jointly considering inter- and intra-system interference, which is an important aspect in unlicensed radio bands. The framework’s applicability to optimize system designs is demonstrated for a typical low-power wide area network scenario. We study the tradeoff between reliability and code rate for a point-to-point link and present achievable throughput regions. The analysis reveals the superior performance of coded time-hopping spread spectrum systems. They reach very low error probability even under strong interference for wide ranges of practically relevant load regions. <s> BIB004 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> Low power wide area (LPWA) networks are making spectacular progress from design, standardization, to commercialization. At this time of fast-paced adoption, it is of utmost importance to analyze how well these technologies will scale as the number of devices connected to the Internet of Things inevitably grows. In this letter, we provide a stochastic geometry framework for modeling the performance of a single gateway LoRa network, a leading LPWA technology. Our analysis formulates the unique peculiarities of LoRa, including its chirp spread-spectrum modulation technique, regulatory limitations on radio duty cycle, and use of ALOHA protocol on top, all of which are not as common in today’s commercial cellular networks. We show that the coverage probability drops exponentially as the number of end-devices grows due to interfering signals using the same spreading sequence. We conclude that this fundamental limiting factor is perhaps more significant toward LoRa scalability than for instance spectrum restrictions. Our derivations for co-spreading factor interference found in LoRa networks enables rigorous scalability analysis of such networks. <s> BIB005 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> Long Range (LoRa) is a popular technology used to construct Low-Power Wide-Area Networks (LPWAN). Given the popularity of LoRa it is likely that multiple independent LoRa networks are deployed in close proximity. In this situation, neighbouring networks interfere and methods have to be found to combat this interference. In this paper we investigate the use of directional antennae and the use of multiple base stations as methods of dealing with inter-network interference. Directional antennae increase signal strength at receivers without increasing transmission energy cost. Thus, the probability of successfully decoding the message in an interference situation is improved. Multiple base stations can alternatively be used to improve the probability of receiving a message in a noisy environment. We compare the effectiveness of these two approaches via simulation. Our findings show that both methods are able to improve LoRa network performance in interference settings. However, the results show that the use of multiple base stations clearly outperforms the use of directional antennae. For example, in a setting where data is collected from 600 nodes which are interfered by four networks with 600 nodes each, using three base stations improves the Data Extraction Rate (DER) from 0.24 to 0.56 while the use of directional antennae provides an increase to only 0.32. <s> BIB006 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> Abstract Low-Power Wide Area Networks (LPWANs) have arisen as a promising communication technology for supporting Internet of Things (IoT) services due to their low power operation, wide coverage range, low cost and scalability. However, most LPWAN solutions like SIGFOX™or LoRaWAN™rely on star topology networks, where stations (STAs) transmit directly to the gateway (GW), which often leads to rapid battery depletion in STAs located far from it. In this work, we analyze the impact on LPWANs energy consumption of multi-hop communication in the uplink, allowing STAs to transmit data packets in lower power levels and higher data rates to closer parent STAs, reducing their energy consumption consequently. To that aim, we introduce the Distance-Ring Exponential Stations Generator (DRESG) framework, 1 designed to evaluate the performance of the so-called optimal-hop routing model, which establishes optimal routing connections in terms of energy efficiency, aiming to balance the consumption among all the STAs in the network. Results show that enabling such multi-hop connections entails higher network lifetimes, reducing significantly the bottleneck consumption in LPWANs with up to thousands of STAs. These results lead to foresee multi-hop communication in the uplink as a promising routing alternative for extending the lifetime of LPWAN deployments. <s> BIB007 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> A huge increase of machines attached to wireless networks is expected in the next few years. A large part of these machines will be covered by some wireless wide area networks. The arrival of cellular M2M (machine-to-machine) communication poses new requirements due to its specific characteristics. For most of the cellular M2M applications, the essential requirement is low energy consumption level or high energy efficiency. This survey provides a global view of the network technologies previewed for cellular M2M. In this survey, we study the existing classifications of M2M applications according to