A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> New Internet of Things (IoT) technologies such as Long Range (LoRa) are emerging which enable power efficient wireless communication over very long distances. Devices typically communicate directly to a sink node which removes the need of constructing and maintaining a complex multi-hop network. Given the fact that a wide area is covered and that all devices communicate directly to a few sink nodes a large number of nodes have to share the communication medium. LoRa provides for this reason a range of communication options (centre frequency, spreading factor, bandwidth, coding rates) from which a transmitter can choose. Many combination settings are orthogonal and provide simultaneous collision free communications. Nevertheless, there is a limit regarding the number of transmitters a LoRa system can support. In this paper we investigate the capacity limits of LoRa networks. Using experiments we develop models describing LoRa communication behaviour. We use these models to parameterise a LoRa simulation to study scalability. Our experiments show that a typical smart city deployment can support 120 nodes per 3.8 ha, which is not sufficient for future IoT deployments. LoRa networks can scale quite well, however, if they use dynamic communication parameter selection and/or multiple sinks. <s> BIB001 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> LoRaWAN is a promising low power long range wireless communications technology for the Internet of Things. An important feature of LoRaWAN gateways is related to so-called capture effect: under some conditions the gateway may correctly receive a frame even if it overlaps with other ones. In this paper, we develop a pioneering mathematical model of a LoRaWAN network which allows finding network capacity and transmission reliability taking into account the capture effect. <s> BIB002 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> LoRa is a long-range, low power, low bit rate and single-hop wireless communication technology. It is intended to be used in Internet of Things (IoT) applications involving battery-powered devices with low throughput requirements. A LoRaWAN network consists of multiple end nodes that communicate with one or more gateways. These gateways act like a transparent bridge towards a common network server. The amount of end devices and their throughput requirements will have an impact on the performance of the LoRaWAN network. This study investigates the scalability in terms of the number of end devices per gateway of single-gateway LoRaWAN deployments. First, we determine the intra-technology interference behavior with two physical end nodes, by checking the impact of an interfering node on a transmitting node. Measurements show that even under concurrent transmission, one of the packets can be received under certain conditions. Based on these measurements, we create a simulation model for assessing the scalability of a single gateway LoRaWAN network. We show that when the number of nodes increases up to 1000 per gateway, the losses will be up to 32%. In such a case, pure Aloha will have around 90% losses. However, when the duty cycle of the application layer becomes lower than the allowed radio duty cycle of 1%, losses will be even lower. We also show network scalability simulation results for some IoT use cases based on real data. <s> BIB003 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> As LoRaWAN networks are actively being deployed in the field, it is important to comprehend the limitations of this low power wide area network technology. Previous work has raised questions in terms of the scalability and capacity of LoRaWAN networks as the number of end devices grows to hundreds or thousands per gateway. Some works have modeled LoRaWAN networks as pure ALOHA networks, which fails to capture important characteristics such as the capture effect and the effects of interference. Other works provide a more comprehensive model by relying on empirical and stochastic techniques. This paper uses a different approach where a LoRa error model is constructed from extensive complex baseband bit error rate simulations and used as an interference model. The error model is combined with the LoRaWAN MAC protocol in an ns-3 module that enables to study multichannel, multispreading factor, multigateway, bi-directional LoRaWAN networks with thousands of end devices. Using the LoRaWAN ns-3 module, a scalability analysis of LoRaWAN shows the detrimental impact downstream traffic has on the delivery ratio of confirmed upstream traffic. The analysis shows that increasing gateway density can ameliorate but not eliminate this effect, as stringent duty cycle requirements for gateways continue to limit downstream opportunities. <s> BIB004 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> The article provides an analysis and reports experimental validation of the various performance metrics of the LoRa low-power wide-area network technology. The LoRa modulation is based on chirp spread spectrum, which enables use of low-quality oscillators in the end device, and to make the synchronization faster and more reliable. Moreover, LoRa technology provides over 150 dB link budget, providing good coverage. Therefore, LoRa seems to be quite a promising option for implementing communication in many diverse Internet of Things applications. In this article, we first briefly overview the specifics of the LoRa technology and analyze the scalability of the LoRa wide-area network. Then, we introduce setups of the performance measurements. The results show that using the transmit power of 14 dBm and the highest spreading factor of 12, more than 60% of the packets are received from the distance of 