# LoRaWAN Communication Protocols: A Comprehensive Survey under an Energy Efficiency Perspective
## 3 Communication Protocols
### Routing Protocols
#### 3.3.3 Ad Hoc Multi-Hop Communication  
A wireless ad hoc is a decentralized type of network. In this topology, every node participates in routing by forwarding data to other nodes. Therefore, the routing algorithm and network connectivity are used to determine which nodes advance data. In this section, ad hoc multi-hop communication protocols are examined in terms of energy efficiency techniques. In [9], a combination of hybrid wireless mesh protocol (HWMP) and ad hoc on-demand distance vector (AODV) routing was reported. The approach was adapted to the demands of LoRaWANs and end-devices. Routing protocol messages from AODV and HWMP, for instance, make use of MAC headers in order to be recognized by GWs. In this case, lightweight nano-GWs are used to relay data packets between and devices and the main GW that is connected to the NS. Note that these GWs partially implement LoRaWAN, and the main purpose of their use is the limited internet coverage in remote areas. In the protocol, HWMP is used to forward packet through the nano-GWs, if a route is already established between the end-devices and the NS; otherwise, through existing AODV routing, a path with minimum number of hops is constructed. Another ad hoc transmission scheme is presented in [78], which aims to monitor the ancient underground water distribution systems in Siena, Italy. In more detail, the authors presented the underground coverage of LoRaWAN as a challenge, reported that traditional star-of-stars topology is not feasible, due to the high total power losses present in this harsh environment. Therefore, they presented a synchronization linear LoRaWAN multi-hop communication protocol. In this protocol, every node only receives and transmits packets of its direct neighbor. Synchronization is the first phase of the protocol, followed by data collection and a sleep phase to conserve energy. For setting the schedule for transmission and reception, the GW sends a SYNCH message that is flooded into the linear network. During synchronization, the GW begins by initializing the elapsed time (ET) to zero, which is the number of SYNCH packets transmitted in the chain. Upon reception of the SYNCH message, the immediate neighbor increments the ET by one, overhears the message retransmitted, implicitly classifying the retransmission as an ACK, and forwards the message to the next node. The process iterates until the SYNCH message reaches the last node of the linear network. After transmitting the SYNCH message, switching to low power listening mode (LPL), each data source node sleeps for a short period of time, wakeups and transmits the data packet. The data will be flooded until it reaches the GW. Numerical results showed that the proposed wake-up time optimization leads in the best case to a 50% reduction in power dissipation required to acquire SYNCH with respect to a scheme that evaluates the wake-up time in a non-optimal way. In conclusion, Table 6 summarizes the basic features, as well as the main limitations of each routing algorithm in multi-hop communication in LoRaWAN. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & \multirow{2}{*}{**Topology**} & \multirow{2}{*}{**Test**} & \multicolumn{1}{c}{**No.