A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> VI. NEW PROPOSED ACK METHODS <s> Low-Power Wide Area Networks (LPWANs) are continuously gaining momentum as fundamental enablers of the Internet of Things (IoT) paradigm. These networks provide longrange coverage to end nodes, exploiting license-free frequency bands. The focus of this work is on one of the most prominent LPWAN technologies: LoRa™. We implemented a new ns-3 module to study the performance of a LoRa-based IoT network in a typical urban scenario. Simulation results show that a LoRa network can scale well, achieving packet success rates above 95% in presence of a number of end devices in the order of 104. <s> BIB001 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> VI. NEW PROPOSED ACK METHODS <s> In this paper, we propose a scheduling algorithm to connect a massive number of IoT devices in LoRaWAN. The proposed algorithm supports time-synchronized transmissions to mitigate the problem of scalability due to random channel access used in LoRaWAN. Our proposed algorithm schedules spreading factors, frequency channels, and timeslots for wireless links connecting end devices and gateways. Also, the group acknowledgement is employed to improve channel efficiency by aggregating acknowledgements of uplink transmissions received simultaneously from multiple end devices. We evaluate the performance of the proposed algorithm and compare it with the performance of ALOHA used for uplink transmissions in LoRaWAN. Our numerical results show that the proposed algorithm improves the connectivity of end devices over ALOHA. Compared to ALOHA, the proposed algorithm provides more than 60% increase in the number of end devices connected to a gateway, whereas more than 90% data messages originating from all end devices are delivered successfully. <s> BIB002 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> VI. NEW PROPOSED ACK METHODS <s> In this paper, we analyze the throughput of LoRaWAN Class-A data transfer with acknowledgement (ACK) under duty-cycle restriction which is imposed on gateways and end-devices. We show that ACK traffic limits the throughput of data traffic. We propose a method for aggregating ACKs from multiple users to improve the throughput and reliability of data transfer. The proposed method cumulates ACKs for multiple packets for multiple user-nodes. It also has schedules receive windows of user-nodes'. We show via performance analysis and simulation results that our method improves data transfer throughput by ten to thirty times. <s> BIB003 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> VI. NEW PROPOSED ACK METHODS <s> Low-power wide-area networks are continuously gaining momentum as enablers of massive machine-type communication, thanks to long-range wireless technologies operating either on unlicensed spectrum like, e.g., Long-Range (LoRa) and Sigfox, or on licensed spectrum, as Narrowband-IoT. For the former category of technologies, stringent requirements in terms of channel utilization and radiated power must be fulfilled, thus algorithms and protocols for enhanced radio-resource management need to be investigated. In this paper, we propose a disruptive approach for LoRa systems, exploiting a novel operational mode at the gateways side called time-power multiplexing, which fully exploits the degrees of freedom of the unlicensed bands regulations. The simulation results show that the proposed solution boosts the number of users that a LoRa network can support with respect to the default operational mode. <s> BIB004 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> VI. NEW PROPOSED ACK METHODS <s> Long range (LoRa) wide-area network (LoRaWAN) protocol is a promising candidate for Internet of Things devices that operate on Low Power Wide Area Networks (LPWAN). Recent studies on this protocol are mostly focused on uplink (UL) communication. In particular, the performance of the pure ALOHA type MAC protocol used in this protocol is not fully observed. Investigations have revealed that the pure ALOHA type MAC protocol and downlink (DL) traffic may cause significant negative impacts in terms of the scalability and the reliability of the network. For this respect, in this paper, we propose an extension to LoRaWAN MAC layer, called Aggregated Acknowledgment Slotted Scheduling LoRaWAN (A2S2-LoRaWAN), to improve the scalability and reliability of LoRaWAN. A2S2-LoRaWAN contains time-slotted ALOHA-based periodic frame structure, which is supported by aggregated acknowledgment methods, for scheduling transmissions. A2S2-LoRaWAN reduces UL-DL traffic and increases the scalability of LoRaWAN. For instance, while A2S2-LoRaWAN decreases UL-DL traffic five times compared to LoRaWAN, its success rate with 10,000 End-Devices (EDs) transmitting 10-byte payload is twice better than the success rate of LoRaWAN with a single gateway (GW). <s> BIB005 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> VI. NEW PROPOSED ACK METHODS <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> BIB006