30 km on water. With the same configuration, we measured the performance of LoRa communication in mobile scenarios. The presented results reveal that at around 40 km/h, the communication performance gets worse, because duration of the LoRa-modulated symbol exceeds coherence time. However, it is expected that communication link is more reliable when lower spreading factors are used. <s> BIB005 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> The need for low power, long range and low cost connectivity to meet the requirements of IoT applications has led to the emergence of Low Power Wide Area (LPWA) networking technologies. The promise of these technologies to wirelessly connect massive numbers of geographically dispersed devices at a low cost continues to attract a great deal of attention in the academic and commercial communities. Several rollouts are already underway even though the performance of these technologies is yet to be fully understood. In light of these developments, tools to carry out `what-if analyses' and pre-deployment studies are needed to understand the implications of choices that are made at design time. While there are several promising technologies in the LPWA space, this paper specifically focuses on the LoRa/LoRaWAN technology. In particular, we present LoRaWANSim, a simulator which extends the LoRaSim tool to add support for the LoRaWAN MAC protocol, which employs bidirectional communication. This is a salient feature not available in any other LoRa simulator. Subsequently, we provide vital insights into the performance of LoRaWAN based networks through extensive simulations. In particular, we show that the achievable network capacity reported in earlier studies is quite optimistic. The introduction of downlink traffic can have a significant impact on the uplink throughput. The number of transmit attempts recommended in the LoRaWAN specification may not always be the best choice. We also highlight the energy consumption versus reliability trade-offs associated with the choice of number of retransmission attempts. <s> BIB006 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> Providing low power and long range (LoRa) connectivity is the goal of most Internet of Things networks, e.g., LoRa, but keeping communication reliable is challenging. LoRa networks are vulnerable to the capture effect. Cell-edge nodes have a high chance of losing packets due to collisions, especially when high spreading factors (SFs) are used that increase time on air. Moreover, LoRa networks face the problem of scalability when they connect thousands of nodes that access the shared channels randomly. In this paper, we propose a new MAC layer—RS-LoRa—to improve reliability and scalability of LoRa wide-area networks (LoRaWANs). The key innovation is a two-step lightweight scheduling : 1) a gateway schedules nodes in a coarse-grained manner through dynamically specifying the allowed transmission powers and SFs on each channel and 2) based on the coarse-grained scheduling information, a node determines its own transmission power, SF, and when and on which channel to transmit. Through the proposed lightweight scheduling, nodes are divided into different groups, and within each group, nodes use similar transmission power to alleviate the capture effect. The nodes are also guided to select different SFs to increase the network reliability and scalability. We have implemented RS-LoRa in NS-3 and evaluated its performance through extensive simulations. Our results demonstrate the benefit of RS-LoRa over the legacy LoRaWAN, in terms of packet error ratio, throughput, and fairness. For instance, in a single-cell scenario with 1000 nodes, RS-LoRa can reduce the packet error ratio of the legacy LoRaWAN by nearly 20%. <s> BIB007 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> LoRaWAN is gaining momentum in the arena of IoT connectivity technologies thanks to the low cost, ease of deployment, and support for adaptable transmission rates and bidirectional communications. The research community has then been working to develop suitable analytical models that can be instrumental in the study of this technology. In this work we propose a mathematical model that makes it possible to accurately estimate the packet success probability of a LoRaWannetwork in presence of bi-directional traffic, i.e., both uplink (UL) and downlink (DL) transmissions, and that accounts for the most critical features of the LoRa chipset and the LoRaWanstandard. The proposed model, furthermore, makes it possible to study the effect of different parameters configurations, thus offering a valid tool to investigate possible improvements to the system configuration. The proposed model is first validated by comparison with some accurate simulation results and, then, its potential is exemplified by analyzing the system settings that yield the best network performance. <s> BIB008 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> LoRa is a leading Low-Power Wide-Area Network (LPWAN) technology for Internet of Things (IoT). While LoRa networks are rapidly being deployed around the world, it is important to understand the capabilities and limitations of this technology in terms of its throughput, coverage and scalability. Using a combination of real-world measurements and high fidelity simulations, this paper aims at characterizing the performance of LoRa. Specifically, we present and analyze measurement data collected from a city-wide LoRa deployment in order to characterize the throughput and coverage of LoRa. Moreover, using a custom-built simulator tuned based on our measurement data, we present extensive simulation results in order to characterize the scalability of LoRa under a variety of traffic and network settings. Our