## 4 Results and Discussion
### Simulation Results  
The Simulink platform code can be easily adapted to the register transfer level (RTL) code and joint test action group (JTAG) co-simulations and can be ported to the field-programmable gate array (FPGA) platform for hardware implementation. The emulator possesses additional capabilities, such as data symbols, filtering, sampling, up-converting, and down-converting signals to be processed through the emulator. Example plots for channel responses over narrowband and wideband channels are provided in Figure 6.  
In order to generate a simulated version of channel samples for different UWAC scenarios using our Simulink-based platform, we chose two different centre frequencies, i.e., 1 MHz (low) and 100 MHz (high), and two different channel bandwidths, i.e., 10 KHz (narrowband) and 20 MHz (wideband). Of these, a 1 MHz centre frequency with 10 KHz of bandwidth has already been established as the selectable range for UWAC [23] (refer to Figure 6 bottom right-hand side). By increasing the bandwidth to 20 MHz, we obtain a predictive view of whether reliable communication is achievable within this frequency range and bandwidth. Please refer to Figure 6, the top right-hand side, for further details. On the other hand, UWAC systems often utilise bandwidth that is relatively wide compared to the centre frequency of the signal, thus qualifying as wideband systems in a relative sense [24]. Therefore, we also investigate the behavior of the channel when employing a broader bandwidth with a high centre frequency for communication (see Figure 6, top left-hand side). However, it is widely recognised that the path loss of acoustic signals is frequency-dependent, and higher centre frequencies experience increased path loss, as depicted in Figure 6, bottom left-hand side, and Figure 6, top left-hand side. Figure 5: Block diagram of the Simulink model for the channel emulator. We have not yet included noise in our generated results; however, the emulator platform is very flexible and any noise model and distribution can be added (a generalization that we will explore in future work). In Figure 6, top left-hand side, only one or two dominant paths are visible and the delayed versions of channel responses are highly attenuated. In Figure 6, bottom left-hand side, we can observe multiple dominant paths along with several low-strength reflected micro-paths. These arise due to the narrowband characteristics of the channel, which leads to the presence of multiple unresolved micro-paths. Unresolved micro-paths result in frequency-selective destructive and constructive interference, although micro-paths themselves do not suffer from frequency-dependent attenuation. Now, let us look again into the low-frequency responses in Figure 6 (bottom right-hand side) and Figure 6 (top right-hand side). Although the scenario presented in the bottom right-hand side has been experimentally validated as suitable for long-distance transmission underwater, this scenario suffers from a delayed version of reflected paths. Reflected paths can be strong in some instances, resulting in destructive interference; this necessitates the use of robust filtering on the receive side. However, based on the observation in the top right-hand side, a wider bandwidth for low-frequency range communications is recommended. In terms of the predictive behavior obtained from our model, a wider bandwidth can result in a lower number of sparse reflected paths and a strong dominant signal path. In the absence of actual measurement data, in order to establish the validity of our simulation platform, we compare our results with those generated by the well-established BELLHOP platform. We generated channel impulse responses (CIRs) using the BELLHOP simulator for four different transmitter depths transmitting hyperbolic frequency modulated pulses at centre frequencies of 4 kHz, bandwidths of 1 kHz, and pulse durations of 0.68 s (refer to Figure 7). A similar set of parameters is used to generate acoustic signal paths using our proposed simulator.