# A Visitor Assistance System Based on LoRa for Nature Forest Parks
## 5 The Proposed Visitor Assistance System
### Infrastructure  
Totems communicate with mobile terminals using LoRa technology, developed by Semtech for LPWAN [4; 26]. LPWAN is designed to connect equipment over long distances, with a low transmission rate and low power consumption. They are therefore suitable for the use of battery-operated terminals, small lightweight and low cost, as required in this work. Only the physical layer (PHY) of LoRa is used, operating at the frequency of 915 MHz. LoRaWAN specifies a star network topology, with the gateway being the central node. Thus, since point-to-point communication between terminals is required in our project, LoRaWAN is not used.  
The LoRa PHY uses Forward Error Correction and Chirp Spreading Spectrum modulation (CSS) [6], which varies the frequency without changing the phase between adjacent symbols. The Spread Factor (SF) defines the ratio between bit rate and chirp rate. LoRa specification defines six different spreading factors (SF7, SF8, SP9, SF10, SF11, and SF12), which allow the formation of orthogonal channels. Links with different spreading factors do not collide [3]. The SF defines the symbol duration and, together with the Bandwidth and Code Rate (CR) parameters, defines the data transmission rate. The CR defines how many bits are used for redundancy in the message, to perform error recovery. A higher SF increases the sensitivity of the reception threshold in terms of power, but also increases the duration of symbols and, as a consequence, decreases the transmission rate of the link [27].  
Figure 6: Network scheme.  
Totems announce their coordinates through beacon messages and serve as communication gateways to mobile terminals, forwarding their data to the cloud. Figure 7 illustrates the scenario. We assume that communication between totems and the cloud will use any available means: cellular, Wi-Fi, etc. The specification of this communication is beyond the scope of this article.  
The fixed infrastructure must be reduced, favoring the low cost of the whole system and considering that the area to be covered by the monitoring system is typically large--ecological conservation units. The totems must be positioned in places that allow optimizing the coverage, considering the signal on the trails, the attenuation caused by the height difference between the antennas and the risks of each stretch of the trail.  
Nevertheless, the difficulties of propagation in the forest inevitably lead to a smaller radio range. Thus, the communication network in the forest will present connectivity gaps. Therefore, the proposed monitoring system is based on a DTN composed of fixed and mobile points. The mobile points are the visitors' terminals which, operating collaboratively, forward packets between themselves and to the fixed points, the totems. Figure 8 shows mobile terminals moving toward each other and exchanging records. Totems perform as data kiosks, concentrating information and interconnecting with the park's server. Packet forwarding between DTN nodes is done opportunistically when terminals have contact with each other or with a totem.  
To keep the solution as simple as possible, we do not employ the whole DTN protocol stack [28], or the bundle protocol [29], in the proposed monitoring system. Instead, the main DTN techniques are implemented directly on top of LoRas physical layer, saving memory and processing resources. The terminals exchange data with each contact, using a store-carry-forward approach. The DTN concept of custody transfer is partially implemented since it only occurs in the communication between a mobile terminal and a totem. Mobile terminals exchange location data periodically with the totems and with each other, allowing the visitor's movement to be monitored.