measurement results show that as few as three gateways are sufficient to cover a dense urban area within an approximately 15 Km radius. Also, a single gateway can support as many as 105 end devices, each sending 50 bytes of data every hour with negligible packet drops. On the negative side, while a throughput of up to 5.5 Kbps can be achieved over a single 125 KHz channel at the physical layer, the throughput achieved at the application layer is substantially lower, less than 1 Kbps, due to the network protocols overhead. <s> BIB009 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> Although LoRaWAN offers many advantages, one of its main limitation is the insufficient downlink capacity in large-scale scenarios. This makes reliable communication impractical as, in LoRaWAN, reliability is achieved through the acknowledgement of confirmed messages. The contribution of this paper is to provide a comprehensive description of the problems associated with the presence of downlink traffic and to propose solutions to enable the use of acknowledgments for low to medium downlink load. The proposed mechanisms build on a more adequate and flexible choice of the gateway for each downlink transmission by the network server. Thus, they are entirely compatible with existing LoRaWAN technologies and deployments. Our simulation results show that the proposed solutions enable the successful realization of a higher percentage of LoRaWAN confirmed messages. Thus, the common practice of strongly discouraging the use of confirmed messages could be reconsidered to allow applications that require reliable communication. <s> BIB010 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> Low-power wide-area network (LPWAN) technologies are gaining momentum for Internet-of-things applications since they promise wide coverage to a massive number of battery operated devices using grant-free medium access. LoRaWAN, with its physical (PHY) layer design and regulatory efforts, has emerged as the widely adopted LPWAN solution. By using chirp spread spectrum modulation with qausi-orthogonal spreading factors (SFs), LoRa PHY offers coverage to wide-area applications while supporting high-density of devices. However, thus far its scalability performance has been inadequately modeled and the effect of interference resulting from the imperfect orthogonality of the SFs has not been considered. In this paper, we present an analytical model of a single-cell LoRa system that accounts for the impact of interference among transmissions over the same SF (co-SF) as well as different SFs (inter-SF). By modeling the interference field as Poisson point process under duty cycled ALOHA, we derive the signal-to-interference ratio distributions for several interference conditions. Results show that, for a duty cycle as low as 0.33%, the network performance under co-SF interference alone is considerably optimistic as the inclusion of inter-SF interference unveils a further drop in the success probability and the coverage probability of approximately 10% and 15%, respectively, for 1500 devices in a LoRa channel. Finally, we illustrate how our analysis can characterize the critical device density with respect to cell size for a given reliability target. <s> BIB011 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> Low power wide area networks (LPWANs), such as the ones based on the LoRaWAN protocol, are seen as enablers of large number of IoT applications and services. In this work, we assess the scalability of LoRaWAN by analyzing the frame success probability (FSP) of a LoRa frame while taking into account the capture effect and the number of parallel demodulation paths of the receiving gateway. We have based our model on the commonly used SX1301 gateway chipset, which is capable of demodulating up to eight frames simultaneously; however, the results of the model can be generalized to architectures with arbitrary number of demodulation paths. We have also introduced and investigated three policies for Spreading Factor (SF) allocation. Each policy is evaluated in terms of coverage probability, FSP, and throughput. The overall conclusion is that the presence of multiple demodulation paths introduces a significant change in the analysis and performance of the LoRa random access schemes. <s> BIB012 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> Both physical and MAC-layer parameters impact the autonomy of IoT devices. We present an open-source cross-layer assessment framework for Low Power Wide Area networks (LPWANs) in this paper. It extends the state-of-the-art with energy models, downlink messages, and adaptive datarate features. Hence, hypotheses and transmission schemes can be tested and evaluated. As a representative case, the LoRaWAN protocol is assessed. The findings demonstrate that a cross-layer is imperative to effectively realize LPWANs in terms of energy efficiency and throughput. For instance, up to a factor of three reduction in energy consumption can be achieved by transmitting longer packet on quasi-static channels. Yet, under adverse dynamic conditions, an energy penalty will occur. <s> BIB013 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> IV. IMPACT OF DOWNLINK TRAFFIC ON NETWORK PERFORMANCE <s> LoRaWAN promises to provide wide-area network access to low-cost devices that can operate for up to ten years on a single 1000-mAh battery. This makes LoRaWAN particularly suited for the data collection applications (e.g., monitoring applications), where device lifetime is a key performance metric. However, when supporting a large number of devices, LoRaWAN suffers from a scalability issue due to the high collision probability of its Aloha-based MAC layer. The performance worsens further when using acknowledged