## 2 State of the Art
In addition, various works in the literature [15; 16; 17; 18] investigate the applicability as well as criticism of DL in LoRaWAN networks and its possible negative impact on performances when not well implemented. To sum up, LoRaWAN technology has limitations that need to be carefully considered for its use in the considered FMAR scenario. In particular, one of the most critical issues is related to the use of the CONF mode to provide link reliability. Indeed, as discussed in [19], the use of ACK in DL can significantly drain the network capacity since GWs must be compliant with duty-cycle regulations. This problem is also studied in detailin [20] where the authors propose a solution called _sub-band swapping_ where a first receiving window is opened on the dedicated downlink (DL) channel and a second one on the uplink (UL) channel to alleviate the duty cycle's bottleneck. Another line of investigation deals with the use of ad-hoc control schemes which deviate from the standard LoRaWAN ACK mechanism [21; 22]. One of the most closely paper to ours is [6], where collision free mechanism based on clustering of EDs is provided with the aim of increasing the reliability. The authors provide various optimized solutions by maximizing the number of EDs in a service area via maximum possible channel utilization. However, the considered test scenario is built with the aim of enabling massive IoT (mIoT) and it differs then from ours (FMAR) in the following aspects:  
* The considered solution is more generally focused on a air pollution monitoring system where there is the requirement of sending RPs containing the measurement report at every pre-defined interval within a given time window (400 s-1600 s). * The EDs should be perfectly synchronized with the GW in time so that the transmission times for the users are accurate. In this case, they have introduced some guard times which require major infrastructural modifications both at ED and GW level. * It does not consider the FMAR scenario; especially the situations of UP delivery is not considered in their work: this aspect deals not only with the reliability but also with the stringent latency constrains. * The EDs are assumed fixed. In this paper, we propose a reliable and low-latency communications solution which is suitable for the considered FMAR scenario and that is fully LoRaWAN compliant. On the other hand, to the best of our knowledge all the previous works in the literature addressing the problem of reliability in LoRa deviate from LoRaWAN standard and, as such, would require a brand-new firmware update at both nodes' and GW level. Conversely, the aim of the solution proposed in this work is to integrate the standard LoRaWAN configurations: this would make the implementation of this system almost straightforward since only minor modifications on the server side of the network infrastructure are required. As a matter of fact, the proposed solution may exploit already existing LoRaWAN networks and thus may be installed without any need for major infrastructural integration. Accordingly, this work proposes a fully operating experimental setup and the viability of the proposed scheme is demonstrated by means of a fully operating LoRaWAN network infrastructure whereas in all the referenced works the results could be achieved only by means of simulations. To sum up, this work does not require any modification in the current LoRaWAN protocol and is fully compliant with the current standard.