different criteria in the literature. The comparison of traffic characteristics between M2M and human-to-human is also proposed. Quality of service (QoS) requirements for typical M2M applications are resumed. The advance of reference M2M network architectures proposed by the Standard Development Organization (SDO) is investigated. We identify two possible effort directions to improve the energy efficiency for cellular M2M. The first one is to evolve the current existing 3rd Generation Partnership Project (3GPP) Consortium cellular networks to effectively support MTC (Machine Type Communication). The other direction is to design M2M-dedicated networks from scratch, which are often called low-power wide-area (LPWA) networks. We review, compare and categorize the proposals related to energy issues of cellular M2M mainly over the period 2011–2015 for the first direction. We introduce the development of LPWA networks for the other research directions. We highlight that the cooperative relaying, the design of energy-efficient signaling and operation, the new radio resource allocation schemes, and the energy-efficient random access procedure are the main points of improvement. It is important to jointly use the aforementioned approaches, for example, joint design of random access control and radio resource allocation, to seek for a trade-off between energy efficiency and other system performances. <s> BIB008 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <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> BIB009 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <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> BIB010 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> The Internet-of-Things (IoT) landscape is expanding with new radio technologies. In addition to the Low-Rate Wireless Personal Area Network (LR-WPAN), the recent set of technologies conforming the so-called Low-Power Wide Area Networks (LP-WAN) offers long-range communications, allowing one to send small pieces of information at a reduced energy cost, which promotes the creation of new IoT applications and services. However, LP-WAN technologies pose new challenges since they have strong limitations in the available bandwidth. In general, a first step prior to a smart object being able to gain access to the network is the process of network access authentication. It involves authentication, authorization and key management operations. This process is of vital importance for operators to control network resources. However, proposals for managing network access authentication in LP-WAN are tailored to the specifics of each technology, which could introduce interoperability problems in the future. In this sense, little effort has been put so far into providing a wireless-independent solution for network access authentication in the area of LP-WAN. To fill this gap, we propose a service named Low-Overhead CoAP-EAP (LO-CoAP-EAP), which is based on previous work designed for LR-WPAN. LO-CoAP-EAP integrates the use of Authentication, Authorization and Accounting (AAA) infrastructures and the Extensible Authentication Protocol (EAP) protocol. For this integration, we use the Constrained Application Protocol (CoAP) to design a network authentication service independent of the type of LP-WAN technology. LO-CoAP-EAP represents a trade-off between flexibility, wireless technology independence, scalability and performance in LP-WAN. <s> BIB011 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> Internet of Things (IoT) technology has attracted much attention in recent years for its potential to alleviate the strain on healthcare systems caused by an aging population and a rise in chronic illness. Standardization is a key issue limiting progress in this area, and thus this paper proposes a standard model for application in future IoT healthcare systems. This survey paper then presents the state-of-the-art research relating to each area of the model, evaluating their strengths, weaknesses, and overall suitability for a wearable IoT healthcare system. Challenges that healthcare IoT faces including security, privacy, wearability, and low-power operation are presented, and recommendations are made for future research directions. <s> BIB012 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <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> BIB013 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> In 3GPP LTE Release 13, Narrowband Internet of Things (NB-IoT) was standardized for providing wide-area connectivity for massive machine-type communications for IoT. In LTE Release 14, NB-IoT was further developed to deliver enhanced user experience in selected areas through the addition of features such as increased positioning accuracy, increased peak data rates, the introduction of a lower device power class, improved non-anchor carrier operation, multicast, and authorization of coverage enhancements. In this article, we provide an overview of these features introduced for NB-IoT in LTE Release 14. An analysis is given on the applicability of these features and their benefits to enhance the NB-IoT radio access technology. <s> BIB014 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> Abstract Internet of Things (IoT) technologies can provide various intelligent services by