# A Survey of LoRaWAN for IoT: From Technology to Application
## 8 Extensions
### LoRa Multihop Networks  
Current star topology of LoRaWAN cannot be exploited in every type of application. For end nodes that are located deep indoors or experience bad receiver quality from gateways, multi-hop communication can be a solution. To this end, LoRa multi-hop network topologies are considered as a method to boost up performance especially for such end nodes [19; 54; 123; 124; 125]. In other studies, the LoRaWAN end nodes were extended with relying capabilities [126]. [PERSON] and [PERSON] [19] focused in mesh communication for LoRa nodes by introducing a system architecture where the gateway is the central sink. Gateway will start beaconing to advertise its presence. Once a node asks to join the network, the gateway will stop transmitting beacons. Rather, gateway will poll periodically the joined node for the information and these packets will be used by other end nodes as beacons. The other nodes that can hear polling packets from gateway or replying packets from the joined nodes they can ask to join network to certain nodes. The nodes can accept or refuse the join request. If they accept, they inform the gateway for all their \"children\". Thus, gateway will have the general network view and will poll each node in the network periodically. It is shown that in such a network topology the PDR for end nodes far away from gateway will improve, for some nodes from 0% to 91% [19]. The tested setup employed only 19 nodes in total deployed indoor and outdoor in an area of 800 m by 600 m [19]. Even that it is seen as a good solution for deep indoor end nodes, for larger deployments network scalability of such a solution might be an issue as each of the nodes has to be polled by the gateway. Moreover, accounting for radio duty cycle and usage of different SFs, the solution might end up using single SF that will decrease the network capacity. If multiple SFs are used in different hops, network can be fragmented. Another LoRa based multi-hop network was presented in [123]. In this case, traffic flooding in the network is used, by using concurrent transmission (CT) protocol. As LoRa exhibits capture effect, this will help in reception of the highest-power packet at each relay hop. Authors of [123] demonstrated the impact of the power offset, timing offset as well as carrier frequency offset in reception of one of the concurrent transmissions. Based on this empirical evaluation, they proposed a new transmission method, offset-CT, where each node in the relay hop will delay the retransmission by a randomly generated time offset between 0 and \(T_{s}\), \(T_{s}\) being the symbol time. They showed that the offset-CT improves the packet reception ratio (PRR) from 51% to 77% in case of relay groups with four nodes in a four-hop network topology. Same observations are valid for this solution too, only single SF can be used, otherwise network can end up being fragmented. In [54], a multi-hop MAC protocol on top of LoRa, LoRaBlink, was proposed. A time slotted channel access scheme is used where time slots are organized in frames. Each time slot frame is divided in beaconing slots and data slots. In the beaconing slots, the first slot is used by sink to transmit its beacon, the second slot is used by the second group of end nodes to transmit their beacons and so on until the last node. The number of beacon slots determines the network depth. For data communication, the leaf node will transmit the uplink packet first, then the next hop will transmit concurrently the data packet. Due to capture effect that LoRa exhibits, one of the packets will succeed and will reach the sink. However, this approach has a drawback if multiple SFs will be used in the network. In such a case, end node must be able to listen simultaneously for multiple SFs and in multiple channels, which will increase the end node price. Otherwise, for single SF and single channel deployment approach network throughput will decrease. In all the studies published until now for multi-hop network topologies, radio duty cycling is not considered.