A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> 5) MULTI-GATEWAY NETWORKS <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> BIB001 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> 5) MULTI-GATEWAY NETWORKS <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> BIB002
In a multi-gateway LoRaWAN, the NS is responsible for choosing which gateway must transmit a downlink frame to a node. A gateway selection algorithm is thus required and two approaches are explored in BIB002 . The first is a simple Signal to Noise Ratio (SNR) based approach currently used by the open-source LoRaWAN NS ChirpStack (previously called LoRaServer)  . The recorded SNR values of a node's previous packets are used to select the gateway with which it is best to respond with. If one gateway is preferred over others for a large number of nodes, it could quickly become overworked by the NS. As the algorithm only uses the best gateway and doesn't consider DC restrictions, other gateways, with slightly lower SNR values, end up being underused BIB002 . The second approach attends to correct this flaw by also considering the DC restrictions placed on gateways. The addition of more gateways was found to result in a significant improvement, even when the SNR based approach is used. In a quad versus single gateway scenario, the additional gateways halved the packet loss experienced in the network. The main cause of this was that downlink traffic was now split amongst multiple gateways, each with their own DC restrictions, allowing more ACKs to be successfully sent. The results revealed that this can be improved upon, as with the SNR based approach the first gateway was assigned to transmit 60 % of the ACKs. This gateway was only able to send 24 % of these requests due to DC restrictions, whilst two of the four gateways receives nearly no requests. The second approach, which attempts to balance the load between the gateways, was able to reduce the packet loss by 25 % due to its better load distribution. An open-source LoRaWAN ns-3 module developed for a scalability analysis is presented in BIB001 . Part of this analysis was investigating confirmed traffic in a single uplink channel network with a fixed NbTrans of four with either one, two or four gateways. In a one gateway network, confirmed traffic under-performed unconfirmed traffic with the under-performance increasing as the number of devices or the traffic volume increases. The decrease in network PDR is attributed to the number of receive windows missed by the gateway due to DC restrictions. Increasing the number of gateways did improve the PDR, but had a bigger impact on unconfirmed traffic than confirmed traffic. For unconfirmed traffic, adding gateways is very effective as decreases the distance between nodes and their closest gateway, allowing them to use faster data rates and thus decreasing congestion. For confirmed traffic, traffic volumes also play a role as adding gateways does allow more ACKs to be sent but it also allows more packets than previously to be successfully received who now require an ACK. This increases the number of required ACKs, causing DC restrictions to remain a problem. In networks with low traffic volume, additional gateways are very effective, as the volume is low enough that the only bottleneck was DC restrictions.