# LoRaWAN Communication Protocols: A Comprehensive Survey under an Energy Efficiency Perspective
## 4 Discussion and Research Directions towards Green LoRaWAN Protocol  
Nowadays, LoRaWAN is rendered one of the most widely adopted LPWANs, increasingly attracting the interest of research community, capable of providing long-range connectivity with low power consumption, which is required by a diverse range of IoT applications; however, a number of challenges and critical issues have not still been addressed. Firstly, collisions frequently occur in dense networks, which contribute significantly to packet loss and degrade network performance in terms of scalability, throughput, and reliability. Scalability depends on several factors, such as number of available radio resources, duty cycle restrictions, transmission parameters, and the number of end-devices. Further, reliability and its effect on scalability is also an important research topic. The extent of LoRaWAN coverage is another significant issue, especially in case of high interference and significant reduction in signal strength of end-devices, such as in large cities where many antennas and/or underground areas exist. Finally, the energy efficiency remains a focal issue that LoRaWANs faces. The energy consumption of end devices and GWs is a crucial factor that can affect the network lifetime. Several studies have discussed various critical aspects of LoRaWAN across PHY, MAC, and network layers. We have classified the research works into three categories, namely: (i) energy efficiency protocols, (ii) multiple access protocols, and (iii) routing protocols. Firstly, energy efficiency protocols aim to reduce the rate of energy consumption per volume of data sent over a network. In this category, the impacts of transmission parameter selection on energy consumption are examined. For example, increasing TP and SF is expected to increase the energy consumption. Therefore, the optimal parameter configuration that minimizes the energy consumption, while satisfying the required communication performance, is important for LoRaWANs. Based on this, we presented a comprehensive literature review that aims at the energy optimization of the network by presenting resource allocation policies appropriately assigning communication parameters. A different approach that could be followed is to adopt a dynamic state transition that takes advantage of the reduced energy demand in some states, trying to occupy as much time as possible in relation to the most energy-intensive states. Finally, there are some contributions that approach the problem by presenting hardware improvements. Secondly, we have investigated the multi-access protocols and how they could optimize the performance of LoRaWANs, especially in cases of dense networks. The mechanisms include splitting the channels into time slots and synchronizing the end-devices (enhanced ALOHA, TDMA) or adopting a frequency multiple access technique (FDMA) and checking the channel before transmission (CSMA). In particular, they aim to minimize collisions in LoRaWAN transmissions and, consequently, improve the reliability, throughput, and scalability. Furthermore, we have examined how the multi-access protocols can affect the energy consumption. The packet collisions result in a waste of the limited energy resources. Thus, the reduction in collisions, as well as potential retransmissions, results in a reduction in energy consumption. CSMA protocols introduce additional energy cost due to regular channel listening. However, through the reduction in collisions, the overall energy consumption is improved. Furthermore, the integration of CAD in CSMA was introduced to reduce energy consumption. Performing fewer CAD can lead to a further reduction in energy consumption, which is achieved through proper SF selection. Enhanced ALOHA protocols require significant energy because they continuously listen to the channel, maintaining the synchronization with the GW.