collecting information from objects. To collect information, Wireless Sensor Networks (WSNs) are exploited. The Low Power Wide Area Network (LPWAN), one type of WSN, has been designed for long-range IoT services. It consumes low power and uses a low data rate for data transmission. The LPWAN includes several communication standards, and Long Range Wide Area Network (LoRaWAN) is the representative standard of the LPWAN. LoRaWAN provides several data rates for transmission and enables adaptive data rate control in order to maintain network connectivity. In the LoRaWAN, the wireless condition is considered by the reception status of the acknowledgement (ACK) message, and adaptive data rate control is performed according to the wireless condition. Because the judgment of the wireless condition by the reception status of ACK messages does not reflect congestion, adaptive data rate control can lead to inefficiency in data transmission. For efficient data transmission in long-range IoT services, this paper proposes a congestion classifier using logistic regression and modified adaptive data rate control. The proposed scheme controls the data rate according to the congestion estimation. Through extensive analysis, we show the proposed scheme’s efficiency in data transmission. <s> BIB015 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> As the Internet-of-Things is becoming a reality, the need for a new Low-Power Wide-Area (LPWA) network emerged in the last few years. Numerous low-cost devices will be connected, and this requires an optimization of the link budget: the physical layer needs to be designed highly energy efficient. The combination of M-ary orthogonal Frequency-Shift-Keying (M-FSK) modulation and coding in the same process has been shown to be a promising candidate when associated with an iterative receiver (turbo principle). In this work, we study this new digital transmission scheme, called Turbo-FSK. An EXtrinsic Information Transfer (EXIT) chart analysis is realized. The influence of the packet length is investigated, and the scheme is shown to stay energy efficiency even with short packet sizes. Comparison with LPWA current technologies is performed, showing the potential of this technology. <s> BIB016 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> In the last few years, traffic over wireless networks has been increasing exponentially due to the impact of IoT. IoT is transforming a wide range of services in different domains of urban life, such as environmental monitoring, home automation, and public transportation. The so-called smart city applications will introduce a set of stringent requirements, such as low latency and high mobility, since services must be allocated and instantiated on demand, simultaneously, close to multiple devices at different locations. Efficient resource provisioning functionalities are needed to address these demanding constraints introduced by smart city applications while minimizing resource costs and maximizing QoS. In this article, the CoT framework is presented, which provides not only data collection and analysis functionalities but also automated resource provisioning mechanisms for future smart city applications. CoT is deployed as a smart city testbed in Antwerp, Belgium, which allows researchers and developers to easily set up and validate IoT experiments. A smart city use case of air quality monitoring through the deployment of air quality sensors in moving cars is presented showing the full applicability of the CoT framework for a flexible and scalable resource provisioning in the smart city ecosystem. <s> BIB017 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> Abstract By 2020, more than 50 billion devices will be connected through radio communications. In conjunction with the rapid growth of the Internet of Things (IoT) market, low power wide area networks (LPWAN) have become a popular low-rate long-range radio communication technology. Sigfox, LoRa, and NB-IoT are the three leading LPWAN technologies that compete for large-scale IoT deployment. This paper provides a comprehensive and comparative study of these technologies, which serve as efficient solutions to connect smart, autonomous, and heterogeneous devices. We show that Sigfox and LoRa are advantageous in terms of battery lifetime, capacity, and cost. Meanwhile, NB-IoT offers benefits in terms of latency and quality of service. In addition, we analyze the IoT success factors of these LPWAN technologies, and we consider application scenarios and explain which technology is the best fit for each of these scenarios. <s> BIB018 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> Abstract Wireless sensor and actuator networks play a central role in the Internet of Things, and a lot of effort is devoted to enable energy efficient and low latency communications. In the recent years, low power communications has evolved towards multi-kilometer ranges and low bit-rate approaches such as LoRa™. However, the medium access layer protocols rely on the well-known duty-cycling schemes, which require a trade-off between power consumption and latency for message transfer from the gateway to the nodes. Domains such as industrial applications in which sensors and actuators are part of the control loop require predictable latency, as well as