# Performance Optimization of Underwater Communication Links at Different Ranges for AIS Relay to AUV
## 5 Simulations Using Realistic UAC (Watermark)  
To test the modem and compare the performances of different parameters, watermark channels were used [20]. Watermark is a benchmark for UAC that offers different realistic acoustic channels to test a communication system. Channel generated by watermark are created using real measured channels to faithfully reproduce their effects. Watermark channels are time and frequency selective. In the Watermark database, two selected scenario NOF1 (Norway Oslofjord) and NCS1 (Norway--Continental Shelf) channels have been considered to evaluate the performance of the proposed modem.  
Different configurations of the modem were tested on the NOF1 channel. The modem is using 256 subcarriers and a 4 kHz passband with no channel coding and no pulse shaping filter. An OFDM symbol lasts 80 ms with a chip rate of 4 ksymb/s.  
In Figures 8 and 9, \(Es\) denotes the energy per chip symbol which includes both the data and pilots, N0 is the noise power spectral density. Results reported in Figures 8 and 9 were obtained by averaging the BER (bit error rate) over 300 realizations of the NOF1 channel. Each realization is a 30-s time and frequency selective channel. The signal goes five times through each 30-s channel of the 60 available. BER is calculated for each realization, and only the average over all realizations is shown. No channel coding for error detection and correction were used. Each modem has a 4 kHz bandwidth. OVSF and Gold sequences were used respectively for the spreading and scrambling process. Performances of the proposed modem is compared to an OFDM modem using 256 subcarriers and an LS estimation with half of its subcarriers being pilots.  
Figure 7: Simulation results over an 8-path Rayleigh block fading channel with a SF of \(L=8\).  
Watermark channel can be very selective in frequency and time domain. For instance the NCS1 channel has a half coherence time of 100 ms meaning that each OFDM symbol (80 ms) needs its own pilots to properly estimate the channel frequency response. Parameters of the simulation are summarized in Table 1.  
As can be observed, increasing SF makes system more resilient to noise, hence improving BER at low \(\frac{E_{8}}{N0}\). Depending on transmission conditions, greater SF may be used. However, increasing SF reduces the throughput of the mod  
\begin{table}
\begin{tabular}{c c c c} \hline  & \(Q\)\(\times\)\(M\) & **Bits in 1 MC-SS-2D** & **User Bit Rate** \\ \hline MC-SS-2D & 2 \(\times\) 4 & 128 & 400 bps \\ MC-SS-2D & 4 \(\times\) 4 & 64 & 200 bps \\ MC-SS-2D & 4 \(\times\) 8 & 64 & 100 bps \\ OFDM & 1 & 256 & 3200 bps \\ \hline \end{tabular}
\end{table}
Table 1: Signals description.  
Figure 8: BER vs. \(\frac{E_{8}}{N0}\) for a communication on NOF1 channel (frequency passband: 10.5 kHz–14.5 kHz).  
Figure 9: BER vs. \(\frac{E_{8}}{N0}\) for a communication on NCS1 (Norway Continental Shelf) channel [20] (frequency passband: 10.5 kHz–14.5 kHz).  
longer to be sent. Furthermore, in Figure 8, 2 MC-SS-2D modems have a SF of 32 but different performances. Indeed, unlike to NCS1 channel, NOF1 channel varies slowly in time reducing the impact of time diversity. Therefore, a greater SF in frequency domain brings better results. However, SF in frequency domain is limited by number of subcarriers and the passband. These results show the robustness of the designed modem. In addition, SF can easily be modified to change the processing gain and adapt to SNR and range. Such characteristics are interesting for AUVs applications. The distribution of the SS in the time and frequency domains can affect the performances of the modem and its optimization is important and challenging.