## 4 The Proposed Communication Strategy
### Coordination of ED Transmissions through Downlink Control Packets (DCPs)  
We refer to the Class A DL operation in which the Network Server transmits a DL packet to an ED after the reception of an UP packet precisely at the beginning of one of two possible receiving windows. More precisely, the ED opens Class A RX1 and RX2 receiving windows after RECEIVE_DELAY1 and RECEIVE_DELAY2 secs respectively. The DL data rate for RX1 depends on the corresponding UL whereas RX2 uses a fixed data rate depending on the region.  
In the considered scenario, each node transmits RPs periodically every predetermined (long) time intervals (e.g., several minutes) in UNCONF mode. We program the server in such a way that for every received RP a corresponding DCP is scheduled. Specifically, the DCP message is intended to control the eventual transmission of UPs. The adopted control mechanism, which is discussed in detail in the next section, acts independently on each cluster since it is highly unlikely that in the considered scenario EDs of different clusters have to transmit an UP at the same time (the potentially dangerous event is local and infrequent).  
Hence, upon the necessity of delivering an UP to the system, the ED transmits according to the control information specified in the last received DCP. More specifically, we opted  
Figure 8: A service area with different clusters represented by different colors.  
to choose the UNCONF mode also for UPs. The rationale for this choice will be given in the next section.  
One of the important aspects that has to be taken into consideration while designing any LoRaWAN system is to comply with the duty cycle regulations as discussed in Section 3. This poses some stringent constrains in the process of allocating the resources to the EDs. Owing to the per sub-band duty cycle regulations, we have various possibilities to assign the resources to the EDs for the next UPs. In particular, one of the feasible choice is to differentiate the sub-bands for the two types of packets, i.e., allocating fixed non-overlapping sub-bands for the RPs and UPs. Indeed, this not only allow the isolation in terms of frequencies but also address the issue of duty cycling in the case when it is necessary to transmit an UP when the time elapsed form the last RP is lower than the minimum time established by duty cycling restrictions. In particular, the server assign different sub-bands for RPs and UPs so that duty cycling restrictions are independently established for the two kinds of transmissions. An illustrative example is given in Figure 9, where 5 EDs in close proximity, i.e., belonging to the same cluster, are allocated sub-band g (Channel (Ch0-4) with frequencies 867.1, 867.3, 867.5, 867.7, 867.9 MHz) and g1 (Channel (Ch5-7) with frequencies 868.1, 868.3, 868.5 MHz) for UPs and RPs respectively. To elaborate, each EDs transmit RPs by randomly selecting one of the available frequencies from g1 whereas the UPs are transmitted using different frequencies in sub-band g to avoid collisions. Such frequencies can be selected by each ED according to the last DCP received from the server which is in charge of isolating the UP transmissions of the same cluster. Considering 5 channels in sub-band g, it is worth noting that we can allocate a maximum of 5 different channels to 5 EDs in each cluster. However, we also have the possibility to accommodate more users in the cluster by assigning different SFs as shown in the next section.