# A Weighted Turbo Equalized Multi-Band Underwater Wireless Acoustic Communications
## 5 Conclusions  
For improving the performance in UWAS communications, multi-band techniques, and turbo equalization models have been used, in recent years. The multi-band communication technology is effective in performance and processing efficiency because it can overcome selective frequency fading, by allocating the same data to different frequency bands, in an environment that has a rapidly changing UWA channel transmission characteristic. In others, at the receiver side, we resorted to powerful turbo equalization algorithms that iteratively exchanged probabilistic information between the DFE and the turbo decoder, thereby, reducing the error rates significantly. We have analyzed the efficiency of multi-band transmission schemes in the UWAS communication, by applying 1-4 number of multi-bands, using a turbo pi code with a coding rate of \(1/3\). This was carried out in an actual underwater experiment which was conducted at the sea, in front of the Korea Maritime and Ocean University, Korea, in March 2018. Experimental results showed that, in general, the performance increased as the number of multiple bands and iterations of turbo equalization increased. However, a multi-band configuration can have poorer performance than a single-band, or double-band  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{\(N_{b}\)} & \multirow{2}{*}{\(f_{c}\)} & \multirow{2}{*}{**Preamble BER**} & \multirow{2}{*}{**Weighting**} & \multicolumn{6}{c}{**Total Number of Iterations**} \\ \cline{5-10}  & & & & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline \multirow{4}{*}{4} & \(f_{1}\) & \(10^{-0.463}\) & \(w_{1}=0.2\) & & & & & \\  & \(f_{2}\) & \(10^{-0.718}\) & \(w_{2}=0.8\) & & & & & \\  & \(f_{3}\) & \(10^{-0.806}\) & \(w_{3}=0.9\) & & & & & & \\  & \(f_{4}\) & \(10^{-0.876}\) & \(w_{4}=0.9\) & & & & & & \\ \hline \multirow{4}{*}{4} & \(f_{1}\) & \(10^{-0.701}\) & \(w_{1}=0.8\) & & & & & \\  & \(f_{2}\) & \(10^{-0.718}\) & \(w_{2}=0.8\) & & & & & \\  & \(f_{3}\) & \(10^{-1.595}\) & \(w_{3}=1.0\) & \(10^{-0.634}\) & 0 & 0 & 0 & 0 & 0 \\  & \(f_{4}\) & \(10^{-0.247}\) & \(w_{4}=0.1\) & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Experimental results for preamble error rate-based threshold-setting algorithm.  
configuration. This problem could be solved by using a receiving end that analyzed the error rates of each band, set the threshold values, and allocated the lower weights to the inferior bands. In this paper, we proposed an algorithm to set the threshold value using the preamble error rate which was known data that had to be transmitted and received, based on the relationship between the BER of un-coded preamble sequence and the coded data field. We confirmed that when the total iteration number of turbo equalization was 1 or 2, all of the data could be decoded without any error. Based on the experimental results, multi-bands with a weighted turbo-equalized algorithm is a useful technology for performance enhancement, in the environment of UWAS communications.  
For future studies, based on the proposed algorithm, we will apply it to the multicarrier OFDM system to compensate for the multi-path effect and increase the throughput. Finally, it will be applied to long-range UWAS communications that will further increase the distance between the transceivers.  
Conceived and designed the experiment, [PERSON]-[PERSON], [PERSON]-U.B. and [PERSON]-[PERSON]; Analyzed the experimental data, H.-S.L., C.-U.B. and [PERSON]-[PERSON]; Wrote the paper, H.-S.L.; Revised the paper and offered useful suggestion to write, [PERSON]-U.B., and [PERSON]-[PERSON].  
This research was funded by the Agency for Defense Development grant number [UD170022 DD].  
The authors declare no conflicts of interest.