## 2 System Overview
### Transmission Protocol  
In the actual UWVLC process, due to the interference of environmental factors such as underwater vehicle jitter and underwater turbulence, packet loss and bit errors will inevitably occur. In order to ensure an error-free transmission, we introduce an error retransmission mechanism to mark transmission error packets and lost packets at the receiver and feed them back to the transmitter for retransmission. The traditional ARQ protocols can be divided into three folds: Stop-and-Wait ARQ (SW-ARQ), Go-Back-N ARQ (GBN-ARQ), and Selective Repeat ARQ (SR-ARQ) [24]. In these traditional ARQ protocols, the uplink and downlink are usually equivalent. Furthermore, if they are directly applied to UWVLC, a series of problems will arise. First of all, the VLC has the characteristic of line-of-sight transmission, which requires strict link alignment [11; 25]. In the application scenario of this paper, there is a dynamic communication node. If a bidirectional VLC link is introduced, the ROV control system requires extremely high precision, which is extremely difficult to implement. In addition, in order to ensure a larger receiving angle and a longer transmission distance, high-power LEDs and photoelectric sensors with higher sensitivity are often used in the system. The optical signal may interfere with its receiver due to backscattering. When the backscattered light is strong enough, the photoelectric sensor will be saturated or even damaged. Separating the transmitter and the receiver of the VLC and using a half-duplex communication mode can effectively solve the problem of backscattering interference [26]. When using the half duplex communication mode, the switching of transmission and reception modes will also reduce the throughput of the system. The error-free data transmission protocol proposed for underwater optical-acoustic cooperative communication is based on our previous work [19], which we named Combinative-Selective-Repeat ARQ (CSR-ARQ). The protocol ensures error-free data transmission by separating the forward data transmission and backward ARQ data. The key motivation of the proposed CSR-ARQ scheme is to transmit high-speed data through visible light communication, while underwater acoustic communication transmits ARQ and low-speed control commands. Because the transmission media of the two communication methods are different, there is no problem with self-interference and backscattering. Since the downlink only needs to work in the simplex mode and does not need to establish a bidirectional optical link, the alignment requirements of the UWVLC link can be reduced, which is easy to implement in engineering. \"Combinative-Selective-Repeat\" means combining multiple optical communication data frames into one packet. Since the length of the underwater acoustic communication data frame is only 42 bytes, the flag array for one feedback is limited. Therefore, by reorganizing multiple data frames into data packets the receiver can feedback the reception status of all data frames at one time. The working principle of CSR-ARQ is that the transmitter continuously sends data through visible light communication. After all data frames are sent, the receiver feeds back ARQ data packets through underwater acoustic  
Figure 8: (**a**) ROV. (**b**) The internal structure of the ROV. communication. In the ARQ data packet, the combined packet is the smallest unit. After receiving the ARQ packet, the transmitter determines whether to retransmit the data of the entire packet by analyzing the integrity of the corresponding packet location data. Until the next feedback comes, the transmitter will retransmit all the failed packets in a loop.