A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> D. OTHER CHALLENGES <s> Recently, the role of Low-Power Wide Area Networks (LPWANs) as enablers of the Internet of Things (IoT) is emerging. However, many concerns arise on the performance of LPWANs if higher Quality of Service (QoS) requirements are needed by certain classes of IoT services. In this work, we will focus on one of the most prominent of these technologies, i.e., Long-RangeTM (LoRa). Our aim is to study to what extent a LoRa network is able to support uplink (UL) traffic that requires to be confirmed by the Network Server (NS). The simulation results show that the network performance is severely impacted by the downlink (DL) traffic generated by feedback packets. <s> BIB001 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> D. OTHER CHALLENGES <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> BIB002 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> D. OTHER CHALLENGES <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> BIB003 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> D. OTHER CHALLENGES <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> BIB004 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> D. OTHER CHALLENGES <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> BIB005 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> D. OTHER CHALLENGES <s> LoRaWAN is an emerging low-power wide-area network (LPWAN) technology, which is gaining momentum thanks to its flexibility and ease of deployment. Conversely to other LPWAN solutions, LoRaWAN indeed permits the configuration of several network parameters that affect different network performance indexes, such as energy efficiency, fairness, and capacity, in principle making it possible to adapt the network behavior to the specific requirements of the application scenario. Unfortunately, the complex and sometimes elusive interactions among the different network components make it rather difficult to predict the actual effect of a certain parameters setting, so that flexibility can turn into a stumbling block if not deeply understood. In this article, we shed light on such complex interactions for a single-gateway (GW) system by analyzing the effect of some built-in features and configurations, including the GW’s limitations in terms of duty cycle and the number of parallel reception paths, the number of allowed retransmissions for confirmed traffic, and the preconfigured data rate used in downlink transmissions. The simulation-based analysis reveals various tradeoffs and highlights some inefficiencies in the design of the LoRaWAN standard. Furthermore, we show how significant performance gains can be obtained by wisely setting the system parameters, possibly in combination with some novel network management policies (e.g., enabling selective prioritization of downlink transmissions at the GW). <s> BIB006
A challenge when determining the viability of confirmed traffic is how accurate the requirements of a specific use case was captured. More information is required than simply an estimate of how many nodes, a target PDR and expected packet length. Another challenge is that as the protocol is still fairly new, LoRaWAN simulators are not very advanced. They are also not necessarily regularly updated once new research comes to light. Using these tools to examine a complicated project involving a ratio between confirmed and unconfirmed traffic, multiple gateways and the radio environment of a planned deployment site are currently not very feasible. LoRaWAN gateways have limited scalability as only eight simultaneous signals can be received BIB004 . This limitation is not always considered by researchers, for example, BIB001 states their gateways have infinite receive paths. The use of ACKs in large networks cause further scalability issues as LoRaWAN gateways are half-duplex and hence miss uplink messages when transmitting ACKs BIB005 . In small networks, the traffic volume is low and thus the probability that devices transmit at the exact time interval that an ACK is sent is quite small. This is not the case for large networks, and due to the eight parallel demodulation paths of the gateway, multiple transmission could be lost for each ACK sent. The use of multiple gateways can help alleviate this problem, as the messages can be received by another gateway assuming it is in receive mode. The coverage areas of these gateways should carefully be considered, to ensure adequate overlapping zones to also assist with sharing the load of downlink traffic. The potential performance of full-duplex gateways was investigated in BIB006 , which found them to be very effective in improving uplink traffic's PDR. LoRaWAN nodes do support multiple channels, although most evaluations stuck to the default channel allocations. The protocol already supports informing nodes of additional channels supported by the gateway. As suggested in BIB002 , some of these additional channels could be dedicated as retransmission only traffic channels for critical packets. The maximum number of transmissions attempts has been showed to improve the PDR for confirmed and unconfirmed traffic. This increase is seen in small networks and disappears in larger networks. It is however dependent on the traffic volume of a network and not simply on the number of devices. Evaluations examining this configuration parameter typically statically examined this, i.e. fixing the traffic volume and the number of transmission attempts. As suggested in  , BIB003 , a dynamic system is required in which the maximum number of transmissions attempts are adjusted as the network's traffic volume changes. There exists an asymmetry in the receiver sensitivity between gateways and end nodes and this could be problematic  , BIB006 . A situation can occur in which an end device can receive a gateway using a certain SF, but the gateway would be unable to successfully transmit an ACK using the same SF, as the end device's receiver is not sensitive enough. The gateway is then forced to transmit the ACK using the second receiver window, as this would make use of the maximum spreading factor. This problem can be alleviated by either informing an end device that its first window cannot be used to receive an ACK or inform it to expect a reply using a different SF than it originally used  .