# A Weighted Turbo Equalized Multi-Band Underwater Wireless Acoustic Communications
## 3 Experimental Conditions and Parameters  
The actual underwater test was conducted in the same environment, as shown in Figure 3. The experiment was conducted, at sea, in front of the Korea Maritime and Ocean University, Korea, in March 2018, as shown in Figure 3a. The experimental setup schematic is shown in Figure 3b. The water depth was approximately 3 m. The hydrophone was at 2 m, below the water surface, and the horizontal range from the transmitter was 360 m. We used NEPTUNE D/17/BB transmitter and a B&K 8106 receiver. A photo of the measurement equipment-setup is shown in Figure 3c,d. In Figure 4a, the UWAS channel response is shown during a 0.02 s interval. This response was measured by using an LFM (Linear Frequency Modulation) signal with a bandwidth of 2 kHz. The channel gains fluctuated for the secondary and third arrivals. This sparse channel was affected by a multi-path propagation, by reflection from the surface and the bottom. Figure 4b shows the Doppler-spreading for the different arrival times of the LFM signal; a tiny Doppler spreading in the range of \(-1\) Hz to 1 Hz had occurred. Figure 3: Illustration of the sea trial: (**a**) Area map of the sea trials, (**b**) experimental setup schematic, (**c**) transmission equipment, (**d**) receiver equipment. This study conducted an underwater experiment where the parameters shown in Table 1 were used to increase the number of bands. As presented in Table 1, there was a total of 592 bits, consisting of 256 preamble bits and 336 coded bits. QPSK (Quadrature Phase Shift Keying) modulation was applied to form 296 symbols. The SRRC (Square Root Raised Cosine) filter with the roll-off factor of 0.35 was applied as the filters of the receiving and the transmitting units. The inner iteration of the turbo decoder was five times. The total iteration between the turbo decoder and the DFE was six times. The transmission rate was 1 kbps. \begin{table}
\begin{tabular}{c c} \hline Source & 112 bits \\ \hline Preamble bit & 256 bits \\ \hline Number of inner iteration (Turbo decoder internal iteration) & 5 \\ \hline Number of total iteration (Iteration between turbo decoder and & 6 \\ DFE) & \\ \hline Channel coding & Turbo Pi code \\ \hline Coding rate & 1/3 \\ \hline Modulation & QPSK \\ \hline Packet size & 296 symbols \\ \hline Equalizer & LMS DFE \\ \hline Number of bands (\(N_{b}\)) & 1–4 \\ \hline  & \(f_{1}=12\) kHz \\ Center Frequency of each Band & \(f_{2}=16\) kHz \\  & \(f_{3}=20\) kHz \\  & \(f_{4}=24\) kHz \\ \hline Number of samples (\(N_{s}\)) & 384 \\ \hline Sampling frequency & 192 kHz \\ \hline Bit rate & 1 kbps \\ \hline Distance & 360 m \\ \hline Water depth & 3 m \\ \hline Depth & Transmitter: 2 m, Receiver: 2 m \\ \hline \end{tabular}
\end{table} 
Table 1: Experimental parameters. Figure 4: Underwater experimental environment: **(a)** CIR (Channel Impulse Response); **(b)** scattering function. The center frequencies of each band were 12 kHz, 16 kHz, 20 kHz, and 24 kHz, respectively, and the sampling frequency was 192 kHz. When four multi-bands were used, the center frequency was allocated, as shown in Figure 5.  
The transmission signal had a bandwidth of 1.35 kHz. To prevent the inference of adjacent bands, a guard band of 0.65 kHz was allocated between each band. When the four multi-bands were used, the total bandwidth was 13.35 kHz. Figure 6 illustrates the packet structure of the transmission signal. Figure 6a shows the packet structures, inside each band. As is clear from Figure 6, a single packet was composed of LFMB (LFM Begin) signal, for notifying the beginning of transmission for 0.5 s, the silence section for 2 s, the preamble bit and the transmission data, another silence section for 2 s, and the final LFME (LFM End) signal, for notifying the end of the signal, for 0.5 s. Figure 6b presents the packet structure of the transmission signal in the case of \(N_{b}=4\). The packet structure of each of the bands, shown in Figure 6a, was located at \(N_{b}=1\), \(N_{b}=2\), \(N_{b}=4\), in Figure 6b.