## 5 Results and Discussion  
In this Section the results of the two measurement campaigns performed with the setup and the methodology described in Section 4 are presented. The transmission performance of the LoRa technology, implementing the LoRaWAN MAC protocol, is investigated in terms of mean and standard deviation of received signal strength indicator (RSSI) and signal to noise ratio (SNR), and packet loss (PL) percentage. In particular, the SF setting is varied for each tested configuration from 7 to 12 while the other radio settings are left fixed (i.e., CR = 4/5, BW = 125 kHz). This decision was motivated by the fact that, as proved in [8], varying the CR and the BW has no substantial effect in improving the radio performance except for a better error decoding in case of the highest CR, which at the same time leads to higher power consumption. Indeed, the CR setting specifies the ratio between transmitted information bits and redundant parity bits used for forward error correction, hence a lower CR entails more transmitted bits, longer time on air and consequently higher current absorption. On the other side, the SF, which defines the ratio between chip rate and symbol rate, is a relevant parameter for the evaluation of the trade-off between demodulation capability, receiver sensitivity, PL, and power consumption, thus varying the SF in its full range offers a thorough investigation of the LoRa transmission feasibility. The outcomes of the laboratory tests for the configurations of Table 1 are numerically reported in Table 2 and graphically depicted in Figures 6-8. As expected, the change in the SF does not affect the RSSI magnitude but rather the PL, which increases as the SF decreases, in accordance with the LoRa documentation [43]. Moreover, the SNR decreases in conjunction with the increase of the PL, proving the growth of the noise level in the radio channel. In particular, up to a salinity of 30 g/L the PL percentage is consistent with the systematic losses of the radio channel, while for higher salinity the lower SFs start experiencing higher PLs which however, depending on the reliability requirement of the specific application, can still be acceptable. The most critical tested configuration is #3 of Table 1, since the packet reception is hindered for SFs lower than 9 and seriously compromised for the other SFs because of the severe attenuation and the SNR degradation. For this reason we decided not to test lower depths but to increase the salinity fixing the \(d_{\text{{L}W}}\) to 6 cm. Considering that the seawater salinity ranges between 31 g/L and 39 g/L, the results for configurations #7-#10 can be assumed representative of realistic deployments in marine water demonstrating that the transmission is feasible only with the receiver submerged by a few cm of water. This limits the use of this technology to almost superficial monitoring applications such as the monitoring of water sport athletes or tides warning systems, or in combination with other techniques with the aim of implementing hybrid networking [44]. Figure 6: RSSI mean values and standard deviations for SF from 7 to 12 (as per the legend) for the laboratory test configurations reported in Table 1.  
Figure 7: SNR mean values and standard deviations for SF from 7 to 12 (as per the legend) for the laboratory test configurations reported in Table 1.  
These results are also compared with the link budget model proposed in Equation (27), considering that in the examined application \(P_{r}\) is the RSSI downstream of the demodulator, \(P_{t}=14\,\mathrm{dBm}\), \(G_{r}=2\,\mathrm{dBi}\) and \(G_{t}=2\,\mathrm{dBi}\). Equations (28)-(30) are derived assuming \(d_{UW}\) as in Table 1, \(d_{AW}=2\,\mathrm{m}\), \(f=868\,\mathrm{MHz}\) and \(\eta_{A}=376.73\,\mathrm{\Omega}\).