low power consumption. Emerging ultra-low-power wake-up receivers enable pure-asynchronous communications, allowing both low latency and low power consumption, but at the cost of a lower sensitivity and lower range than traditional wireless receivers and LoRa™. In this work, we propose an energy efficient architecture that combines long-range communication with ultra low-power short-range wake-up receivers to achieve both energy efficient and low latency communication in heterogeneous long-short range networks. A hardware architecture as well as a protocol is proposed to exploit the benefits of these two communication schemes. Experimental measurements and analytical comparisons show that the proposed approach remove the need for a trade-off between power consumption and latency. <s> BIB019 </s> Low Power Wide Area Networks: A Survey of Enabling Technologies, Applications and Interoperability Needs <s> VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS <s> Indoor localization has recently witnessed an increase in interest, due to the potential wide range of services it can provide by leveraging Internet of Things (IoT), and ubiquitous connectivity. Different techniques, wireless technologies and mechanisms have been proposed in the literature to provide indoor localization services in order to improve the services provided to the users. However, there is a lack of an up-to-date survey paper that incorporates some of the recently proposed accurate and reliable localization systems. In this paper, we aim to provide a detailed survey of different indoor localization techniques, such as angle of arrival (AoA), time of flight (ToF), return time of flight (RTOF), and received signal strength (RSS); based on technologies, such as WiFi, radio frequency identification device (RFID), ultra wideband (UWB), Bluetooth, and systems that have been proposed in the literature. This paper primarily discusses localization and positioning of human users and their devices. We highlight the strengths of the existing systems proposed in the literature. In contrast with the existing surveys, we also evaluate different systems from the perspective of energy efficiency, availability, cost, reception range, latency, scalability, and tracking accuracy. Rather than comparing the technologies or techniques, we compare the localization systems and summarize their working principle. We also discuss remaining challenges to accurate indoor localization. <s> BIB020
There have been extensive efforts by the research community to optimize or evaluate LPWA operation. Table 4 summarizes these contributions. These works, however, have not addressed all issues associated with the LPWA network systems. In this section, we discuss current LPWA challenges and present directions for future research in this field. In 2016, nearly 17.6 billion devices, including smartphones, tablets, and computers  , were connected via IoT technologies, and this number is set to hit 25.1 billion by 2021  . However, what if each of these 25.1 billion devices could be easily attacked and compromised by hackers? According to a Business Insider survey, 39% of respondents regard this possibility as a great concern. This concern is understandable because these devices collect and VOLUME 6, 2018 transmit data that are valuable and often sensitive and should be secured as such. Since LPWA communications mostly operate in the free unlicensed spectrum, LPWA-based IoT applications will then have security concerns. Although SigFox applies different techniques to secure the network, such as sequencing, message scrambling and anti-replay, it does not encrypt the transmitted payload, and thus it relies on the upper layer for this purpose. SigFox devices rely on a fixed secret key for online registration, which can be exploited for sending forge messages to the network. This may lead to a node being blacklisted if more than the limited number of messages is exceeded BIB001 . RPMA is also reported to be vulnerable to security BIB001 . LoRaWAN end devices, on the other hand, can be exploited when activated Over-The-Air through the nonce that is used to send an unencrypted join request BIB001 . These issues can be addressed by applying better security techniques or by operating in the licensed spectrum. Applying up-to-date security patches and protocols that allow device mobility while enforcing security such as the locator identity separation protocol (LISP)  and network mobility (NEMO)  are some of these techniques BIB009 . The extensible authentication protocol method for 3rd generation authentication and key agreement (EAP-AKA)  and EAP method for GSM subscriber identity modules (EAP-SIM)  are designed for cellular devices while lightweight authentication protocols such as the pre-shared key EAP method (EAP-PSK)  are proposed for devices that require little computing power and memory. A lightweight bootstrapping service is proposed for IoT devices in  using CoAP, EAP and authentication, authorization and accounting (AAA) architecture. Currently, works are in progress within IETF which encourage the use of EAP such as EAP-AKA  (a small revision of EAP-AKA), EAP-transport layer security (EAP-TLS1.3)  and nimble out-of-band authentication for EAP (EAP-NOOB)  . EAP-TLS is an EAP authentication method which is widely supported