## 4 OFDM Transmission Tests  
In order to protect the signal transmitted in the UAC system against the ISI and ICI, the physical layer of data transmission should be matched to the transmission parameters of the channel. The subcarrier spacing should be smaller than the coherence bandwidth and much larger than the Doppler spread. On the other hand, the duration of the OFDM symbol should be longer than the delay spread, and much shorter than the coherence time of the channel [8]. The transmission parameter values described in Section 3 differ significantly depending on the method of their determination. For example, the coherence bandwidth determined as the width of SFCF at the threshold level of 0.5 of the maximum value is almost four times larger than the coherence bandwidth determined at the threshold level of 0.9. Depending on the particular transmission parameters set, different subcarrier spacing and symbol duration values meet the physical layer designing rules. Thus, four possible OFDM modulation schemes were chosen for the inland-water experiment, differing in the number of subcarriers, which was equal to 64, 128, 256, or 512. The number of subcarriers determines the values of subcarrier spacing \(B_{OFDM}\) and symbol duration \(T_{OFDM}\), as it is shown in Table 2. The reliability of the data transmission with the use of such a configured OFDM waveform was tested during the inland-water experiment. The OFDM signal bandwidth \(B\) was the same as in the case of the PRBS probe signal, that is 5 kHz, around the carrier frequency \(f_{c}\) of 30 kHz. For each of the OFDM parameter configurations, 20 transmission tests were performed and the mean BER was calculated (Table 2). The lowest BER was obtained in the case of 256 subcarriers with subcarrier spacing equal to 19.53 Hz and symbol duration equal to 51.20 ms. In the case of 512 subcarriers, the BER was significantly higher than for other subcarrier configurations. As can be seen, in the case of using pilot tones as the only technique of ISI suppression, it is possible to obtain data transmission with a BER less than \(10^{-1}\). The data transmission rate, in this case, is equal to 1.42 kbps. The next experimental tests were conducted using BCH and Reed-Solomon Error-Correction Coding of different number of information bits \(L_{i}\) and message length \(L_{msg}=2^{m}-1\), where \(m\in\{5,6,7,8\}\). The results of BER values depending on the code rate \(C_{r}=L_{i}/L_{msg}\) are shown in Figure 7. From among the ECC parameter configurations, those were chosen that allow obtaining the BER less than \(10^{-2}\), \(10^{-3}\), or \(10^{-4}\) with the lowest possible redundancy, and thus the highest possible coding rate. The transmission rates obtained with these encoding parameters are shown in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline
**No of Subcarriers** & **Subcarrier Spacing** & **Symbol Duration** & \
\begin{tabular}{c} **Symbol Duration with** \\ **Cyclic Prefix** \\ \end{tabular} &
\begin{tabular}{c} **BER** \\ **(no ECC)** \\ \end{tabular} \\ \hline
64 & 78.13 Hz & 12.80 ms & 16.00 ms & 0.0471 \\
128 & 39.06 Hz & 25.60 ms & 32.00 ms & 0.0308 \\
256 & 19.53 Hz & 51.20 ms & 64.00 ms & 0.0249 \\
512 & 9.77 Hz & 102.40 ms & 128.00 ms & 0.1615 \\ \hline \end{tabular}
\end{table} 
Table 2: Configurations of OFDM signal parameters and BER achieved during the inland-water experiment.