# Smart City Pilot Projects Using LoRa and IEEE802.15.4 Technologies
## 5 A Smart City Testbed: The Rigers Project
### Path-Loss Characterization via Experimentation  
A preliminary testbed was realized at WiLAB to get an insight on the actual performance of a LoRa-based LPWAN deployed in the Saragozza district of Bologna. In particular, two Emb Development boards EMB-LR1272-EVK hosting the LoRa module EMB-LR1272 were adopted. Each module was equipped with a half-wavelength dipole antenna with a gain of 2 dB. Figure 5: LoRa gateway setup. One of them, acting as gateway, was placed on top of the Engineering School tower at a height of 71 m above the ground, as shown in Figure 5, whereas the second device, powered by batteries and equipped with a GPS receiver, was carried around in the city at a pedestrian speed, 1.5 m above the ground. Every 8 s, the mobile terminal transmitted to the gateway one packet, whose payload carried the device coordinates, with an output power of 14 dBm (the maximum allowed by the board) in a 125 kHz bandwidth centered on 868.1 MHz. The spreading factor was set to 12 (the highest possible choice), to improve the receiver sensitivity. Upon the reception of each packet, the gateway i): recorded the correspondent Received Signal Strength (RSS), along with the coordinates of the transmitting device, and ii) responded with an ACK packet, to inform its counterpart that it was within the coverage range. A total amount of 2425 RSS measurements was thus collected, with a maximum distance from the gateway of 2390 m. For larger distances, in fact, the received power was below the devices sensitivity, making communications almost or completely impossible. The outcome of the measurement campaign is summarized in Figure 6a, which provides a pictorial representation of the RSS values experienced by the gateway. In particular, the blue circle denotes the gateway location, whereas the colored dots (colour shades from green to red) represent the positions of the transmitter along the traveled paths. The specific color of a dot in a given position is representative of the RSS experienced by the gateway when a device was transmitting in such position. It is worth observing that, despite the favorable position of the gateway (71 m above the ground level), which resulted in propagation conditions cleared from obstacles for most of the signal path, the service coverage is around 2.5 km, which is below the distance that LoRa devices are supposed to cover. This is due to the low elevation of the mobile device (1.5 m from the ground), which suffered from the shadowing introduced by nearby buildings. This phenomenon is well evident in Figure 6b, where the blue dots represent the path-loss measured in correspondence of the transmitter-receiver distance reported in the abscissa. The solid curve refers to the path-loss law \(PL\) that best fits the measurements, assuming for \(PL\) [dB] the one-slope model [37]  
\[PL[dB]=PL_{0}[dB]+10\alpha\log(d)\, \tag{1}\]  
where  
* \(d\) is the distance between the transmitter and the receiver in meters,
* \(PL_{0}\) is the path-loss at 1 meter distance, whose measured value is 31 dB,
* \(\alpha\) is the power decay index. In the specific case plotted in Figure 6b, the best-fit curve has been obtained with \(\alpha=3.25\). This value, quite far from the ideal (free-space) value of 2, confirms the relevant impact of obstacles nearby the mobile device, despite the fact that the signal experiences free space propagation for most of the flying time. Even more relevant is the path-loss spread that can be observed in Figure 6b for a given distance (vertical spread). This phenomenon is summarized by the Root Mean Square Error (RMSE) with respect to the best-fit curve, which is 7.28. This value, quite large indeed, confirms once more the significant role played by the surroundings of the mobile terminal for the experienced performance. In view of the RIGERS project, it is, therefore, of paramount importance a proper choice of the gateway position, especially considering that most of LoRa devices will operate indoor, with additional propagation losses.