# LTM-LoRaWAN: A Multi-Hop Communication System for LoRaWAN
## 1 Introduction  
The LoRa has gained much attention recently, from both academic and industrial fields, due to its capability to facilitate efficient Device-to-Device (D2D) communications and provide applications for different areas, including smart cities, health care, smart agriculture, smart metering, and so forth [1; 2]. LoRa operates using the unlicensed ISM sub-GHz band and uses Chirp Spread Spectrum and Frequency Shift Keying modulation that makes it more robust to interference while transmitting to longer distances. It supports low power consumption and long-range transmission reaching up to several kilometers range. However, the long coverage, along with minimum transmission power, comes at the cost of a low data rate [3]. Long Range Wide Area Network (LoRaWAN) forms a star-of-star topology that contains end devices transmitting traffic to a gateway in a single-hop communication approach. Its basic design consists of end devices, gateways, and network servers, as shown in Figure 1. End devices are in charge of collecting data by interacting with the surrounding environments based on the purpose of the deployed application. The collected data are transmitted by end devices to one or more gateways using Spreading Factors ranging from 7-12. Gateways take the responsibility of forwarding the received traffic to the network server. Gateways are connected to the network server via an IP network. The network architecture implies that an end device must be in the radio range of the gateway to transmit its traffic successfully. However, the radio range can be reduced due to different possible obstacles in the deployed environment, such as buildings, Trees, and mountains, hence affecting the overall efficiency of the network. In addition to that, some deployment scenariosrequire much longer coverage based on the purpose of the employed applications. Being out of the gateway range can also be a result of mobility if the end devices are mobile [4]. Therefore, multi-hop LoRaWAN can fill in this gap, allowing out-of-the-range devices to transmit their traffic to the gateway utilizing other end devices that are in the range of the gateway without installing additional gateway devices. The use of additional gateways is considered impractical sometimes since a gateway requires main power that would be difficult to supply in some remote locations. Moreover, employing multi-hop LoRaWAN would be justified for avoiding collisions since faraway devices can use a smaller Spreading Factor (SF). The use of lower SF reduces the Time-on-Air (TOA) a packet requires to travel to its destination. Thus decreasing the chance of collisions. LoRaWAN provides support for three different Medium Access Control (MAC) modes to satisfy different application requirements. An end device should operate according to one of the three classes: class A, class B, or class C [5]. Class A functions based on the application need. It offers bidirectional communication between the device and the gateway based on the ALOHA protocol. The communication starts with an uplink transmission, which is followed by two receive windows that can be used for downlink communication. It is considered the lowest power device mode. Class B, on the other hand, adopts synchronization of end devices with the network server to enable more receive windows in addition to the two receive windows of class A. A time synchronization beacon is sent periodically by a gateway to allow end devices to synchronize their clocks and, hence, entitle the network server to know the moment that an end device is awake. It targets reducing the delay for the network server to transmit downlink data to an end device. Class C, finally, provides the least latency for downlink traffic since it opens an almost continuous receive window. The receive window is closed only when the device is transmitting.