# Performance Optimization of Underwater Communication Links at Different Ranges for AIS Relay to AUV
## 7 Underwater AIS Relay
### Reducing the Load for Underwater Communications  
An underwater acoustic channel is characterized at least by three major factors: an attenuation proportional to the signal frequency, time-varying multipath and low speed of propagation (\(c\) = 1500 m\(\cdot\)s \({}^{-1}\)) inducing greater multipath and Doppler effects [6]. Due to properties of channel, its capacity is limited at large distances, the raw BR of the modem will be quite low (a few hundred bps at tens of kilometers). Therefore, it is not possible to simply relay all AIS messages to the AUVs. Messages must be compressed to obtain the target BR without loss of important information. To reduce the amount of data, one has to select the relevant information. The goal is to reduce the collisions likelihood between submarines and ships. Therefore, information, such as the type of shipment or origin of ship, are not needed. The important information are:  
* Latitude, longitude for the ship position;
* Maritime Mobile Service Identity (MMSI) for identification purpose and, thus, differentiating the vessels;
* Course and speed of the ship to predict its path. All these information are all contained in five different messages types (1, 2, 3, 18, and 19) [30]. Latitude, longitude, MMSI, course over ground and speed over ground are coded, respectively, on 27 bits, 28 bits, 30 bits, 12 bits, and 10 bits. Hence, by extracting only these information, we need to send 107 bits instead of the 256 bits of the time slot. Further improvements on the load to relay can be made on some fields. As said previously, the main goal of the application is to reduce the risk of collision for AUVs. AUVs only need to know the location of surrounding ships. Hence, we can reduce the size of MMSI, latitude, and longitude fields without any accuracy lost. The MMSI is coded on 30 bits and can, therefore, identify a lot of ships. However, during a mission, the area covered by the AUVs will be quite '_small_' compared to earth scale, a few tens of kilometers for instance. Being able to identify a lot of ships is not needed. However, MMSI also contains information, such as country of ship. It might be interesting  
Figure 11: AIS data packet structure. in some applications to create classes of vessels. Still, with a 10 bits long identifier, it is possible to create 8 classes of 128 boats each which should be enough for most applications. Since the covered area is limited, the latitude and longitude can be reduced without losing position accuracy. The most significant bits will remain the same for a given area. Therefore, they carry no information and can be removed without affecting the accuracy of the position [33]. Analysis of a lot of real AIS messages collected at shore in Brittany (France), show that only 21 bits can be used for both latitude and longitude. Through the described methods, 182 bits of the 256 bits can be saved by message for messages types 1, 2, 3, and 18 giving a compression ratio of 3.46. Further compression could be achieved but at the cost of localization accuracy loss. It would be interesting to study the accuracy needed for AUVs and how it impacts the surfacing time of AUVs. Reducing the payload to relay is important to increase the number of ships the modem can handle. Even with these improvements, the BR difference between AIS and underwater communication stays important. Indeed, an underwater modified AIS packet may take more than 1 s to be transferred depending on the parameters. For example, with \(Q=4\) and \(M=8\), the modem needs 1.28 s (2 MC-SS-2D symbols) to send 1 AIS packet. Presented watermark simulations can be used to analyze the performances of the modem in that scenario. Considering an AIS packet has 74 bits, the packet error rate (PER) can be measured. One packet will be a round number of MC-SS-2D symbols with at least 74 bits. Table 2 shows the PER at an SNR of 3 dB for different configurations and with no channel coding. This table confirms the performance results of the Figures 8 and 9.