# A Novel Transfer Learning-Based OFDM Receiver Design for Enhanced Underwater Acoustic Communication
## 4 Experimental Setup
### Channel Setup
#### 4.2.2 Target Channel Setup
Measured parameters of the Qingdao Lake channel are given in Table 7 and the configuration is given in Figure 8. In addition, the CIRs and time-varying impulse response (TVIRs) of different channels used in the simulations are given in Figure 9 and Figure 10, respectively. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Parameter** & **Value/Observation** & **Unit/Description** \\ \hline Distance (Range) & 170 & meters (m) \\ \hline Transmitter Depth & 34 & meters (m) \\ \hline Receiver Depth & 34 & meters (m) \\ \hline Sound Speed Profile (SSP) & 1480–1495 & m/s (dependent on depth) \\ \hline Maximum Multipath Spread (\(\tau_{max}\)) & 0.0275 & seconds (s) \\ \hline RMS Multipath Spread (\(\tau_{rms}\)) & 0.0293 & seconds (s) \\ \hline Maximum Doppler Spread (\(v_{max}\)) & 2.3396 & Hz \\ \hline RMS Doppler Spread (\(v_{rms}\)) & 3.1487 & Hz \\ \hline Environmental Temperature Range & 9–12 & \({}^{\circ}\)C \\ \hline Salinity & Low (Freshwater) & Qingdao Lake is a freshwater lake \\ \hline Probe Signal Type & LFM & Frequency: 8000–16,000 Hz \\ \hline Number of Sounding Signals per Group & 272 & Each group lasts 30 s \\ \hline Testing Frequency & 3 times per day & Interval \(\approx\) 3 min between groups \\ \hline Number of Data Samples Collected & 34 & Effective measured channel \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Measured parameters and environmental variability of Qingdao Lake channel. Figure 8: Configuration of Qingdao Lake channel. The channel characteristics in Figure 11 clearly illustrate that the UWA channels are time-varying in nature due to the dynamic and harsh environmental characteristics. The time series evolution of the signal received shows time-varying signal strength. In Figure 11a,b, the signal remains almost stable with a gradual fading behavior, which is normal in underwater environments. This attenuation, most probably due to multipath propagation, depends on reflections from surfaces such as the water surface or lakebed, causing delays in the received signal. With time, different color variations in the plots reflect changes in signal strength, indicative of environmental conditions such as water currents that alter the propagation of sound waves. Delay Doppler spread plots give a clearer view of the signal strength. Figure 10: The TVIR of different channels. Figure 9: The CIR of different channels. of the distribution of the signal frequency and time delay. From Figure 11a,b, the energy is concentrated at low Doppler shifts and short time delays, indicating dominance of the direct path. However, for Figure 11c, we see significant Doppler shifts, indicating there is a relative motion between transmitter and receiver. This introduces frequency shifts characteristic of underwater communication, where water movements or objects take their toll on signal frequency. Instead, the delay Doppler spread becomes more sophisticated, with the energy spread around a wider area of frequencies and time delays in Figure 11e, indicating larger object movements and/or increased water turbulence. The power delay profile plots give the distribution of the signal's energy over the time delays. Figure 11a-c have a dominant peak due to the direct path, but with smaller peaks indicating some multipath components, meaning reflections are present though the direct path is primary. Accordingly, the profiles represented by Figure 11e,f depict higher complexity and multi-peaks due to strong multipath propagation, hence proving high reflections on multiple surfaces of various distances.