# Optimization of Acoustic Communication Links for a Swarm of AUVs: The COMET and NEMOSENS Examples
## 5 Physical Layer
### Adaptive Parameters  
The modern previously described is embedded in each AUV. When the AUV is operating in a swarm mode, the system dynamics are constantly changing. So, it would be beneficial if each vehicle could automatically adapt some physical layer parameters according to the channel. When a feedback link is available, the physical layer adaptation is getting some kind of channel state information from the receiver [6; 7; 8; 9; 12; 13]. Such an approach can be efficient but is costly in terms of overhead and energy. Apart from that, a feedback link might not be feasible for some underwater applications, such as long range communications [14]. In our context, no feedback is provided. However, we can take advantage of the information of the AUVs location and speed that is periodically broadcasted. Based on this knowledge, we propose to run, at the transmitter side, an on-board channel simulator. The outputs of this simulated channel are then used to predict some metrics on the modem performance. OFDM parameters that can be adjusted include the number of subcarriers (\(N_{\text{sub}}\)) and the cyclix prefix length (\(T_{\text{CP}}\)). In order to correctly adapt these parameters, it is important to first understand how they impact the system performance. Considering a fixed bandwidth, the usage of a large number of subcarriers will decrease the subcarrier spacing. If this spacing is small enough, the information recovery at the receiver side can be compromised. For example, the Doppler effect might cause intercarrier interference (ICI), and as a consequence the subcarriers orthogonality is lost, hindering the information recovery. On the other hand if we choose a low number of subcarriers, the resulting throughput is lower than the one that could be achieved (due to the CP that is added in each OFDM symbol). So, a proper choice of \(N_{\text{sub}}\) is of paramount importance for an efficient resource usage. Knowing that the underwater acoustic communication is impaired by the multipath channel with a large delay spread, one must consider these effects when designing a communication system. A possible solution to tackle this issue is to add a cyclic prefix (CP) at the signal before transmission. The main function of CP is to remove the interblock interference (IBI) at the receiver side. Although, if the CP length is lower than the channel delay spread, the IBI is not eliminated. Still, if the CP is much longer, we are wasting power and transmission time. A possible way of choosing the number of subcarriers and the CP length would be through the minimization of the IBI and the ICI [27]. As the CP length has a direct impact on the transmission rate, we decided to constraint the system overhead coming from this parameter. So, this optimization problem can be mathematically written as  
\[\min_{N_{\text{sub}}\cdot T_{\text{CP}}} (P_{\text{ICI}}+P_{\text{IBI}})\] s.t. \[\rho\leq\rho_{\text{max}}\]where \(P_{\text{ICI}}\), \(P_{\text{IBI}}\) are the signal power containing ICI and IBI, respectively. The value \(\rho_{\text{max}}\) is chosen by the user, and it represents the maximum overhead due to CP. The overhead \(\rho\) is defined as  
\[\rho=\frac{T_{\text{CP}}}{T_{\text{CP}}+T_{\text{sym}}}, \tag{7}\]  
where \(T_{\text{sym}}\) is the useful symbol duration. The low speed propagation of the acoustic waves increases the round-trip time [26], which may reflect in variations of the channel impulse response. So, it might not be feasible to use channel state information (CSI) for computing \(P_{\text{ICI}}\) and \(P_{\text{IBI}}\). A possibility is to predict the interference power due to ICI using [28]. In addition, since no feedback link is available, the interference can only be predicted but cannot be estimated.