# Using Fast Frequency Hopping Technique to Improve Reliability of Underwater Communication System
## 3 Results of Underwater Experimental Tests  
In order to determine the quality of data transmission, experimental tests were conducted on 4-5 May 2017, on Lake Wdzydze/Poland near the headquarters of the Hydroacoustic Research Station of the Sonar Systems Department/Gdansk University of Technology. Figure 5 shows the general configuration of the measuring stations. Each of the transmitted signals was generated in the MATLAB environment, and the conversion of this signal to analogue form was performed using the digital-to-analogue converter available in the National Instruments USB-6363 recording and generating device. The signal was then amplified and transmitted through an underwater channel using a HTL-10 underwater telephone [15]. Its design is based on the assumptions of programmable radio (SDR--Software-Defined Radio). This assumption makes it possible to use it for research purposes with various signals in the bands from 1 kHz to 60 kHz. The HTL-10, which I am a co-author of the hardware and software part, has been successfully used by the Polish Navy on its helicopters for several years. It uses hydroacoustic antenna of the OKA-2M dipping sonar that is designed to search for submarines because the helicopter devices allow it to descend into the water using a several-hundred-meter-long cable when the helicopter is hovering. The transmitting measuring station was placed on board a motor boat. The same measuring equipment was used on the receiving side, but it was configured differently. The receiving measuring  
Figure 5: Configuration of measuring stations. station was placed in a measuring container placed on a floating platform moored to a bridge with a fixed position during each of the measuring sessions. The received signal was sampled and archived. The transmitting transducer was lowered from the motor boat to a depth of \(h_{Tx}=10\) m regardless of the depth of the water reservoir, which for all transmitting positions of the measuring stations was from 20 to 30 m. The receiving transducer was lowered to a depth of \(h_{Rx}=4\) m, in a place where the water depth was equal to 7 m. In subsequent sessions, the distance \(d\) between the transmitting and receiving stations changed and amounted to 340, 550 and 1035 m, respectively. On the receiving side, the analogue signal was subjected to initial analogue processing by the analogue processing unit of the underwater telephone HTL-10. The obtained signal is digitized by an analogue-to-digital converter with a sampling frequency of \(f_{s}=200\) kHz. Then, the digital receiver uses the digital down converter technique (DDC) to directly convert the real passband signal around the selected carrier frequency into a complex baseband signal around zero frequency [26]. In the conversion process, the receiver uses optimized algorithms for digital signal processing. Before performing the series of measurements, the sound velocity distribution profile was measured using a sound velocity distribution meter. The measured profile and the determination of the acoustic wave propagation paths based on it made it possible to estimate the expected range obtainable of the communication system, which is presented in Figure 6. Propagation paths for the hydroacoustic transducer submerged to a depth of 10 m were determined. On the presented sound velocity distribution profile in Figure 6, the sound velocity gradient is negative, hence the drawing of propagation path clearly indicates that sound rays are refracted towards the bottom. In the considered system, the fast frequency hopping technique assumes the use of BFSK modulation due to the availability of a narrow system frequency band of approximately 5000 Hz. In this technique, hops are made between available subbands during the transmission of a single symbol. The jumps are made according to the assumed pattern, and their maximum number in the tests was set at \(L=9\). The transmissions of subsequent symbols with the hop duration \(T_{H}=\{4\) ms, 16 ms, 64 ms\(\}\) were subjected to experimental tests.