## 6 Evaluation Results  
The UDMSim was validated in two different seawater scenarios. The first scenario--_docked_--considers that the DMU is physically docked or is very close (\(\leq\)10 cm) to the SU through the use of a docking station or an umbilical antenna, similar to the refuel system of a jet plane. This scenario is more suitable for stationary SUs, such as a sea lander or an observatory, and allows the usage of a 20 MHz radio channel based on IEEE 802.11 g/n point-to-point link that may operate at a carrier frequency ranging from 40 MHz to 2.4 GHz, as demonstrated in [30]. The second scenario--_1 m apart_--considers that the DMU approaches and tries to maintain a 1 m distance from the SU, which can be fixed or mobile; an example of a fixed SU is shown in Figure 10. In this case, according to the attenuation of RF signals underwater, especially in seawater [28], carrier frequencies in the range 10-20 MHz should be used. To minimize the SNR differences of the OFDM subcarriers across the IEEE 802.11 channel and since it is not possible to use a 20 MHz channel on a 10 MHz carrier, the bandwidth was reduced in this case to 5 MHz. In order to evaluate the impact of the localization errors on the communications system, two sets of simulations were run. The first considered perfect localization--_PerfectNav_. The second relied on a state estimator using imperfect measurements from sensors--_RealisticNav_--an enhanced version of the ImperfectNav presented in [52]. The outputs of the simulator on the realistic localization are the true poses of the DMU and SU, which are imported to ns-3 by using the _TraceBasedPropagationLossModel_. Figure 11 shows the distance and SNR when the DMU travels 1000 m from the CSU to the SU on the _1 m apart_ scenario and the corresponding SNR variation for the _PerfectNav_ and _RealisticNav_. We can observe that the distance between the DMU and the SU decreases in a linear manner across the journey, and there is no connectivity until the two nodes are close to each other. Figure 12 shows a closer view of the final approach, where we can see the position error, the strong SNR variations, and even connection losses (SNR \(\leq\) 0), which have a negative impact on the short-range throughput. In Figure 13, we can also observe the effect of different antenna alignments on the SNR. UDMSim is able to consider the radiation pattern of the antenna on the DMU and SU and compute the SNR along the mission. We then compare different dipole positions to the isotropic antenna case, i.e., considering that the antenna has a uniform, unitary gain in all directions. If two vertical dipoles are used, perpendicular to the DMU and SU and similar to the ones used in [55], we can observe that the SNR decreases when compared with the isotropic antenna. Since the DMU has a negative pitch, the antennas are not aligned; thus, its gain is reduced. We should also take into consideration the azimuth, since the H-plane of the antenna might change from freshwater to seawater [55]. In this case, the azimuth did not have a significant impact on the SNR. By averaging the negative pitch of the AUV and performing an elevation compensation of the dipole on the AUV by 65\({}^{\circ}\) (relative to the perpendicular of the vehicle), we were able to achieve an SNR closer to the isotropic antenna. Using two vertical dipoles or one vertical and one horizontal dipole resulted in significant SNR losses. Depending on the scenario considered, other type of antennas can be analyzed by UDMSim, such as loop antennas [56]. The optimization of the antenna beam, either physically or by electrical beamforming, is out of the scope of this work. Figure 11: Distance and SNR of a DMU travelling 1000 m from the CSU to the SU using _PerfectNav_ and _RealisticNav_. Figure 12: A closer look on DMU approaching the SU for a 1000 m distance using _PerfectNav_ and _RealisticNay_. Figure 10: 3 Three-dimensional simulation of a DMU approaching an SU.