# A Novel Transfer Learning-Based OFDM Receiver Design for Enhanced Underwater Acoustic Communication
## 4 Experimental Setup
### Channel Setup
#### 4.2.2 Target Channel Setup  
To better understand the time-varying characteristics of the UWA channel and their impact on network performance, a comprehensive experiment was carried out on Qingdao Lake, Hangzhou, Zhejiang Province. The experiment aimed to collect real-world data for testing the TL-based OFDM receiver model proposed. The environmental conditions in the lake were carefully monitored through the deployment of several sensors, which provided a rich dataset of critical environmental parameters, including water temperature, salinity, pressure, and wind speed at different times of the day (morning, noon, and evening), as shown in Table 6. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Morning** & **Noon** & **Evening** \\ \hline Water Temperature (\({}^{\circ}\)C) & 9.2 & 12.5 & 10.1 \\ Salinity (ppt) & 0.12 & 0.14 & 0.13 \\ Pressure (Pa) & 32.5 & 32.2 & 32.9 \\ Wind Speed (m/s) & 3.1 & 4.2 & 3.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Environmental Measurements in Qingdao Lake. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline Name & NOF1 & BCH & KAU1 & KAU2 & NCS1 \\ \hline Environment & Fjord & Harbour & Shelf & Shelf & Shelf \\ \hline Time of year & June & June & July & July & June \\ \hline Range & 750 m & 800 m & 1080 m & 3160 m & 540 m \\ \hline Water depth & 10 m & 20 m & 100 m & 100 m & 80 m \\ \hline Transmitter depl. & Bottom & Suspended & Towed & Towed & Bottom \\ \hline Receiver depl. & Bottom & Suspended & Suspended & Suspended & Bottom \\ \hline Frequency range & 10–18 kHz & 32.5–37.5 kHz & 4–8 kHz & 4–8 kHz & 10–18 kHz \\ \hline Sounding duration & 32.9 s & 59.4 s & 32.9 s & 32.9 s & 32.9 s \\ \hline Delay coverage & 128 ms & 102 ms & 128 ms & 128 ms & 32 ms \\ \hline Doppler coverage & 7.8 Hz & 9.8 Hz & 7.8 Hz & 7.8 Hz & 31.4 Hz \\ \hline Type & SISO & SIMO & SIMO & SIMO & SISO \\ \hline Element spacing & - & 1 m & 3.75 m & 3.75 m & - \\ \hline Cycles & 60 & 1 & 1 & 1 & - \\ \hline Total play time & 33 min & 1 min & 33 s & 33 min \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The experimentation setup of different watermark channels. Figure 8 depicts the setup for the lake experiment. The measurement has lasted six days, and channel measurements are taken over the selected intervals to capture the variability in the underwater communication. These data gathered include all key parameters that characterise the underwater channel: distance between transmitter and receiver, depth, sound speed profile, and the measured SNR. Other important underwater communication factors, such as multipath spread and Doppler spread, were measured. Maximum multipath delay spread was 27.5 ms and the Root Mean Square (RMS) multipath delay spread was 29.3 ms. These values give the magnitude of the reflected signal from various underwater surfaces, the water surface and lakebed along with its delay. The Doppler spread, which gives a measure of the frequency shift due to the relative motion of the transmitter and receiver with respect to the environment, was also quantified. The maximum Doppler spread was 2.3396 Hz, while the RMS Doppler spread was 3.1487 Hz, showing moderate motion-induced variations. Figure 7: The configuration of different watermark channels. The data acquired in this experiment are used for testing the proposed TL model. The pre-trained model, here trained using a diverse set of underwater channels, would be fine-tuned with real-world data from Qingdao Lake. It is crucial for the TL model to be capable of adapting to the Qingdao Lake environment. Fine-tuning the pre-trained model with real channel data will enable it to handle the dynamic changes of the underwater channel and improve performance under real conditions. This experimental setup and data collection process provide practical validation of the transfer learning-based OFDM receiver and make sure that the model will be able to handle the complexities of other real underwater channels.