transmissions due to the duty-cycle restriction at the gateway. For this, we propose FREE , a fine-grained scheduling scheme for reliable and energy-efficient data collection in LoRaWAN. FREE takes advantage of applications that do not have hard delay requirements on data delivery by supporting the synchronized bulk data transmission. This means data are buffered for transmission in scheduled time slots instead of transmitted straight away. FREE allocates spreading factors, transmission powers, frequency channels, time slots, and schedules slots in frames for LoRaWAN end-devices. As a result, FREE overcomes the scalability problem of LoRaWAN by eliminating collisions and grouping acknowledgments. We evaluate the performance of FREE versus different legacy LoRaWAN configurations. The numerical results show that FREE scales well and achieves almost 100% data delivery and the device lifetime is estimated over ten years independent of traffic type and network size. In comparison to poor scalability, low data delivery and device lifetime of fewer than two years for acknowledged data traffic in the standard LoRaWAN configurations. <s> BIB014
Downlink traffic in a LoRaWAN can be in the form of transmitting an ACK for a confirmed uplink message, sending MAC commands, application layer downlink data or a combination of all of these BIB010 . Accurate network models are key when determining the impact of downlink traffic on network performance, and modelling should include the PHY and MAC layers. Due to their Aloha like MAC configuration and the DC limits imposed by operating in ISM bands, LoRaWANs suffer from scalability issues BIB014 . They do, however, perform better than pure Aloha networks, in part due to the capture effect BIB001 , BIB002 and SF pseudo-orthogonality BIB011 , BIB003 . Network models evolve and earlier models become outdated as new information on the capture effect and SF orthogonality come to light. Earlier work such as  , modelled LoRa with the assumption that simultaneous transmissions sent on the same frequency but at different SFs are orthogonal and can thus be simultaneously decoded by the gateway. Research has however shown that the orthogonality is not perfect as should rather be considered quasi-orthogonal. Models should rather consider the interference caused by transmissions using the same SF (co-SF) and different SFs (inter-SF) BIB011 , BIB012 . Another aspect that models should include is the capture effect. This effect describes the possibility that a packet can successfully be received even if it collides with other packets if its power is sufficiently high BIB007 . This is problematic for packets from faraway devices, as any collisions with packets sent from nearby devices will result in the far away device's packet not being received. Even when these factors are considered, authors interpret the protocol differently as can be seen in the case of BIB002 . In their theoretical model gateways always transmit two acknowledgements (one for each receive window) but this behaviour is not specified in the protocol  . This approach to acknowledgements was investigated in BIB008 , which found that transmitting in both windows hurts performance. In BIB008 , the point is made that all three of the main approaches of evaluating LoRaWAN performance has limitations: experimental work is limited by a large number of variations in cost, size and parameters. Simulation-based approaches are limited by computational power, assumptions made and the level of detail achievable. Mathematical models use ideal assumptions which impact the accuracy, but can be used to focus the efforts of the other options BIB008 . LoRaWAN simulators have been developed used from a variety of existing WSN platforms such as ns-3, OMNET++ and SimPy. Publicly available LoRaWAN simulators that support downlink traffic are LoRaSim  , the expanded version of LoRaSim used in BIB014 , the ns-3 modules presented in BIB004 and  , LoRaEnergySim BIB013 and FLoRa  . LoRaWAN is very popular under European researchers, and as a result, most simulators are built assuming operation in the EU863-870 MHz band. One exception to this is BIB009 , which simulates 915 MHz North American networks but does not support downlink traffic. This simulator is, however, open-source, and thus can be modified to investigate ACKs. Simulators also need to remain updated with changes to the LoRaWAN protocol as well as relevant research findings such as pseudo-orthogonality and the impact of the Doppler effect on LoRa transmissions as described in BIB005 . The current LoRaWAN literature focuses almost exclusively only on ACKs during downlink performance evaluations. Notable exceptions to this are the work presented in BIB006 and BIB004 . The simulations performed in BIB006 , showed that the addition of downlink data traffic increases the competition experienced by ACK transmissions and as the DC restrictions are tight, this further increases the number of retransmissions. When downlink data traffic was investigated in BIB004 , a small decrease in the Packet Delivery Rate (PDR) for uplink transmissions was noted due to more packets being missed. This is due to the increased time a gateway now spends in transmission mode. Their analysis showed that adding additional gateways can help reduce this negative impact. The current LoRaWAN literature is also dominated by works examining networks containing only class A nodes. This is logical as class A is expected to be the most common, but does leave the performance of the other classes and networks containing a mix of classes unexplored. One exception to this is the work on class B networks in  . They found that the data rate and the number of channels have a significant impact on the delay of downlink traffic.