## 4 Performance Analysis
### Simulation Assumptions and Conditions  
For the simulation, we consider a network scenario consisting of two BSs and one AUV, as depicted in Figure 2. There exists an overlapping region between the network areas of these two BSs, where handover can be conducted. Under this scenario, the simulation considers the following assumptions:  
* Underwater acoustic communication can suffer from underwater background noises, which induce severe interferences and, thus, frequent transmission failures. However, to investigate the pure effect of BS motion on the handover decision, it is assumed that the received signal degradation due to background noises is not considered in simulations. * The AUV moves along its programmed path with a uniform speed and direction. * The AUV receives sufficient messages from the two BSs to determine the distances to them, and it can calculate the AUV-BS distance based on the transmission and reception timestamps of these messages. * In the simulation, we only consider a one-way scenario where the AUV moves from the network edge of BS1 to the network edge of BS2, as illustrated in Figure 2.
* We assume buoys floating on the water surface that have variable positions over time caused by diverse maritime environmental conditions, including winds, waves, and currents, as well as the methods employed for maintaining their position, such as mooring or anchoring. This situation can have a substantial impact on the triggering of handovers. The simulation was executed using MATLAB software (ver. R2023a) under the following conditions:  
* In simulations, the AUV-BS distance is primarily derived considering the BS motion response over time. Here, the BS motion response is determined considering various environmental conditions, such as wind and waves, as explained in Section 3. Therefore, by varying the direction and magnitude of BS motion, the BS motion response is derived and applied to the simulations. If the BS moves in the same direction as the AUV, the AUV-BS distance reduces, possibly leading to decision errors, in contrast to a stationary BS. Additionally, the relative motion directions of BS1 and BS2 can also cause errors, since BS motion affects the difference between \(D_{BA1}\) and \(D_{BA2}\). Consequently, this variability could result in the AUV erroneously selecting a BS, since it aims to connect to the closest network. The direction of BS motion entails four possible combinations based on (1) whether the BS moves in the same direction as the AUV and (2) whether the motions of BS1 and BS2 are directionally the same. We consider these four cases in analyzing the AUV-BS distance. These four cases are described in the caption of Figure 4.
* In addition, the magnitude of BS motion can affect the extent of positional changes in the x, y, and z coordinates; that is, the larger the BS motion, the greater the potential variability in the AUV-BS distance. This paper considers three cases according to the WMO sea state codes. Among the codes defined by the WMO, ranging from 0 to 9, SS3 (sea state code = 3, slight), SS6 (very rough), and SS8 (very high) are considered [43]. * To check the influence of network radius on handover decisions, we consider four network radii of 100, 300, 500, and 1000 m.
* The operating depth of the AUV (\(d_{A}\)) is set at 50 m.
* As shown in Figure 2, the initial positions of BS1, BS2, and AUV are given as \((r,-r,0)\), \((r+L,-r,0)\), and \((0,-r,-d_{A})\), respectively. The straight-line distance between the two BSs on the water surface, denoted as \(L\), is set using the overlap rate value \(\delta\) and \(r\) (i.e., \(L=(1-\delta)\times 2r\)) where \(\delta\) is set to 0.1. * The moving speed of the AUV is set at 5 knots, where 1 knot is approximately equal to 0.514 m/s. * In the simulations, we varied the conditions of BS motion direction, magnitude, and network radius. To ascertain the unique impact of each factor on the handover process, the simulations were conducted by altering only one condition at a time.