# A Method for Underwater Wireless Data Transmission in a Hydroacoustic Channel under NLOS Conditions
## 5 Results
### The Transmission Quality in Laboratory Conditions  
When assessing the transmission quality, the bit error rate for reception from single hydrophones and the diversity combining from two and three reception paths was determined. The tests were carried out with the following transmission parameters: \(N_{sf}=5\), \(N_{bs}=2000\), \(F_{d}=10\) kHz, \(\Delta f_{t}=20\) Hz, \(\Delta f_{bits}=80\) Hz, \(\Delta f_{r}=1\) Hz, \(\tau_{guard}=0.04\) s, \(F_{c_{rememble}}=55\) kHz, \(N_{premble}=511\), \(S_{premble}=20\) kBd. The sampling frequency was 500 kHz (simultaneous). According to the above parameters, the symbol duration was 50 ms, the bandwidth occupied was 10-170 kHz, and the transmission rate was 19.397 kbit/s.  
Figure 11 shows an example of a fragment of the spectrum of the signal transmitted and received at measurement point number 3. Figure 11a shows the spectrum of transmitted signal for 19 bits. In Figure 11b, the spectrum of the signal received by individual hydrophones for the same 19 bits is shown. Figure 11c shows the spectrum after diversity combining. The obtained result indicates that, at the same frequencies, each of the hydrophones received a signal of a different strength. At some measurement points, we observe multipath-induced decays. In Figure 11b, the special case where the hydrophone No. 2 did not register the signal at a certain frequency is marked with an arrow. It should be noted that in the case of diversity combining, due to summing the spectra from the three reception paths, it is possible to detect all bits.  
Figure 12 shows the result of the bit error rate for the individual measurement points. At individual measurement points, the results obtained from individual hydrophones are similar to each other, i.e., a similar error rate was obtained. As the set of the receiving hydrophones was moved further down the corridor, the reception quality expressed in BER deteriorated. Diversity combining improves BER in all cases. As expected, the lower BER was obtained using three hydrophones. It should be noted that the greatest gain from diversity combining, even two orders of magnitude, was obtai  
Figure 11: An example of fragments of amplitude spectrums (**a**) transmitted signal, (**b**) received signals, (**c**) after diversity combining.  
points located near the entrance to the corridor. For the points at the end of the corridor, the gain from diversity combining was two times smaller.  
For comparison, an experiment was carried out in which the DSSS transmission with BPSK modulation [22] was performed under the same propagation conditions as the MFSK transmission described earlier. The transmission parameters were as follows: sampling frequency 500 kHz, carrier frequency 55 kHz, modulation rate 20 kHz, 4 chips per bit, therefore the transmission speed was about 5 kbit/s. This speed is comparable to the speed of the signal transmission in the method used in the present work, assuming the same bandwidth. Assuming that the bandwidth occupied by the MFSK signal would be 40 kHz then it would give a transmission rate of 4.85 kbit/s, which is comparable to the speed of the performed DSSS transmission. The results of the experiment are shown in Figure 13.  
Figure 13 shows that the diversity combining carried out in accordance with the method for DSSS signals described in [22] resulted in the improvement of the reception  
Figure 12: Quality of data reception.  
Figure 13: Quality of data reception for DSSS transmission.  
quality. However, comparing the obtained BER error rates with the results presented in Figure 12, the proposed method based on MFSK is much better than the traditional DSSS. This is mainly due to the greater resistance of the MFSK method to the effects of the multipath phenomenon.