in WiFi, MulteFire [110] and 5G networks. EAP-NOOB is proposed for the registration, authentication and key derivation of IoT devices that have a minimal user interface and no pre-configured authentication credentials, and could also be a candidate for authenticating LPWA devices due to similar device characteristics. On the other hand, IETF recommends AAA framework  to tackle some of the security issues of LPWA networks  , and it has been considered as one of the technologies to secure IoT deployments in BIB009 . Authentication of the massive number of nodes that number in the hundreds of thousands in some scenarios is a challenge for LPWA technologies. This is also true for data encryption  , BIB010 ,  , BIB011 . Furthermore, security mechanisms of systems that rely on the cloud for data storage and processing demand further investigation. This should not be overlooked for healthcare systems, where privacy is mandatory BIB002 ,  , BIB012 . Some of the LPWA solutions are currently based on the cloud, and the future trend is likely towards more cloud dependence. The large amounts of data collected from sensors deployed in end devices must be processed, and information should be generated in a way that is meaningful to humans. A framework that provides mechanisms for data collection, analysis and resource provision in smart cities is presented in BIB017 . Traditional approaches to data processing are no longer appropriate with respect to the IoT environment BIB012 . This task requires extensive research that can lead to developing efficient and secure data mining algorithms and implementing machine learning for managing big data. The LPWA technologies surveyed in this paper provide basic support for roaming, which allows network deployment on a large scale. This matter is being investigated by the LoRa-Alliance in the LoRaWAN 1.1 specification BIB013 . SigFox makes it possible for devices to communicate with multiple base stations without the need for roaming or handovers BIB018 . However, mobility support must be further developed to include services such as end-to-end secure communication, inter-operator billing, device location and transparent provisioning throughout roaming periods. Although LPWA uses a number of robust and reliable techniques, the massive number of LPWA devices that are mainly operating in shared radio spectrum are error-prone. This is a serious issue for data transmission (as is the case in the LPWA networks where short messages are often transmitted), which requires a high level of accuracy. Message integrity concerns the LoRaWAN DL as frames are not checked for CRC and NB-IoT supports a single adaptive asynchronous hybrid automatic repeat request (HARQ) for both links BIB014 . This raises the issue of message integrity, which needs to be properly addressed. The low-end device cost is a potential economic factor driving the deployment of LPWA systems by businesses and governments. This one-time cost, however, is not as significant as the cost of service subscription. Thus, LPWA operators must keep this cost as minimal as possible. On the one hand, the use of licensed spectrum resources by some LPWA technologies will increase their operational costs due to the license subscription requirement. On the other hand, the large number of objects, whose number will increase even more in the future, operating in the free unlicensed spectrum increases the probability of packet collision.  has found that although LoRa is robust to packet collisions resulting from concurrent transmissions, introducing timing offsets between relaying packets to add a random timing delay can further improve the receiver performance. CSMA/CA or cellular like multiple access protocols seem to be very complicated for simple and cheap LPWA devices. Although the use of Sub-GHz band mitigates the possibility of interference at low power budgets and duty cycle regulations limits device activity on the network, more efficient protocols are vital for better managing access to the network BIB010 . For LPWA technologies that operate in the licensed spectrum, the issues of frequency licensing and spectrum management must be addressed. There is a potential for new radio schemes to be introduced as needs and technology evolve. The initial target of LPWA was applications and services that are not sensitive to delay or data loss. To be widely deployed, LPWA networks must be able to satisfy the requirements of a wide range of services. Examples of services that currently do not have sufficient support of LPWA include video surveillance applications, which require high data rates. Although RPMA and NB-IoT, with their relatively high data rates and large payload length (10 kB BIB010 and 317 B BIB014 , respectively) can be tentative solutions for these applications, they must be permitted to communicate at higher data rates and larger payloads (depending on the encoding rate). This, however, is not possible with convolutional codes in devices that replaced turbo codes for the sake of simple decoding BIB014 . Furthermore, because LPWA networks are often capable of supporting low data rates and small frame sizes, the data need to be compressed robustly. It has been proposed to take advantage of compression techniques at the application layer such as CoIP for this purpose BIB009 . LPWA technologies communicate over low data rates that limit the size of frames. This shows the inefficiency of classical compression techniques for these technologies and the need for more efficient techniques. To address the requirements of different IoT applications, LoRaWAN offers a number of classes and adapts the data rate using spreading factors. A congestion classifier was pro-VOLUME 6, 2018 posed by BIB015 for data rate control. Although most LPWA technologies support DL communications, some (e.g., SigFox and Telensa) are not effective for use-cases that require communication with end devices due to low data rates on the DL. LPWA technologies deploy modulation (such as UNB) and spread-spectrum techniques (such as CSS and DSSS) that are resilient to interference. The analysis by BIB004 showed a low error probability for coded time-hopping spread spectrum systems, even under strong interference. LoRa modulation exhibits good resistance to interference  . However, interference from signals using the same spreading sequence drops the coverage probability exponentially as the number of devices increases BIB005 . Furthermore, the popularity of LoRa has resulted in the deployment of multiple networks in close proximity, causing high interference. To ensure the reception of the message, SigFox allows nodes to send the same message up to three times. It has been observed that the use of multiple base stations or directional antennas improves performance in an environment characterized by interference BIB006 . Turbo-FSK is a new UL scheme for LPWA networks. The transmitter performs FSK modulation, and the receiver turbo-decodes the FSK waveforms. M-ary orthogonal FSK modulation is associated with an iterative receiver (the turbo principle) in BIB016 . An extrinsic information transfer (EXIT) chart analysis  was used to optimize the parameters and find the best values. The scheme was reported to be energy efficient for different packet sizes when compared with the IEEE 802.15.4k standard  , the LoRa physical layer  and the serial concatenation of turbo codes (TC)  . The work extended the initial Turbo-FSK analysis presented in BIB003 . Whereas LPWA are required to provide higher data rates and larger payloads to support realtime applications, these come at the expense of increasing the encoding rate and eventually less robust radio links. Whereas positioning can bring benefits to IoT applications, it is vital in some use-cases. Health condition and environmental monitoring, smart transportation and cities are among use-cases that require the nodes to be able to determine their physical position or logical location. Currently, NB-IoT and LoRaWAN allow the nodes to be located based on received signals and time BIB010 , BIB014 . Basic positioning of Release 13 NB-IoT is based on cell identity (CID), and advanced positioning of Release 14 is supported through the observed time difference of arrival (OTDOA) technique BIB014 . A similar approach with an ultra-high resolution time-stamp is exploited by LoRaWAN; however, its inaccuracy in indoor environments has been argued BIB020 . Mechanisms such as the combination of LoRa and GPS were proposed for more accurate positioning [121] . This articulates the need for new techniques that can provide precise positioning services to IoT applications. There are cases where a message needs to be sent to a number of nodes simultaneously, for example, when the firmwares of a large number of end devices are upgraded or street lights are turned on or off. NB-IoT provides multimedia broadcast multicast services (MBMS) using single-cell point-to-multipoint (SC-PtM) data BIB014 . RPMA and LoRaWAN also support multicast  ,  . However, LPWA systems must address issues related to multicast communications such as security and data flooding. A new provisioning class was recently added to Release 14 NB-IoT to reduce the power consumption of end devices to a maximum of 14 dBm compared to 20 dBm in Release 13 BIB014 . LoRa was combined with wake-up receivers in an architecture to improve both power consumption and latency BIB019 . The combination of optimal-hop routing and the transmission configurations algorithm yielded a longer network lifetime for multi-hop communication compared with single-hop communication due to the reduction in the energy consumption of far-end devices in an LPWA network with up to thousands of end devices BIB007 . Cooperative relaying, signaling, radio resource allocation, and random access schemes are identified as main areas of energy efficiency improvement in BIB008 . More energy-efficient algorithms are particularly vital for LPWA technologies whose inherent energy efficiency is not high such as RPMA. Scalability will most likely be a serious issue of LPWA technologies since the number of devices are exponentially increasing. Although LoRa allows transmitters a range of communication options, such as center frequency, SF, bandwidth, and coding rates, it limits the number of supported transmitters. Studies have shown that the number of end devices has a greater impact on LoRa scalability than spectrum restriction BIB005 . According to  and  , LoRa can scale well only if it is supported by dynamic transmission parameter selection and/or multiple sinks; otherwise, up to 120 nodes can be handled in 3.8 ha.