## 3 New Link Adaption Technique in Maritime Environment
### Maritime Auto-Rate Fall-Back
where \(R_{s}\) indicates the symbol rate; \(\log_{2}M\) indicates the number of bits in each modulation symbol; \(r\) indicates the channel coding efficiency; \(L\) indicates the total data length of each packet; and \(C\) indicates the data overhead, such as the header packet of each packet [30; 31]. One critical consideration for the design of mARF is the occurrence of a phenomenon known as \"deep null\" in maritime communication channels, where signal attenuation occurs rapidly, and then, suddenly disappears. In other words, a technique that can address the deep null phenomenon is required. When experiencing a deep null, there is sharp signal attenuation; therefore, the most reliable communication technique must be used. Thus, mARF first determines that communication errors occur because of signal attenuation caused by the deep null phenomenon, not by data collision. Consequently, even if a single communication error occurs, mARF performs a fast drop-out operation. For example, when a communication error occurs once in link ID 19, it immediately switches to the lowest MCS profile--link ID 13--to ensure stable communication. The mARF technique increases the MCS profile as the channel conditions improve to enhance the channel utilization. As described earlier, communication situations deteriorate rapidly in maritime communication channels because of the deep null phenomenon; however, paradoxically, they can also improve dramatically. Thus, when using the most reliable MCS profile for communication and observing \(n\) consecutive successful communications, the algorithm switches to the next profile for high-throughput communication. In addition, after achieving \(\lfloor n/2\rfloor\) consecutive successful communications in the current MCS profile, it changes to the next higher MCS profile. We define the process of increasing the MCS profile as fast recovery. Algorithm 1 shows the pseudocode of the algorithm used in mARF to direct the rate of increase in the transmitter rate with fast recovery. ```
\(\mathit{MCS}_{nact}\):=\(\mathit{MCS}_{current}\): \(\mathit{MCS}_{high}\):= the MCS type higher than \(\mathit{MCS}_{current}\); \(\mathit{MCS}_{low}\):= the lowest MCS type among MCS profile; \(\mathit{MCS}_{br}\):= \(\mathit{MCS}_{high}\) \(\mathit{attempt\_success}\):= 0; while (\(\mathit{MCS}_{nact}\):= \(\mathit{MCS}_{current}\) \(\mid\)\(\mathit{attempt\_success}==0\)) { if(\(\mathit{xmit\_success}\)(\(\mathit{MCS}_{nact}\))== 0) then\(\mathit{MCS}_{nact}\):= \(\mathit{MCS}_{low}\) / rate decreasing attempt by fast drop-out/ else {  attempt_success:= attempt_success + 1; if(\(\mathit{attempt\_success}\geq n\)) then { \(\mathit{MCS}_{next}\):= \(\mathit{MCS}_{high}\) / rate increasing attempt by fast recovery/ break; } } if(\(\mathit{MCS}_{next}==\mathit{MCS}_{br}\)) thenattempt_success:= attempt_success/2; elseattempt_success:= attempt_success; }
```  
**Algorithm 1** Pseudocode for MCS adaptation procedure in mARF. In addition, \(n\), which is an MCS-changing counter, can be adaptively updated based on previous communication history. When updated adaptively, it can be updated through the weight relationship between the previous \(n\) values, as shown in Equation (6):  
\[n_{t}=\alpha\cdot n_{t-1}+(1-\alpha)\cdot n_{t-2} \tag{6}\]  
where \(\alpha\) is a weighted factor and ranges between 0 and 1. To determine the \(\alpha\) value, we performed the simulation by changing \(\alpha\) from 0 to 1 in 0.1 increments, and confirmed that giving 70% weight to the immediately previous value had the best performance. Thus, the mARF technique uses a default \(\alpha\) value of 0.7. Additionally, the \(\alpha\) value can be changed by the user. In addition, the mARF technique determines the MCS profile based on existing communication history and adjusts the MCS change threshold, so it is very efficient as it can greatly reduce the computational complexity for MCS change in systems such as the VDES with a limited number of usable MCS profiles.