# LoRaWAN Communication Protocols: A Comprehensive Survey under an Energy Efficiency Perspective
## 4 Discussion and Research Directions towards Green LoRaWAN Protocol
TDMA and FDMA schemes introduce additional energy consumption due to end-device scheduling and synchronization as the end-devices need to listen for the beacon from the GW before sending a packet. In TDMA, a beacon-skipping approach has been proposed to improve energy efficiency. Additionally, it has been combined with sleep mode transition, whenever they do not transmit or receive during a particular time slot, taking into account the fact that the energy consumption in sleep mode is negligible. Finally, routing protocols were also investigated in multi-hop LoRaWANs. This topology does not only aim to improve network range by adding some extra nodes (i.e., GWsor/and end devices, as relay nodes) but also in some cases to achieve better energy efficiency via multiple energy efficiency techniques (e.g., optimal resource allocation, dynamic state transition). Several mechanisms have been documented in order to maximize network coverage and in many cases by proposing some modifications to LoRaWAN PHY layer. We divided these routing approaches into three categories, such as clustering, non-clustering approaches and ad hoc multi-hop communication strategies. Then, we classified these approaches in subcategories according to their topology (tree of flooding). In more detail, clustering nodes into layers is an effective solution to manage energy consumption. On the other hand, non-clustering approaches can also manage energy efficiency by transmitting packets to the nearby neighbors. At this point it should be noted that all proposed works considered the duty cycle constraint. However, we observed that a small number of hops was reported in both simulations and practical testbeds. This restriction indicates that recent literature has not yet dealt with the development of new routing protocols aimed at increasing the number of hops, and, therefore, extend the coverage and scalability of the network. Finally, it is crucial to achieve synchronization so that new routes can be discovered. For that reason, several protocols extend LoRaWAN PHY layer and define new MAC protocol messages. Motivated by the aforementioned recent literature survey of LoRaWAN communication protocols, in Figure 5 we depict a classification of the basic methods that could be adopted when designing a new communication protocol focusing on energy efficiency along with their interrelation with critical issues/aspects to be addressed, highlighting potential solutions. Optimal resource allocation yielding the best transmission parameters is one of the most promising techniques' categories to reduce energy consumption. Related mechanisms could be combined with dynamic state transition in order to conserve energy, when possible, hardware improvements and energy harvesting. However, in a large-scale scenario, where multiple end-devices use the same physical parameters (e.g., SF and TP), a large number of collisions will occur. Thus, retransmissions along with ACK messages in case of acknowledged transmission in downlink communication will increase energy consumption, while resulting in severely degrading network performance, constituting robustness and scalability a major challenge. As a result, several mechanisms could complement resource allocation for minimizing collisions. For example, some techniques are proposed in MAC layer, as well as some routing algorithms that both use SYNCH messages with less frequent beacons in order to reduce energy, also satisfying DC restriction. Additionally, increasing  
Figure 5: Critical aspects to be considered in the design of LoRaWAN communication protocol with focus on energy efficiency. the number of gateways could serve as a potential solution to the scalability challenge. Furthermore, an end-device association to a single gateway concerning both uplink and downlink transmissions will minimize energy consumption at the GW side due to the fact that a GW will receive and forward only packets from the associated end-devices.