# Optimization of Acoustic Communication Links for a Swarm of AUVs: The COMET and NEMOSENS Examples
## 4 Channel Access  
The communication among the AUVs is essential to accomplish any underwater task. During a mission, these vehicles need to exchange information for coordinating the navigation. Each AUV needs to compute its own position and velocity, and broadcast them to the other vehicles. So, the manner that these information will be exchanged among them, i.e., the channel access method, should be previously defined.  
Many multiple access methods exist in the literature, such as time-division multiple access (TDMA), frequency-division multiple access (FDMA), code-division multiple access (CDMA), and Aloha. The Aloha protocol is efficient for the case the message length is proportionally smaller than the transmission periodicity. Although, this condition might not be satisfied in our case, specially when the number of AUVs increases.  
In the CDMA method, each user is assigned with a spreading sequence to be used before transmitting the data. As the user can send information during all the time using the whole bandwidth, this system is highly susceptible to near-far effect. Despite that, it needs a power control mechanism at the transmitter side, which requires a feedback link. However, this is not desirable due to this additional resource consumption.  
In the FDMA method, the frequency resources are shared among users. As the projector bandwith is small (only a few kHz), each user will be assigned an even smaller bandwidth. This causes a reduction of the bit rate and subsequently an increase on the communication duration.  
The TDMA method has an easy implementation and consists of sharing the time among users. One drawback of TDMA is that each AUV might have a distinct internal clock shift. Another inconvenience of utilizing TDMA for underwater communications is that the acoustic wave has a significant propagation delay. For coping with this channel feature, one should insert a guard interval, which reduces the information throughput. Nevertheless, TDMA was selected due to its robustness and resilience to near-far effects.  
In the TDMA approach adopted in this project, each AUV can transmit information during a designated time slot, as shown in Figure 5. The communication ordering is previously defined, and each user needs to wait its turn for communicating. In this network, there is no central node, and each user has the knowledge of the total number of nodes.  
All time slots have the same duration and its time length is defined as  
\[T_{\text{slot}}=T_{\text{frame}}+T_{\text{guard}} \tag{1}\]  
where \(T_{\text{frame}}\) is the frame duration, and \(T_{\text{guard}}\) is a guard interval. The frame duration is a fixed parameter and it is set to \(T_{\text{frame}}=2\) s [19]. The latter parameter (\(T_{\text{guard}}\)) has the role of guaranteeing no transmission collision in the network. Notice that \(T_{\text{guard}}\) can have a variable value, which depends on the channel and on the network topology. In other words, it depends on the maximum distance between two AUVs and on the highest channel delay spread:  
\[T_{\text{guard}}>\frac{d_{\text{max}}}{c}+\tau_{\text{max}} \tag{2}\]where \(\tau_{\rm max}\) is the highest delay spread.