## 3 New Link Adaption Technique in Maritime Environment
### Maritime Auto-Rate Fall-Back  
Two approaches to LA techniques can be used to adjust the MCS: the auto-rate fall-back (ARF) technique, which adjusts the MCS profile based on the historical communication success record, and the receiver-based auto-rate (RBAR) technique, which measures the actual received signal strength to adjust the MCS profile [28; 29]. The RBAR technique has an advantage over ARF in that it determines the MCS profile based on the actual  
Figure 3: Two-ray and three-ray signal path loss characteristics in marine wireless channel. received signal strength, providing a more accurate channel condition-based MCS profile decision. However, it incurs overheads because it involves measuring the signal strength of the MCS profile for each communication instance. Additionally, it has limitations, as it may inaccurately interpret channel conditions owing to momentary fading phenomena that occur during signal measurements. Therefore, in environments such as maritime communication, where continuous communication is not maintained and deep nulls are present, utilizing the ARF based on historical communication records may be a more efficient approach. The existing ARF technique considers the current channel condition to be excellent if ACK signals for the transmitted data are received within the timeout period; otherwise, it is assumed that the channel condition is not excellent. In other words, when acknowledgments of multiple data transmissions are received, the channel condition is deemed to be sufficiently excellent, prompting a change in the MCS profile. However, existing ARF techniques are designed for terrestrial communication; therefore, they are ineffective in maritime environments. In maritime communication, which is characterized by significant signal distortion, particularly deep null path loss, as opposed to terrestrial wireless communication, it is imperative to introduce an optimized LA technique. Thus, we propose a new ARF technique capable of addressing deep nulls, called the mARF technique. The mARF technique was applied to the VDE-TER in the VDES, and the MCS profiles used in the VDES-TER are listed in Table 1. As indicated in Table 1, the VDE-TER provides nine different MCS profiles, and each profile is identified through the link configuration ID. In addition, each MCS technique has a different packet error rate (PER), and an appropriate link configuration ID must be set based on the channel situation. The PER for each link configuration ID is defined as follows. \[PER=1-\left(1-SER\left(\frac{E_{s}}{N_{o}}\right)\right)^{N_{symbol}} \tag{4}\]  
where \(N_{symbol}\) denotes the symbol length and \(SER\left(\frac{E_{s}}{N_{o}}\right)\) denotes the symbol error rate, which is determined by the modulation and coding rates. Table 2 describes the SER function as modulation scheme. Consequently, we can derive the throughput using  
\[Throughput=(1-PER)\cdot R_{s}\cdot\log_{2}M\cdot r\cdot\frac{L-C}{L} \tag{5}\] \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Link Config ID** & **11** & **12** & **13** & **14** & **15** & **16** & **17** & **18** & **19** \\ \hline Channel BW (kHz) & 25 & 25 & 25 & 50 & 50 & 50 & 100 & 100 & 100 \\ Symbol rate (ksps) & 19.2 & 19.2 & 19.2 & 38.4 & 38.4 & 38.4 & 76.8 & 76.8 & 76.8 \\ FEC rate & \(1/2\) & \(3/4\) & \(3/4\) & \(1/2\) & \(3/4\) & \(3/4\) & \(1/2\) & \(3/4\) & \(3/4\) \\ Modulation & \(\pi/4\) & 8 & \(\pi/4\) & 8 & \(\pi/4\) & 8 & \(\pi/4\) & 8 & 16 QAM \\ QPSK & PSK & 16 QAM & QPSK & PSK & 16 QAM & QPSK & PSK & 16 QAM \\ \hline \hline \end{tabular}
\end{table} 
Table 1: VDE-TER configuration parameters. \begin{table}
\begin{tabular}{c c} \hline \hline
**Modulation** & **SER** \\ \hline \(\pi/4\) QPSK & \(Q\left(\sqrt{\frac{2E_{s}}{N_{o}}}\right)\) \\ \(8\) PSK & \(2Q\left(\sqrt{\frac{2E_{s}}{N_{o}}}sin\frac{\pi}{8}\right)\) \\ \(16\) QAM & \(3Q\left(\sqrt{\frac{4E_{s}}{N_{o}}}\right)\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SER function as modulation scheme.