# A LoRa-Based Linear Sensor Network for Location Data in Underground Mining
## 1 Introduction  
This paper describes our work in designing, implementing and analysing an emergency network for transmitting location data within an underground mine. We consider the situation where existing mining communications infrastructure, typically wireless communications based on IEEE 802.11 with optic fiber backhaul, is severely damaged and temporary communications infrastructure must be installed rapidly while the main infrastructure is repaired. This temporary infrastructure's primary purpose is to support the transmission of low bit-rate location data of people and equipment carrying out repairs or rescue and act as a backhaul. Rescuers or repairers place wireless relays as they advance into the mine. This sequence of wireless relays will form a linear network. Personnel and equipment carry tags that generate location information which is transmitted via the relays to a headend connected to surviving infrastructure. The motivation for this work has come out of our development of a LoRa basesd system as a relay technology for detonating explosives in underground mines [1]. We observed that LoRa propagates very impressive distances underground and that it consequently has great potential for other applications in underground mining. In this paper we explore its capabilities for one of those applications. LoRa has considerable advantages over other network technologies for applications of this kind where high bit rates are not needed because of the great distances it can propagate. WiFi is commonly used underground but for it to cover the full length of an extraction tunnel, an extraction drive of more than 220 m directional antennae must be used. Our observations underground showed that, perhaps surprisingly, LoRa can propagate the full length of an extraction drive and still provide a usable signal a further 70 m without line of sight in a perpendicular access tunnel. We intend reporting on these results in more detail in a future paper but an overview of them can be read in [1]. Linear networks comprise one or more source nodes, one or more relay nodes and a destination node often referred to as the headend. Linear networks are sometimes described as 'long and thin' because topologically they resemble a line of nodes where each node is connected to only its two nearest nodes. In a wireless network, there may well be overlap between more than the nearest nodes. The relays both forward messages from the tags (the source nodes) carried by personnel and equipment as well as forward messages from other relays. Configuration by those installing the relays has to be minimal. Ideally they will just be placed in an appropriate location with no further configuration at the time of deployment. These requirements lead to a number of design decisions which we discuss in detail in Section 3. These decisions are:  
* The system uses the LoRa physical layer because of its long range, low cost and ease of deployment. LoRa (from \"Long Range\") is a low bit-rate proprietary technology from Semtech which we used to transmit messages to support repair or rescue efforts. * Message forwarding is based on flooding rather than unicast. Any relay that receives a message uses minimal state information to decide whether or not to forward it. There is no need for static routing or routing protocols. * Collision management is based on a simplified Carrier Sense Multiple Access (CSMA) mechanism where tags and relays listen to the wireless medium and when it becomes idle, wait a random interval before transmitting. Linear networks can suffer from unfairness. Some nodes will be so busy that messages will be dropped. The more relays a message has to traverse the greater the probability a message will be dropped. Consequently, the further from the headend a message is generated, the greater the probability that the message will be dropped. In our analysis of the network, we explore how this probability is related to number of relays and traffic load.