An alternative approach to data collection and acknowledgement is suggested and compared with standard LoRaWAN in BIB006 . A scheduling scheme called FREE is proposed which requires that nodes store data in bulk and collection takes place periodically. This approach allows devices to send an order of magnitude fewer packets simply by bulking data and sending longer packets. This system is thus not designed for priority data as collection could potentially only occur once every few hours. With FREE, data reception is acknowledged by the gateway using a compressed group ACK scheme. During their evaluations, nodes generated 20 bytes every 5 minutes, could potentially retransmit a packet up to 8 times and simulations considered 1 day's worth of data. For a standard LoRaWAN using confirmed traffic, the device lifetime drops quickly below two years due to the cost of retransmissions and for a network of 1000 devices, the number of transmissions increased 7.4 times the level of a similar network using unconfirmed instead BIB006 . LoRaWAN's overall Data Delivery Ratio (DRR) plummets sharply falling below 50 % at ≈ 200 devices as the number of collisions rise sharply. The researchers note that collisions are not the major cause for the drop in DRR, instead, the gateway's DC restrictions prevent it from responding with ACKs. The loss of either the data packet or the ACK due to channel fading also had an impact, but this was much smaller. Their proposed solution outperformed LoRaWAN significantly as the precise scheduling prevented collisions and the DC restrictions were kept in mind. A method to perform acknowledgement aggregation is proposed in BIB003 . In this method, the NS periodically sends out an ''AggACK'' containing ACKs for several devices and received packets. The proposed method was evaluated a modified version of the ns-3 simulator presented in BIB001 . The results show that this method was able to improve the normalized throughput fand found that transmitting an aggregated ACK every 20 seconds was the optimum waiting time for their evaluated scenario. It is not clear with which SF the aggregated ACK is sent with, or how nodes know on what channel it will be transmitted on. The presented throughput model assumes the number of nodes is large, 1000 nodes, but the ns-3 evaluates only examine up to 200 nodes. An alternative radio-resource management solution is proposed in BIB004 . By assigning 869.525 MHz as the designed channel for both downlink windows, collisions between uplink and downlink traffic can be prevented. An operational mode called time-power multiplexing is presented which relies on new hardware designs allowing a gateway to transmit more than one packet at a time. Additionally, new power amplifier subsystems are required to allow for variable transmit power based on the number of simultaneous transmissions BIB004 . The authors suggest a final modification namely that only one receive window is used, to allow a gateway to maximise the number of downlinks it can group. A comparison, using a MATLAB simulator, between the proposed system and the default protocol showed that higher network capacity is now supported. The improvements did not only improve the capacity for confirmed traffic but also slightly improved unconfirmed traffic's capacity as well. Whilst the scheme did offer improvements, simply reducing the percentage of confirmed traffic would still have a bigger impact on unconfirmed traffic's scalability. In addition to a transmission scheduling algorithm, the use of group acknowledgements is proposed in BIB002 . The scheduling algorithm dictates the SF, channel and timeslot for all transmissions thereby allowing a group acknowledgement to be sent for all packets received after each timeslot. The format of this acknowledgement is not described and the proposed solution was compared with ALOHA using simulations. The results showed that the solution can offer an improvement in packet delivery but is dependent on the number of channels. An extension to the LoRaWAN protocol named A2S2-LoRaWAN is proposed in BIB005 . This extension uses a time-slotted periodic frame structure which is combined with aggregated ACKs to improve the scalability of LoRaWANs. The proposed extension is compared with standard LoRaWAN in simulations, although the gateway is limited to only use one bidirectional channel with a DC restriction of 1 %. This does not quite match the normal operation of an EU based LoRaWAN gateway who would also have access to the downlink only 10 % DC restricted channel used by receive window 2. A. COMPARISON Table 3 shows a comparison between the alternative methods to implement ACKs discussed earlier. Only one of the methods was released as open-source, which would allow other researchers to examine the proposed method further. None of the methods was implemented on currently available LoRaWAN hardware, with BIB004 being unable to do so until a gateway capable of time-power multiplexing is developed. One of LoRaWAN's strengths is its low power consumption as IoT deployments are frequently in remote and inaccessible areas. The impact of the proposed method on energy consumption is a crucial aspect not all works examined.