## 2 System Model
### Communication and Networking  
We consider a three-dimensional B-UWAN, as illustrated in Figure 2. A B-UWAN is normally composed of multiple BSs on the water surface and several AUVs. Each BS communicates with an AUV that is currently located in its cell. AUVs search the target area and obtain data. Then, they transmit their sensed data to their BS. A BS also acts as an underwater wireless backhaul, because the BS transfers all of the gathered data from the AUVs to a land station, as shown in Figure 1. To do this, there are two communication links in the BS: one is the underwater acoustic link between the BS and an AUV, and the other is the maritime wireless link between the BS and the land station. The maritime wireless communication can be exemplified as LTE, VHF, Wi-Fi, or satellite communication [46]. In this paper, we consider two BSs (i.e., BS1 and BS2) and only one AUV to describe the situation in which handover occurs.  
All of the nodes of a B-UWAN are equipped with a digital acoustic communication module that provides the following common characteristics:  
* Multiple channels are available. One is a common control channel used for control signaling, such as channel access or handover. Others are data channels purely employed for data transmission. This ensures sufficient bandwidth for the transmission of underwater data sensed by an AUV in challenging underwater conditions.
* The communication between a BS and an AUV can be half-duplex or full-duplex. The communication method is determined considering operational scenarios, along with the volume and transmission frequency of control and data messages.  
Figure 2: A system model for investigating the impact of BS motion on underwater handovers.  
* An AUV and a BS can communicate with one another within their communication range by employing an omnidirectional antenna. This is because it is challenging to align antenna directivity between fast-moving AUVs and unpredictably moving BSs.
* It is assumed that any AUV can connect to its current BS directly by sending signals with the maximum power. The networking of a B-UWAN is described as follows:
* A remotely operated AUV needs to periodically transmit control signals such as its current survival status and underwater location information to its current BS [47; 48]. In addition, depending on the situation, the AUV may need to transmit large volumes of sensed data to the BS in burst mode. To do this, several control and data messages can be exchanged between the BS and the AUV.
* For a common control channel, the time domain for uplink and downlink transmission needs to be efficiently divided. For uplink transmission of control signaling, a time-based scheduling approach is employed, enabling AUVs to transmit their messages efficiently and without collision. For data channels, an adequate channel access scheme should be employed in order to not only avoid collisions but also enhance spectrum efficiency.
* As any AUV can transmit its acoustic signals to its current BS in one hop, no routing strategies are employed.
* The AUV transitions to the network area of a different BS. To maintain continuous data service for the AUV in the network of a new BS, proper handover procedures are implemented. Detailed descriptions of these handover scenarios are provided in Section 2.3.