A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> III. THE LoRaWAN PROTOCOL <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> BIB001 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> III. THE LoRaWAN PROTOCOL <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> BIB002 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> III. THE LoRaWAN PROTOCOL <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> BIB003 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> III. THE LoRaWAN PROTOCOL <s> Nowadays the IoT paradigm is attracting interest in the scientific and commercial fields. ::: This thesis focuses on the solution combining LoRa and LoRaWAN and on its performance when reliable communication is employed. ::: The discussion presents the implementation of LoRaWAN reliable communication in the ns-3 simulator and proposes a mathematical model for performance evaluation. The performance of the system and the validity of the proposed model are evaluated by means of simulations. <s> BIB004
Unlike some of its competitors, the LoRaWAN standard is an open standard and was developed by the LoRa Alliance. Due to its openness, it has proven to be very popular amongst academics, industry and the maker community. The PHY of a LoRaWAN network is a modulation technique called Long Range (LoRa) and was developed by Cycleo before being acquired by Semtech and remains proprietary. LoRaWANs operate in the Industrial, Scientific and Medical (ISM) bands and its operation thus differs depending on the network's region. Note that this paper was written from the perspective of EU 868 networks with the regional parameters specified in [31] . A LoRaWAN compliant radio offers several data rates and packets have maximum packet lengths dependent on the data rate used for their transmission. When the data rate is set, either statically or dynamically through LoRaWAN's ADR scheme, this is translated into two other settings namely Spreading Factor (SF) and Bandwidth (BW). In the European Union (EU) region, a LoRaWAN compliant radio has two BW settings namely 125 kHz and 250 kHz and offers six different orthogonal SF values namely 7-12. The choice of SF involves a trade-off between the achievable distance and throughput, higher SFs offer more range but also have higher time-on-air durations causing Duty Cycle (DC) restrictions to be reached earlier BIB002 . These DC restrictions are region-specific and must be adhered to in LoRaWAN networks. The LoRaWAN standard adds the required Medium Access Control (MAC) functionality and specifies that the network has a star-of-stars topology which supports uplink and downlink messages. The network is managed by a central Network Server (NS) which is responsible for managing packet duplicates, scheduling acknowledgements, the ADR scheme and will reroute any packets to the Application Server they are destined for BIB001 ,  . Uplink messages are sent by a device to the NS and will be received by one or more gateways who all forward the packet to the NS. Downlink messages are sent from the NS to a specific device and will be transmitted by only one gateway chosen by the NS. LoRaWAN gateways are based on Semtech's SX1301 chip, capable of demodulating 8 LoRaWAN frames simultaneously BIB003 . LoRaWAN gateways are half-duplex and thus a gateway is unable to receive any transmissions whilst it is transmitting downlink messages. Since the release of version 1.0.0 in January 2015, the LoRaWAN specification has continually been improved. The latest minor update namely revision 1.0.3 was released in July 2018. A major update namely 1.1.0 was released in October 2017 and is backwards compatible with 1.0.x end devices and networks. The updates have, amongst other things, added new MAC commands and deprecated old ones, increased security and added frequency plans. The protocol specifies three possible devices classes all allowing bi-directional communication and are referred to as class A, B or C. With class A each uplink from a device to the gateway is followed by two short downlink receive windows as shown in Figure 1 . The device can only receive communication from the gateway during these two windows. This optimises power consumption but causes devices to be unreachable until they sent a message. Class B adds additional scheduled receive windows to make devices more reachable and downlink communication frequency more predictable. This is done through the gateway transmitting a time synchronised beacon and as devices must remain synchronised, increases a device's power consumption. The final class, C, is best used by devices without power concerns as these devices have near-continuous receive windows. This allows downlink communication to class C devices to have lower latency than the other classes but has no impact on uplink latency. The most commonly deployed class in a LoRaWAN is envisioned to be class A. In terms of receiving acknowledgements, this class has the strictest requirements, as class A devices will only be able to receive an Acknowledgement (ACK) during their two downlink receive windows. Whenever an ACK is requested from the device, as required as a response to several MAC commands, the device should transmit an ACK as soon as possible BIB004 ,  . The device may transmit immediately a potentially empty packet with the ACK bit set or simply set the ACK bit of its next message. The protocol specifies three default channels in the EU 868.0-868.6 MHz h1.5 sub-band that all devices on the EU 868 channel plan must support. For these channels, the maximum transmit power is +14 dBi Equivalent Isotropically Radiated Power (EIRP) and a sub-band DC limitation of < 1 % must be adhered to. By default, a device's second receive window is configured to transmit on 869.525 MHz using the maximum spreading factor (SF12) with sub-band h1.7 allowing a DC of 10 % [35], [36] . More channels can be added depending on the region, for example, the popular The Things Network (TTN) uses 10 channels in its EU 863-870 channel plan  . Figure 1 also shows the timing used in the opening of the receive windows after a device's uplink transmission. The first window opens after the uplink's modulation finishes (exact time specified by RECEIVE_DELAY1 +/− 20 µs) and will by default use the same channel and data rate as the uplink. The second window opens RECEIVE_DELAY2 seconds after the uplink and its frequency and data rate are configurable and region-specific [31],  . Transmitting an ACK using the first window could result in a lower transmission time as transmitting an ACK on any of the lower SFs would be faster than doing so on SF12. The first window may, however, not be available as it will have a tighter DC restriction than the second window. The default timing values for these delays as well as the delay between retransmission attempts when an ACK was not received is given in Table 1 . 